Last Modified: April 30, 2013
Contents: Introduction; The Devil's Playground; Power Connections; 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;
Except for the antenna, proper wiring is the most important aspect of installing any amateur radio equipment in a vehicle. Here are the salient points to keep in mind.
• Never 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.
• The wire should be sized large enough to limit voltage drop to less than .5 volts while under full load.
• The wire must be properly fused to protect it from shorts.
• Avoid using circuit breakers, no matter their ratings.
• The wire needs to be protected from abrasion, excess heat, and automotive chemicals.
• The wire should be routed in a neat and tidy manner to avoid interaction with passengers and mechanical devices.
• The temperature rating 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.
We'll be exploring each of these salient points below, along with links to relevant articles when needed.
The Devil's Playground
Never use existing vehicle wiring to power any amateur transceiver! This includes the accessory (cigarette lighter) socket. Typically wired with #16 awg, 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 just because you have managed to get by doing so in the past, doesn't mean you're safe! A quick look at the photo (click to enlarge) should be self evident why existing vehicle wiring shouldn't be used. Note the arrow pointing to what's left of cigarette lighter plug and its cable.
If you're one of those folks who justify this sort of shortcoming, you're playing in the Devil's Playground, and you need to read the next section!
There are 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 as many as 50! 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.
The second school of thought is to connect the ground lead to the battery's chassis ground point. The negative lead doesn't need to be fused in this case. 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, or in the battery's negative lead. 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. In simple terms, when you stop at a traffic light, the engine shuts down. When the light turns green and you 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 may or may not fit a specific case, depending on where the ELD is located. 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 or Voltage Quality Modules. It may be as simple as routing the wiring through the ELD in some cases. In any case, contacting your dealer service personnel may be the correct approach. However, be advised that they may know less about the subject than you do! It always pays to purchase a service manual for your vehicle.
The bottom line is, that the power leads from any amateur gear, must be connected after any Battery Monitoring System. This assures that the ELD will properly measure the current load thus imposed. This does not imply in any way, that the chassis should be used as a ground return—it shouldn't be, period!
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. If not, then the myth becomes reality!
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, twisting the power lead wires has virtually no effect on noise immunity at common amateur frequencies. As for alternator whine, that's typically caused by a ground loop, and is a common issue when using mag mounted antennas.
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. 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, that nominal 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. Also, at low voltages the power output drops, and the IMD increases. Thus is 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.
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 6 AWG, and preferably 4 AWG. Without a second battery, 2/0 wire is needed to achieve the same minimal voltage drop.
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 store 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 flexible 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.
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.
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 ATO 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 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.
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 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 the admonition, 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 there is one admonition about circuit breakers which needs to be followed, it is this. The master wiring protection device should be a 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. That is no doubt true, but how about the transients when the relays are 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.
If you just have to be warm and fuzzy, then place a large Farad-sized super capacitor close to the transceiver side of the power cable. Just make sure it is properly fused as noted herein.
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 ATU 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 ATO 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. It is also ideal 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 outer shoulder stops the drill from going too far. This saw sells for $75 list, but can be purchased from several on-line retailers for about $50.
Once the hole is drilled and you're ready to pull the wire through, you need follow a certain procedure or you may damage the insulation on the wire. Install a grommet before you start pulling the wire through the hole. To make things easier 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 40A 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.