Last Modified: July 27, 2014
Contents: Introduction; The Devil's Playground; Ground Loops; Adequate Power; Power Connections; Load Dump Transients; Power Disconnections; Expelling A Few Myths; What Size Wire to Use; Insulation & Stranding; Wire Terminations; All About Fuses; Circuit Breakers; A Few Words On Relays; Fuse Holders; Wiring Through the Firewall; Hiding the Wiring; Under Chassis Wiring; Power Protectors;
As you read this article, it will become apparent that wiring amateur radio equipment in late-model vehicles is becoming more difficult and exacting. A good example of the increasing complexity is Mazda's i-ELOOP (discussed below). This, and similar subsystems, are being introduced as CAFE (Corporate Average Fuel Economy) increasing strategies. It should be evident that attention to wiring details is becoming more important. With that thought in mind, here are the salient points to remember.
• Never, ever use existing vehicle wiring to power any amateur radio gear.
• The chassis of the vehicle should never be used as a ground return.
• The wire should be connected directly at the battery or jump points as the case may be (more on this below).
• Wiring should be done in a way to avoid circumventing any Battery Monitoring System (BMS).
• Direct connection to the output of a Voltage Stabilizer system should not be done, no matter the current draw.
• The wire should be sized large enough to limit voltage drop to less than .5 volts while under peak load.
• The power wiring must be properly fused to protect it from shorts.
• Avoid using circuit breakers, no matter their ratings.
• The wiring needs to be protected from abrasion, excess heat, and automotive chemicals.
• The wiring should be routed in a neat and tidy manner to avoid interaction with passengers and vehicle controls.
• The wiring insulation temperature ratings should be at least 90°C (195°F) in the passenger compartment, and at least 105°C (220°F) in the engine compartment.
• Terminations (lugs) should be properly crimped and/or soldered.
• Heat shrink connections where apropos
• Regular inspection and/or scheduled maintenance of wiring is an important undertaking.
• Do yourself a favor! Buy a Service Manual for your vehicle.
We'll be exploring each of these salient points below, along with links to relevant articles when needed.
The Devil's Playground
Never, ever use existing vehicle wiring to power any amateur transceiver, especially the accessory (cigarette lighter) sockets. These so-called power taps are typically wired with #16 AWG, and a few with #14 AWG. Either way, they're inadequate to handle even the average current draw of an amateur HF transceiver, much less the peak, no matter its power rating or fuse size. If you do, you run a very great risk of causing an electrical fire—the most costly of vehicle repairs, and the most traumatic as well!
The National Fire Protection Association, publishes a list of vehicle fire causes which includes (in part) sub-section 15-3.2.1:
Overloaded Wiring. Unintended high-resistance faults in wiring can raise the conductor temperature to the ignition point of the insulation, particularly in bundled cables such as the wiring harnesses or the accessory wiring under the dash where the heat generated is not readily dissipated. This can occur without activating the circuit protection.
The process alluded to above, is called pyrolysis. It is the decomposition of the insulation covering the wire, brought about by high temperatures—resistive heating in other words. Pyrolysis may occur within a few minutes, or over an extended length of time.
Again, the key words here are resistive heating. Aside from the small wire size feeding the accessory sockets, all mating plugs use spring loaded contacts. These contacts were never meant to continuously carry more one or two amps. Much more than this, and the center contact can get hot enough (due to resistive heating) to damage the wiring behind the socket. In this specific case, the expressed load was a laptop power supply drawing less than 3 amps!
A ground loop is defined as a voltage differential between any two points, in either a power or ground connection. The most common mobile manifestation is alternator whine superimposed (primarily) on the transmitted signal. It is often a direct result of using a mag mounted antenna. They often appear as an RFI issue, particularly when they cause data corruption in one of the various on-board digital devices. This is the exact reason why proper wiring, and bonding, is so important.
Speaking of bonding, if you haven't read the bonding article, please do. Remember this important point: Excessive bonding—running ground straps hither and yon—can create more problems than they solve! While off the subject, this rule applies to antenna mounts too!
With very few exceptions, any alternator-equipped vehicle made after 1980 has enough electrical reserves to power any modern 100 watt (200 watt input) transceiver. If the vehicle in question has a heated rear glass or heated seats, the reserve is typically large enough for even a 500 watt (1,000 watt input) amplifier. This is possible because we don't run all of the accessories at the same time.
The first American-made vehicle with a factory-equipped alternator was the 1960 Chrysler Valiant. The basic model was rated at 30 amps. By 1975, Chrysler's average alternator had grown to 75 amps. Today, it is not uncommon for OEM alternators to exceed 150 amps, and a few (right photo) are as big as 250 amps! It should be obvious from the first paragraph why alternators have increased in size. Less obvious is the switchover to electric power steering, electrically-boosted brakes, and EIS (Engine Idle Shutoff ). All of this, in an effort to reduce parasitic loads, and increase fuel economy. We're just along for the ride!
There have always been two schools of thought with respect to where the power cable ground of an amateur transceiver should be connected, but both agree that the chassis should not be used as a ground return. While this practice was wide-spread in the past, doing so in a modern vehicle should be avoided at all costs. It is important to remember that today's vehicles are rolling computers, with some models having more than 80 CPUs controlling everything from ABS to VSS! Each one of these CPUs has sensors connected to it which control every facet of operation. Using the chassis for a ground return can cause a ground loop to occur which can corrupt the data read from them.
