Contents: Basics; Insulation Requirements; AWG Ratings; What Size Wire to Use; Wiring Through the Firewall; Hiding the Wiring; Under Chassis Wiring; Connectors (lugs); Terminations; Factory Wiring Guides; Connecting to the Battery; Automatic On/Off; Odds & Ends;
Except for the antenna and requisite bonding, proper wiring is the most important aspect when installing an amateur radio in any vehicle; power rating or frequency of operation notwithstanding. It needs to be properly sized and fused; it needs to be protected from abrasion, heat, and chemicals; and it needs to be neat and tidy to avoid interaction with passengers and mechanical devices.
The most important consideration is where the power comes from. You should never power any amateur radio from any existing vehicle wiring harness, QRP or otherwise. This includes accessory (cigarette lighter) sockets, and using a modified blade fuses as power taps. If you violate these basic rules, you run a great risk of burning the vehicle’s wiring harness which might start a fire. There is absolutely no auto repair more costly than an electrical fire!
Regardless, manufacturers keep coming up with ways to circumvent safe electrical connections, and operation. A good example of this, is the MFJ 4403 (left photo). Touted as a voltage conditioner, it contains six, 2.5 volt, 25 Farad caps in series with appropriate bleeder resistors, and delay circuitry to safely charge the caps. The literature suggests that a 100 watt SSB transceiver can be safely (?) powered from an accessory socket. The key word is safely, and personally, I wouldn't use one on a bet. If you do, you're on your own.
An important point; Never use solid wire for mobile installations. It's lack of flexibility, and its susceptibility to vibration are substandard.
It is preferable to use wire with a temperature rating of 105 degrees C, but 90 degrees C is adequate for under the hood use if the wire is protected with an outer loom. In fact, all exposed wire should be covered with protective split loom no matter the quality or type of wire used.
Scuff resistance is important especially when we're installing the wire. Construction grade THHN and THWN, have an outer nylon jacket making them very scuff resistant. However, construction grade wire is almost always 19 strand, 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. 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 degrees C or better.
Most after-market sound-system wire is insulated with Polyethylene, an inexpensive thermoplastic. It's moisture resistant, but its poor heat and flame resistance makes it a poor choice for mobile installations. It's sold in a variety of transparent, neon-like colors. I don't recommend it for under-hood wiring. Because it is so flexible, 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; they take special connectors to properly terminate them. While you can circumvent problems by solder dipping the bare ends, not too many 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. This isn't a concern with 49 strand automotive wire.
American Wire Gauges, like their European and Asian counterparts, have specific amperage ratings. These ratings are based on the type of insulation, the ambient temperature they are subjected to, their proximity to other wiring, and even their intended use. We often see the acronym ICAS, which stands for Intermittent Commercial and Amateur Service. There are similar standards (both real and assumed) in just about every industry. In our case, it pays to be conservative, and this fact will become more evident later on.
The ARRL Handbook lists the various AWG ratings for bare and enamel coated wire. The PowerStream site also has ratings which differ from the Handbook list, although both are based on the same loose standard of 700 circular mils per ampere (CM/A). It is not uncommon for suggested ratings to range from 250 CM/A to 1,000CM/A. It should be noted that 700 CM/A appears to be very conservative, but it is not as we shall see. Incidentally, at the bottom of the PowerStream page is a voltage drop calculator that comes in handy for folks who don't understand Ohm's law.
There are more standards and rules of thumb applied to wire and fuse selection than just about any other subject you can name. The problem is, very few of them take fuse hysteresis into account. What you read here is purposely conservative, so if you decide to push the envelope, you're on your own.
First of all, you need to know your average current draw. For FM application, this is steady state, and you can use the manufacturer's current specification. For SSB applications, we need to know the average, as well as the peak power as we shall learn shortly.
For example, a 50 watt output FM transceiver will draw about 7 amps, but this isn't the whole story. Both the wire size and length are important. Using the aforementioned calculator, enter the length of your cable run, the amperage draw, and select a size that has less than a half a volt drop, and preferably less than one quarter volt drop. If you want to calculate it yourself, here is the formula:
(2Rw • l) + 2k • A • .001 = Vd where:
Rw = the 1,000 foot resistive value from the Handbook Chapter 7.
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.)
l = Overall length of the cable assembly including connectors.
A = Peak current draw in amperes, or steady state for an FM transceiver.
Vd = Cable assembly voltage drop.
There is a good reason for minimizing voltage drop. The FM transceiver in question only puts out 50 watts when the input voltage is about 13.8 or so. At 12.8 volts, its output will drop to perhaps 30 watts. This same reduction in power also effects SSB transceivers, but it is less noticeable. 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.