The first school of thought is to connect the negative lead directly to the battery. The screen shot at right (click to enlarge) is Yaesu's recommended hookup for their FT-857D. Alinco, Icom, and Kenwood also recommend this method, as does every automobile manufacturer heretofore. Also note the X's indicating the incorrect methods. This method requires that the negative lead be fused to protect the transceiver in the rare case of a negative starter lead failure.
The second school of thought is to connect the ground lead to the battery's chassis ground point (not the engine block!). Both methods work equally well (unless you're running high power). However, on most newer vehicles with idle engine shutoff, both methods require scrutiny, high power or not.
Starting about 2002, manufacturers have begun monitoring the current load, and voltage of the electrical system. These so-called Electrical Load Detectors (ELD) use a Hall device incorporated into the main wiring harness fuse, in the battery's negative lead, or in some cases incorporated into one of the battery connectors (see photos below). The measured data is fed to the engine CPU (known by a variety of names). This not only allows more precise fuel-air adjustments for changing accessory loads (AC for example), the condition of the battery, over its lifetime, can also be accurately monitored. In some cases, BMW for example, if the battery is replaced the engine CPU must be reprogrammed. While a rarity in the industry today, that may change in the near future.
The engine compartments of modern passenger vehicles, are getting rather crowded. Thus some manufacturers are relocating the battery elsewhere in the vehicle. As a result, special connections are installed, called Jump Points. As the name implies, they're used as connection points when jump starting a vehicle with a dead battery. Mercedes Benz, Volkswagon, and others have used jump points for some time, and thus recommend they be used to power ancillary equipment (radios, high-power stereos, etc.). In some cases, they have pre-threaded holes specifically for the purpose. Because these jump points are adequately wired to handle heavy starting loads, they are convenient connections points for amateur radio gear. But that is not the whole story.
In an effort to meet the demand for increased fuel economy, automobile manufacturers are incorporating engine idle shutoff schemes (Fed mandated in most cases). In simple terms, when you stop at a traffic light, the engine shuts down. When the light turns green and you let off the brake or push on the gas pedal, the engine restarts and away you go! Ford Motor Company is no exception. In their own words:
Ford Auto-Start-Stop features an enhanced, durable starter motor to support the increased number of engine starts and a high performance Improved Flooded (IF) Battery with a Voltage Quality Module (VQM). This maintains the energy supply to electrical components such as the entertainment system during engine restart. In addition a Battery Monitoring System (BMS) continuously checks the battery charge and communicates with the Auto-Start-Stop function, so the system leaves the engine running in the event that the battery does not have sufficient charge.
It should be obvious that either school of thought with respect to the ground connection, may or may not fit a specific case, depending on where the ELD is located. Usually, it surrounds the battery's negative lead as shown at left. Ford and Honda have theirs built into the negative lead connection as shown below right. But, until automobile manufacturers update their two way radio wiring suggestions, it is up to us amateurs to use caution when wiring our transceivers and amplifiers to assure we do not circumvent or bypass Battery Monitoring Systems. There are other cautions as well.
This is a good point to bring up an automotive wiring change, few have noticed. In the past, battery wiring harnesses had two wires attached to both posts. The larger ones connected to the starter, and the engine block. The smaller ones to the accessory load, and the chassis near the battery. When running high power, using the chassis connection could cause excessive current to flow through the accessary ground and/or through the drive train with predictable results. With the advent of BMS, there is only one wire on each battery connection. Referring to the photo at left, note the wire extends from the battery's negative post, down to the chassis, and then to the engine block. It should be evident why this is a good thing, especially if you run high power.
First, VQMs should not be used to power amateur radio, no matter the current draw. They're only designed to handle the imposed accessary load, and no more! Further, some General Motors models have a second, but smaller, AGM battery mounted in the trunk area. This battery powers the VQM, and thus the accessories. According to GM, using this battery to power amateur gear is a warranty violation.
If you have any doubts about wiring amateur gear, contact your dealer's service personnel! However, be advised that they may know less about the subject than you do! Thus, it always pays to purchase a service manual for your vehicle.
Starting with the 2014 models, Mazda has added yet another level of frustration with their new i-ELOOP system. It features a large double-layer ultra-capacitor, a varying-voltage alternator (up to 25 volts output), a 50 amp rated DC-DC converter, and a whole lot of computing power! When the vehicle slows, the alternator provides regen-braking which charges up the ultra-capacitor, which in turn goes to the DC-DC converter which runs the accessories. Any left over is used to charge the battery, and provide a ≈10% increase in city driving mileage ratings.
The i-ELOOP system requires connecting the positive lead directly to the battery, and the negative to the battery's chassis ground connection, therefore not bypassing the ELD. However, the extra load may cause the system to change over to a standard alternator-battery charging configuration depending on the start-stop ratio. Incidentally, the system operates hand-in-hand with the EIS ( Engine Idle Shutoff), called i-Stop by Mazda.
The bottom line is this: The power leads from any amateur gear, must be connected in such a way to avoid bypassing any Battery Monitoring System (BMS), and/or overloading any DC-DC converter, and/or interfering with the Electrical Load Detector (ELD).
When connecting the negative lead to the chassis, do not remove the fuse assembly! If the grounding point should lose its integrity for whatever reason, excessive current can still flow through the transceiver's negative lead. It also prevents a ground loop between the leads, albeit small and typically not bothersome.
It is not uncommon for power to a vehicle's electrical system to be interrupted when installing radio equipment. Whether inadvertent or not, this interruption can have consequences. For example. anti-theft protection built into Navi and entertainment systems is often activated. This requires reentering an activation code. In some rare cases, the ignition system can also be effected. While these codes may be readily available (in the Owner's Manual), but in some cases your dealer is the only source.
In addition, vehicles equipped with programmable BMS (battery monitoring system) like BMW vehicles, may have to be reprogrammed by your dealer. And, vehicles equipped with fuzzy-logic control systems, will have reprogram themselves, over time. The bottom line here is, if in doubt, read the Service Manual, or contact your dealer's service department before undertaking your installation.
Load Dump Transients
Proper wiring maintenance is an important undertaking, but one which is often overlooked. One all to common consequence is a failure of a battery lead and/or its connection to the chassis or distribution block. The result is a load-dump transient (LDT). If one occurs, it is not uncommon for the resulting voltage spike to exceed several hundred volts! Fortunately, there is some built-in protection at least on late-model vehicles.
The diodes used in most late-model vehicle alternators are more than just simple diodes. They are designed to break down, and act like a reverse-biased zener diode should the output voltage exceed ≈18 volts. This provides first level protection for the on-board electronics should a battery connection be lost (LDT). This fact may help protect amateur radio gear interconnected to the vehicle's electrical system, but it doesn't alleviate the need to do periodic wiring maintenance especially fuse holders.
As noted below, there is a published standard for the voltage drop across a fuse and its holder. The drop assumes the connections are all secure. But time, dirt, and oxidation can cause these connections to increase in resistance, which increases the voltage drop. So part of the routine maintenance should include removing and reinserting fuses, power pole splices, and even chassis-mounted power connectors.
Expelling A Few Myths
One very popular Internet myth asserts that power wiring and coax cable runs should not be parallel to and/or bundled together with the coax feed line. The truth is, coax doesn't leak RF! What can (and does) happen is common mode current. In this case, RF flows on the outside of the coax shield which can be both radiated and induced into surrounding cabling, whatever it is. It is this fact which no doubt led to the myth in the first place.
There will always be some level of common mode current flow in any mobile installation. The reason is, there is always ground loss even a VHF frequencies. The more ground loss, the higher the level of common mode there will be. Poor antenna mounting techniques (mag and clip mounts) and locations (trailer hitch and bumper mounting) increase the level rather drastically. As the Common Mode article explains, proper choking of these currents must be applied.
Twisting the positive and negative power leads together to enhance noise immunity and/or cure alternator whine are popular myths. So popular in fact, that at least one automobile manufacturer mentions this myth in their two way radio installation guide. The truth is, power cables are not balanced in the same sense CAT5 cable is. Thus twisting the power lead wires has virtually no effect on noise immunity at common amateur frequencies.
Another popular myth is using a brute force filter to cure alternator whine, common mode current, or RF imposed on the power wiring (an exceedingly rare occurrence). For alternator whine, they're a patch at best, and if you wire your installation correctly, you won't need one. However, there is one thing brute force filters will do, and that's increase voltage drop. For no other reason, they should be shunned. The rule of thumb is, if they cure a problem, then something else in the installation is (was) amiss!
Contrary to popular belief, the chassis of an automobile has more resistance than copper wire. While the voltage drop may be tolerable when using the chassis as a ground return in low current applications, it should be avoided, and here's why. As the current increases, the resulting voltage differential between factory-wiring chassis tie points, may cause a ground loop to occur between them thus corrupting on-board remote sensor readouts.
What Size Wire To Use
Modern amateur mobile transceivers universally operate on a nominal 13.8 vdc. In a mobile scenario, the DC voltage actually varies from below battery resting voltage (≈12.2), to as high as 14.4 when the alternator is charging the battery. If we allow the voltage to drop much lower than 12.2, most transceivers will simply shut off. And, at low voltages the power output drops, and the IMD increases. Thus it behooves us to minimize the voltage drop in our wiring. The rule of thumb is not more than .5 volts. With that in mind, we need to know the peak current draw. In both cases the manufacturer's published figures are close enough. For an average 100 watt (200 watts input) transceiver, the peak current is approximately 22 amps which includes some parasitic draw like the cooling fan. A 50 watt FM transceiver is about half that or 11 amps.
Here is the formula for calculating the voltage drop for any given size and length of wire including the slight drop due to the fuses and their holders. For the record, voltage drop is frequently referred to in amateur literature as I2R (current squared times resistance) losses, but I2R actually refers to power loss.
(Rw • 2l • .001) + 2k] • A = Vd where:
Rw = the 1,000 foot resistive value from the Handbook Chapter 7 (#12=1.588Ω, #10=.9987Ω, #8=.6281Ω, #6=.3952Ω, #4=.2485Ω, #2=.1563Ω).
l = Overall length of the cable assembly including connectors.
k = nominal resistive value for one fuse and its holder. Note: most power cables have two fuses. If yours doesn't, use 1k in the formula. (If you don't know the fuse and holder resistance, use a conservative value of .002 ohms.)
A = Peak current draw in amperes.
Vd = Cable assembly voltage drop.
Please note that most on-line calculators do not take into account the voltage drop across the fuses. At rated load, a 30 amp ATC fuse (used in most late-model transceiver power cords) has a voltage drop of 95 mV (.095 volts). A 60 amp Maxi fuse will have a voltage drop of 77 mV (.077 volts).
Another reason to shoot for less voltage drop is temperature rise. Since an automotive environment is hotter than a base station one, over-sizing the wire (less resistance) will keep temperature rise to a minimum. Too high a temperature rise, and the insulation could melt.
Minimal voltage drop is even more important if you're using an amplifier. For example, an HF 500 watt mobile amplifier draws between 25 and 40 amps average with peaks of about 100 amps including the driving transceiver. Again, the wire selection should be based on the peak power draw, not the average, in an effort to minimize IMD (read that as splatter). This is why a second trunk-mounted battery is a mainstay for high-power installations. Even then the main feed from the front SLI battery should be at least number 4 AWG, and preferably 2 AWG if the length is over 20 feet. Without a second battery, 2/0 wire is needed to achieve the same minimal voltage drop. Further, both the leads to the front battery need to be fused on both ends. While two sets of fuses adds slightly to the overall voltage drop, it is still less than using the chassis from a ground return.
Selecting the correct wire size should always be based on the peak current draw, not the average. And, it is always best to error on the safe side when it comes to voltage drop. Selecting the next larger size wire doesn't double the cost of the wire or that of the connectors. If you're planning on running an amplifier in the future, it's always best to plan ahead and install the capacity you're going to need when the time comes.
Insulation & Stranding
As mentioned above, the insulation temperature rating of vehicle wiring should be at least 90°C (195°F) in the passenger compartment, and 105°C (220°F) in the engine compartment. All exposed wire should be covered with protective split loom no matter the quality or type of wire used.
Abrasion resistance is also important. Construction grade THHN and THWN wire has an outer nylon jacket making them very abrasion resistant. However, construction grade wire is almost always 7 or 19 strand (depending on the wire size), and rather stiff. High strand count super-flex wire like that sold at mobile sound stores is almost too flexible. Automotive wire is usually 49 strand, is much easier to work with, and will generally stay where you put it. However, you should still use cable ties to keep it in place.
Cross-Linked polyethylene (XLP), and Polyvinyl Chloride (PVC) are both acceptable products, if they're rated at 90°C (195°F) or higher. However, most after-market sound-system wire is insulated with Polyethylene, an inexpensive thermoplastic. It is moisture resistant, but its poor heat and flame resistance makes it a poor choice for mobile installations. It is sold in a variety of transparent, neon-like colors, and is not recommend for under-hood wiring. Its extra flexibility means it must be well restrained especially near terminals and connections.
There is another consideration with respect to super-flex wire including high strand count welding cable. You should use crimp-type connectors to properly terminate them. While you can circumvent connector problems by solder dipping the bare ends, few amateurs have a solder pot on their workbench. You can solder it with an iron, but it takes a BIG iron to get it properly wetted, and even then it wicks badly creating more problems than it solves. This isn't a concern with 49 strand automotive wire—if you know how to solder!
Wiring terminations are an important part of any mobile installation, yet it is the one item most often short circuited (pun intended). Especially important are the heavy-duty leads directly from the battery or jump points to where the transceiver is located, because it often necessitates a transition between the large and small gauges. While some folks use butt splices for the transition, they are difficult to solder, and in some cases mechanical crimping alone does not provide the best of connections.
Some of the best terminal devices are based around the Anderson PowerPole® connectors. West Mountain Radio, MFJ Enterprises and PowerWerx market these devices (as shown below), but a caveat or two are in order. First, some types are not completely sealed, so they need to be placed in such a manner to prevent any object from dropping on their partially exposed connections (water, dirt, keys, etc.), however unlikely.
Secondly, some of the various models marketed use preinstalled cables which are usually too short (and too small of a gauge), and require butt splices to attach them to the power source. Extending the cable's length also increases the I2R losses, so proper sizing is important. If possible, choose one without a built in primary cable, and add your own in the exact length, and size required.
Their best attribute is the ease of connection and disconnection. The unit in the left photo is a RigRunner® model 4005 from West Mountain Radio. They carry 5 different models in a variety of configurations to suit almost any mobile or base requirement. If you're interested to learn more, there is a good article about Anderson Powerpole devices in the March 2006 issue of QST starting on page 31. Incidentally, some models of the RigRunner® have LED indicators that light when a fuse blows.
Both West Mountain Radio and PowerWerx carry just about any kind of terminal, fuse block, and wire you could think of. Although the biggest wire size they carry is #6 AWG, it has a high strand count which makes it very flexible, and therefore easy to install.
The 4005 pictured above comes with a 6 foot, 10 AWG primary power cord with 45 amp connectors already installed on one end, and 1/4 ring connectors on the other end. If you need a longer (and larger) cable, go to West Mountain Radio's, web site for more information. While you're there, look over the other amateur related devices, especially their wire crimper. By the way, if you're looking for the best possible termination when using Power Pole connectors, the PWR crimper shown right is the only way to get it. And take time to read the instructions which come with the tool. If you don't, you'll invariably end up twisting one or both wires so the terminals will seat properly in the nested housings.
If there is a drawback to power distribution blocks, it is the fact they can't be completely sealed from environmental hazards. So whereever you mount yours, make sure nothing can drop into them. While a coke comes to mind, moisture of any kind, dirt, mud, and a few other things can and do cause problems.
Incidentally, when using a 4005 or similar distribution block, in a vehicle, no matter where the negative lead is attached (battery or chassis), both it and the positive lead need to be fused. This prevents a small ground loop from occurring between the leads, and prevents damage to the block and/or attached devices, should the negative lead connection fail.
As with any wiring, it should be protected from shorts, abrasion and other maladies. Therefore, any excess should be cut off, not bundled up with a cable tie. Incidentally, PowerWerx and DX Engineering both carry all manner of OEM wiring connectors, Anderson Power Pole connectors, and wire for any installation, mobile or other wise.
Power Pole connectors come is a variety of colors, and most match the color of the various fuse amperages used in the aforementioned distribution terminals. Thus, if you select a connector with the same color as the ATC fuse, you're less likely to plug a cable into the wrong amperage fuse slot.
If you run an amplifier and/or second battery, the PP120, and PP180 are good choices for a quick disconnect for wire sizes up to 2/0. The only drawback is the requisite crimper, which sells for about $300 with dies. By the way, it isn't advisable to solder these large connections. The cabling typically used is high strand, and solder easily wicks into the cable weakening the connection, making it more susceptible to vibration fatigue.
Finding connectors for wire sizes over #8 can be a problem. Here are a few places you might not have thought of. Fastenal has stores all over the United States, and they do mail order as well. They also carry fuses and fuse holders. Newark.com carries larger sized connectors than Mouser or DigiKey, but the latter ones will special order.
On a local basis, welding supply shops, electric motor rewinding shops, OTR truck parts houses, and oil field suppliers often carry heavy duty connectors. Most hardware stores, Lowes Home Improvement Centers, and Home Depot stores are a waste of time for anything larger than size 10 AWG.
Wherever possible, I suggest using ring connectors, rather than spade type connectors because the stay put even if the connection loosens. All connectors should be both crimped and soldered to insure strength and low resistive connections. It is also a good idea to use star washers under the lugs to assure a tight connection. As I have said elsewhere, avoid butt connectors.
Rigorous inspection of wiring and fuse holders should be carried out at least once a year. This may require removing and reinserting fuses in their holders. This is especially important when using tubular fuses, housed in spring-loaded, in-line holders. The inspection should also include looking for discolored parts which might indicate a loose connection or overload condition.
As mentioned above, excess cabling should be shortened as required. Unfortunately, some amateurs just can't bring themselves to cut up a $12 factory power cable. The typical response is, "...what if I have to lengthen it when I change vehicles"? Here's the answer; use Power Pole connectors to make your splices. If you need to extend the cable later, take that cutoff piece (you did save it because you're a typical amateur, right?), apply Power Pole connectors to it, and you're back to a full-length cable. What could be easier?
All About Fuses
The definition of a fuse is relatively simple. It is a short piece of enclosed wire, that melts when subjected to too high of a current. When it does, the circuit opens. However, if you've chosen the incorrect size for your application, it may not open. Or, it may open after a very long delay. In any case, you want the fuse to do its job, well before your wiring becomes its own fuse!
Fuses are there to protect the cabling (wiring). For example, the Icom IC-7000 has a 5 amp (system) fuse mounted inside the radio, and 30 amp fuses in the cabling (both plus and minus leads). If you short out a supply connection (pin 3 of the tuner port for example), a circuit board trace and/or switching transistor will fail long before the 5 amp fuse opens. The 30 amp fuses will never open in this particular case. It can be argued that the power cable fuses do protect the radio if something fails catastrophically, a final perhaps, but chances are some other component in the circuitry will be damaged beyond repair before the power cable fuse(s) opens, and here is why.
All fuses exhibit a time delay between any given ampere overload, and when the fuse opens. This delay is called Ampere Squared Seconds, and is expressed as I2T. For example, a nominal 20 amp fuse will handle a 30 amp load for about 90 seconds. It will hold a 100 amp load for about 1 second. Just after the fuse element melts, there is a brief short period of time when an arc occurs, after which the fuse opens the circuit completely.
The chart at left (click to enlarge) covers Littelfuse's 299 series fuses (shown below in colors which match their ratings), more commonly called Maxi fuses. They're popular in amateur mobile installations as they are a modern replacement for the older cylindrical style 5ag fuses. They're also available with LED failure indicators.
Note that a 30 amp Maxi fuse will take about 3 seconds to open when subjected to a 100 amp load! The same fuse will carry 40 amps for about 2 minutes! As the static temperature goes up, the vertical scale compresses slightly, and in very cold temperatures settings it elongates slightly. So, I2T is the time lag between applying an overload, and the fuse opening to protect the wire. In the mean time, the wire being protected is getting rather warm. If it gets too warm, hot really, it could cause a fire. For a better understanding, let's look at some specific cases.
Most amateur transceivers' DC power cords are built using 10 AWG, or an equivalent (e.g.: Metric 6). Further, most are about 9 feet long, and most are fused with 30 amp fuses. If you subject them to 22 amps of current (nominal input for key down full power), they'll exhibit about a half volt drop (including the drop caused by the internal resistance of the fuses, and their holder). This means the power cable will be dissipating about 11 watts.
If we subject the cable to a load of 100 amps (not a dead short) where the fuse would nominally require 3 seconds to open, our voltage drop is 2 volts, and our wire has to sustain 200 watts of dissipation for 3 full seconds! Now you know one of the reasons why it is so important to choose the correct wire size. To reiterate, the wire must be capable of handling the nominal ICAS load with a minimal amount of voltage drop, and yet be capable of handling an impressive overload, albeit briefly.
One often asked question: If the radio draws just 20 amps peak, why not use a 20 amp fuse instead? Here's why. Subjecting any given fuse to instantaneous loads close to their current rating will eventually cause them to fail. Depending on the load characteristics (steady or varying), fuses are sized from 25% to 50% larger than their impressed loads. It is important to remember that a fuse protects the circuit by melting the fuse element. Therefore, ambient temperature, the type of fuse, the fuse holder it is mounted in, and even the contact material all have an affect on how hot the element gets.
Another often ask question: Is it safe to use a higher voltage fuse? The answer is maybe! For any given ampere rating, fuses designed for high voltage (nominal 250 volts maximum) service typically have lower resistance than those designed for low voltage (nominally 32 volts maximum). Thus, their low voltage I2T is elongated, which means they take longer to open under a given overload. While these facts alone don't preclude their use in low voltage applications, the bottom line is, you should select fuses specifically designed for the voltage range in use.
When you buy spare fuses for your transceiver, here is something to keep in mind. The blade fuses supplied with most mobile transceivers are ATC style, while most automotive fuses are ATO. While they look identical, the ATC fuse element is completely sealed in plastic, and the ATO is not. Since the power cable fuse holders are not waterproof, only an ATC fuse should be used. If an ATO is used, and water gets into the fuse, the fuse element corrodes with predictable results.
If you want to read a bit more about automotive fuse applications, Littelfuse's Autofuseology article is for you.
At the start of this article, is a strongly worded warning about using existing vehicle wiring to power an amateur transceiver. Here is why that warning exists! If we apply the same 22 amp load described above, the voltage drop through the same 9 foot length of #16 (average-sized wiring to an accessory socket) would be 1.65 volts. This means the wiring is being subjected to 36 watts. At 100 amps, the voltage drop is 7.5 volts, and the wiring is subjected to 750 watts of dissipation until the fuse opens in about 1 second. It should noted that vehicle wiring is bundled, and the heat dissipation is additive between them. Thus, one second may appear to be a safe time limit, it is not a guarantee, especially if the average current has already warmed the wire up.
While reading the last section, if you were thinking out loud to yourself with this statement—I use circuit breakers, so I don't have this problem—you're kidding yourself! Fact is, circuit breakers exhibit a much longer I2T than any fuse except some specially designed slow-blow fuses. What's more, most circuit breakers will fail closed on dead shorts if the current exceeds 2,000 amps or so. A standard SLI (Starting, Lights, Ignition) car battery in good condition can easily supply 3,000 amps to a dead short, and an AGM as much as 4,000 amps! This includes so-called marine and aircraft circuit breakers, and any other device which uses movable contacts including relays!
If you use circuit breakers as switches, remember this: The main wiring protection device should be a properly-size fuse!
A Few Words On Relays
Using ignition-controlled relays to switch power off and on to amateur radio equipment is a common, but ill-advised, practice. Advocates believe that doing so will protect the equipment in question from starting motor transients. But how about the transients when the relays are de-energized? The truth is, they're a waste of time, and just one more failure point. The argument that they protect the owner's memory when they fail to remember to turn off their transceivers, is just as lame. Fact is, all but a couple of modern transceivers (no matter their frequency coverage) have built in timers which automatically shut off the transceiver after some preset time delay.
An often-cited reason for using a relay is parasitic current draw—this is the current the transceiver draws when switched off. On average, it is about 20 uA (0.00002 amps). At a nominal battery resting voltage of 12.2 , the current draw amounts to less than .0003 watts! Just for the record, most vehicles have a parasitic draw averaging 20 mA (.02 amps), and a few four times this much!
As for starting motor transients: The truth is, a decent-quality SLI battery will have a capacitive equivalent of about 2,000,000 Farads! On average, their ESR (equivalent series resistance) is ≈3 milliohms (.003Ω), overruling that contention. But, if you just gotta have remote relay control, Perfect Switch® is the answer (also see Power Protectors below). They employ FET technology, so there are no contacts which could fuse together in a dead-short scenario. Fact is, drawing current over their rating will cause them to disconnect, and require a reset. This is about as fail-safe as you can get. Proper fusing is still required, however.
If you use a Perfect Switch® (or relay for that matter) to isolate a second battery, and that battery is discharged, turning on the Perfect Switch® or relay can cause a fault to be written to the engine CPU, thus illuminating the MIL (Maintenance Indication Light). This is especially troublesome scenario with vehicles equipped with battery monitoring systems.
When using distribution devices like the aforementioned RigRunner®, there is a need to use a master fuse mounted close to the battery. In-line fuse holders should be avoided for two reasons. First, they require butt splices which are very hard to solder. Secondly, the supplied wire size is universally too small. You can use the PP15-45 connectors to hold an ATC fuse for loads up to 40 amps, and PP75 connectors will accommodate a Maxifuse up to 60 amps. The connectors are a lot cheaper than most decent fuse holders, but they have a drawback. Getting the fuse inserted properly can be difficult if you don't make sure the connectors are pushed all the way into their respective housings. For the smaller ATC fuses, standard 1/4 inch, female blade connectors work well too.
DigiKey, Fastenal, Mouser Electronics, and PowerWerx all carry a wide variety of fuse holders. For high-amperage loads the choices are few, and those available for Maxi fuses have inadequate pigtails. The Littelfuse MAB1 holders are no longer manufactured limiting the choices even further. An alternative is the Bluesea Systems Maxi Fuse Block. They're sold by West Marine, and other full-line marine retailers. It will hold wire sizes up to #4 gauge wire which is adequate for most installations. However, the wire is clamped in using set screws, all but negating the use of high-strand wire, like welding cable, unless you use a crimped-on sleeve made for the purpose. Their protective cover is available in both red and black.
Most sound shops sell fuse holders designed for the older 5 ag fuses. There is nothing wrong with using them, but 5 ag fuses are getting hard to find especially in the larger amperage sizes (≥30 amps). The same goes for CBO and CNL styles (often called fork lift fuses), so their use should be questioned, unless you carry spares. You do carry spares don't you?
Wiring Through the Firewall
With a few exceptions, the vehicle battery is located under the hood. It can be on the left or the right, and in some cases tucked away in front of the fender well or in the trunk area (ala Chrysler). Wherever it is located, the connections to it should be neat, clean, properly fused, and clear of obstructions. Split loom like that sold by Radio Shack and mobile sound shops should be used to cover the wiring. Routing the wire through the firewall must be neat, tidy, as direct as possible, and properly grommeted to avoid abrasion and the possibility of a short circuit.
Unfortunately, automotive manufacturers don't design their vehicles for inclusion of amateur radio. In some cases an extra hole is purposely left unused to facilitate the installation of after-market devices such as high-end sound systems. This is an exception, not a rule. Without this extra hole, most amateurs resort to using an existing wiring grommet—a potentially dangerous practice.
The existing holes allow all manner of control and electrical cables to safely enter the passenger compartment and keep engine fumes and water out. Size wise, most are barely adequate for their intended purposes and leave little margin for additional wires, especially those sized properly for an amateur transceiver. Further, probing these existing holes with an ice pick or screwdriver is a disaster in the making! The only real alternative is to drill your own hole. To some, this may seem as disastrous as drilling a hole in the top of the vehicle to mount an antenna. It isn't if you follow a few precautions. As previously stated, it requires forethought and patience, and the right tools.
Here is a caveat. Most modern vehicles have a fresh air inlet plenum just behind the hood. The plenum sometimes contains the windshield wiper assembly and perhaps an air filter. For these reasons, the upper area of the firewall should be avoided. Further, most diesels and a lot of high-end vehicles have a second firewall designed to minimize engine noise intrusion into the passenger compartment. This second firewall should also be avoided.
The Antenex® NMO hole saw was designed for drilling 3/4 inch NMO mount holes in blind locations, such as the roof of a vehicle, albeit difficult to use. They're no longer available, but can still be found on-line. The outer shoulder stops the drill from going too far. It can be used for drilling through the firewall. The pilot drill is rather short, so it won't do a very good job if the angle is sharp. The one shortcoming is using a cordless drill as a power source. Typically, they turn too slow to do a decent job. If your cordless has a changeable gear drive, set it on the highest speed to assure a clean hole. You'll still have to remove the ragged edge on the back side of the hole.
Once the hole is drilled, and the edge cleaned up, install a 3/4 inch ID grommet (you can buy these from any well-stocked hardware store). To make it easier to pull the wire, place a dab or two of Ideal© #77 wire lubricant on a rag and wipe down the outer surface of the cable. This stuff is slick (excuse the double entendré). In almost all cases, the amount of wire needed from the firewall out will be much less than that behind the fire wall. In other words, feed the wire from the passenger compartment to the requisite under hood fuse blocks first, and then to its final destination.
Neatness counts! Take your time! Use tyraps to secure the wires together as you go, not before hand as doing so makes routing much tougher. Make sure they clear all controls and moving parts like clutch, brake and accelerator pedals, and don't forget the protective wire loom material.
If you decide to drill a hole, here are a few suggestions. Besides avoiding the upper firewall area, wherever you drill the hole, place it outside any area where is might be stepped on or pinched. It should be obvious why. The hole should have a proper-sized grommet, and must be sealed to avoid any intrusion, water or otherwise. Body putty (better known as dumdum) works very well, and doesn't leave a residue like tape, and silicon caulk.
Hiding the Wiring
Correctly installed wiring means it should be out of sight, protected from abrasion and sharp edges, and positioned in such a way to eliminate tripping hazards and interference from or to vehicle wiring and controls. This requires forethought and patience, and should never be done hurriedly. Running power cables hither and yon tied back by tyraps, twist ties, rubber bands, or duct tape are prescriptions for disaster.
The left photo was taken just to the rear of the drivers side kick panel shows two #6 AWG THHN wire runs for the plus and minus feeds to the trunk. It is not evident in these photos, but the wiring trough is large enough to accommodate several more runs.
The second photo shows the cables where they enter the left rear seat trim panel on their way to the trunk. The runs are protected by a door sill cover assembly as well as the door sill trim panel. Most stereo shops run their wiring under the carpet which is a lot more work and not as safe.
Individual runs may be strapped together with commercial-grade tyraps if required. This should be done after the runs are installed. Do not use vinyl electrical tape to secure the cables as it will not stand up well in an automotive environment. Although the vinyl may not fail, the adhesive eventually becomes elastic which allows the tape to unwrap, and the adhesive sticks to everything but itself. The best alternative is good quality polyolefin heat-shrink tubing, but admittedly it can't be used everywhere. If you have to use tape, use Scotch #27 high-temperature glass tape. Be aware it is difficult to remove once it is subjected to heat. The best bet is to use butyl rubber shrink tape which doesn't use an adhesive backing. Dig deep as this stuff costs $9 a roll from DigiKey. You can use Rescue Tape as well, but remember, it cuts easily so avoid sharp edges.
Neatness counts! Make all turns as gradual as you can, avoid sharp edges and pinch areas. Use split sheathing where the wires will be exposed, and at interference points that can not be avoided. Don't get in a hurry. As I've said many times, the phrase "throw the rig in the car and head for..." has no merit in amateur radio. The results are always equal to the effort put into the project. The aforementioned wiring job took me several hours to complete.
Being careful counts too! The photo on the right, courtesy of Mike Higgins of K-Chem Labs, shows the end results of a vehicle electrical fire caused by careless wiring. In this case, a screw used to hold down the door sill trim plate was screwed through the wiring. The resulting fire destroyed the vehicle. This proves the need to know what is on the other side of any attachment screw, existing or not! Incidentally, Mike's site has some very interesting information of vehicle fires, and their causes, including this one. While not directly related to amateur radio, it reenforces the need to be take you time, and do your wiring with prudence.
Heat shrink tubing requires a heat gun. Besides the tubing, All Electronics, DigiKey, Fastenal, and Mouser Electronics, all carry heat guns with prices varying between $40 and $250 depending on both quality, and duty cycle. Since we don't use one all day long, we don't need a high priced one, so here's a suggestion. Hobby Lobby sells (in store and on line) a heat embossing gun for ≈$25. It works perfectly as a light duty heat shrink gun, and its small size and light weight make it easy to use and store. Harbor Freight sells a medium-duty one for about the same price, but it is not nearly as handy for tight spots.
Under Chassis Wiring
More and more, amateurs are opting to run high power wiring under the chassis. If you do, you need to use extra care in protecting the wiring from road hazards, and weather issues. The wiring itself should be rated for wet locations, and should be covered with split loom material. Liberal use of all-weather tyraps is essential, and you almost can't use too many. Don't use run-of-the-mill tyraps! They just won't stand up to the environmental stresses involved. Fastenal is a good source for all-weather tyraps. If the package doesn't say uV protected, you can bet they're not the correct ones.
Thomas & Betts, the inventor of the Ty-Rap®, makes one type which has a thin metal strip imbedded inside. They're available with and without screw eyes, and are very strong. If your application is demanding, these are the best money can buy. Check their web site for retailers.
When routing the wiring, you should stay clear of suspension members, exhaust systems, existing factory wiring, and fuel lines. On most vehicles, it is possible to follow the brake lines and utilize their hard points to secure the wiring. However you do it, frequent inspections is a prerequisite for safety.
If you live in the sticks, or drive on rock-covered roads, or you live in the upper U.S. with lots of ice, under chassis wiring should be a last resort.
West Mountain Radio manufactures two devices for those folks who power their radios directly from a nominal 12 volt supply; mobile in other words, especially stationary mobile.
The PWRcheck® properly assesses, and monitors your backup battery load. It can handle up to a 40 amp load, and has 13 display modes including voltage, current, wattage, and amp-hours, replete with programmable alarms! And of course, it uses 15/45 Powerpole® connectors for both source and load. It has a backlit LCD, stores up to 174,000 sample points for data logging (that's over 4 months worth), and includes PC software for real-time monitoring, data download, and charting. If you're into fixed, and/or portable operation, it is just the ticket!
The device shown at right is their PWRguard®. It contains a 40 amp, high-power FET switch (no relays here!) which control circuitry activates. If the input voltage drops below 11.5 VDC, or goes above 15 VDC, the PWRguard® shuts off the output power. Although aimed at the battery powered crowd, it does in fact have applications in the portable, and mobile market places.
Both devices are available from PowerWerx, and other fine amateur radio dealers.