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.
A more extreme case is an HF, 500 watt mobile amplifier. Most draw between 25 and 40 amps average, and peak at a maximum of about 100 amps. The wire selection should be based on the peak power draw, not the average. Minimal voltage drop is very important, as we're trying to minimize IMD (read that as splatter). Voltage drop considerations are why a second, trunk-mounted battery are a mainstay for high-power installations.
The main feed from the front SLI battery should be at least number 6 AWG, and preferably 4 AWG. Without the second battery, 2/0 wire would be needed to achieve the same minimal voltage drop. Remember too, in most cases the transceiver and the amplifier will be supplied by the same cable, so you need to take both into account when sizing your wire.
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, Honda is an example, and extra grommeted 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 path. It is my consummate opinion that this is 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 as was recently suggested in a thread on eham.net 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.
My 1998 Mercury Mystique had a usable hole, unfortunately it was on the passenger side of the vehicle and the battery was on the left. Remember, I said the wiring should be as direct as possible. In any case, I opted to drill a hole to pass the two #6 AWG wires through. This takes a 3/4" hole to accommodate the requisite grommet as well as the wires themselves. Here's how I did it.
On the engine side of the firewall was a convex dimple apparently used to align the various pieces of sheet metal in a jig (a sharp punch can be used to make your own dimple). After pulling back the carpet and insulation, this dimple was clearly visible from the passenger compartment side. This dimple also allowed me to center the drill bit used to make the hole. I used 3/4" hole drill designed for sheet metal attached to an 18" drill extension I bought at Ace hardware for $6.95. You need to take time drilling this hole, as the cutting edge of the hole saw will be at an angle to the sheet metal making drilling difficult. Again, take your time!
The Antenex saw (left photo) 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 stop the drill from going too far.
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 the 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.
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. I've run cables in the fashion for years and have never had a problem of any kind. I've noticed a lot of stereo shops run their wiring under the carpet. In my opinion this is a lot more work and not as safe.
If it were possible, I'd include photos of the under-dash routing as well. There is a predrilled and plugged hole in the firewall of all late model Hondas and Acuras, coupe and sedan alike. It is located just above and to the right of the accelerator cable entrance point. It's convenient, but care must be taken to assure adequate accelerator pedal clearance. The cables loop over the steering column and parking brake unit, and then down behind the drivers side kicker panel to the wiring trough.
The individual runs may be strapped together with commercial-grade tyraps if required. This should be done after the runs are installed if you're using nineteen strand THHN wire because it is rather stiff. 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 Digi-Key. You can use Rescue Tape as well. Just remember, it cuts easily so avoid sharp edges.
Digressing for a moment; Heat shrink tubing requires a heat gun. Besides the tubing, All Electronics, Digi-Key, 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 just $19.95. 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's not nearly as handy for tight spots.
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.
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 inventer 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, and you 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.
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 Electronics carries larger sized connectors than Mouser or DigiKey, but the latter ones will special order. Currently, Newark is the only stocking dealer of Littelfuse's MAB1 Maxifuse holders. They do have a $25 minimum, but that usually isn't a problem.
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.
Where ever possible, I suggest using ring connectors, rather than spade type connectors because the stay put even if the connection loosens. All connector should be both crimped and soldered to insure strength and low resistive connections. It's also a good idea to use star washers under the lugs to assure a tight connection. As I have said elsewhere, avoid butt connectors.
Running separate, heavy-duty leads directly from the battery to where the transceiver is located, be it under the drivers seat or in the trunk, necessitates a transition between the large and small gauges. While some folks use butt splices for the transition, they are difficult to solder and mechanical crimping alone does not provide the best of connections, so use them as a last resort.
Some of the best terminal devices are based around the Anderson PowerPole® connectors. West Mountain Radio, MFJ Enterprises and PowerWerx market these devices, 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 requirements.
Their best attribute is the ease of connection and disconnection; a boon for those who R & R their transceivers regularly. 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.
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.
By the way, 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 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.
As I stated above, the main fuses are to protect the heavy-duty wiring, not the device(s) connected to it. Those fuses are (almost) always built into the (radio's) factory wiring harness. This presents a potentially dangerous problem when transitioning from large to small wire gauges.
If the wiring between the transition point and the transceiver's factory wiring harness fuses gets shorted, the only fuse protecting the harness is the large fuses located near the battery. This could lead to an electrical fire with catastrophic results! As with any vehicle 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.
Are power distribution terminals worth the expense and installation hassles? Yes, if neatness counts, and I believe it does. As trite as it may sound, the old adage is true; An ounce of prevention is worth a pound of cure. By the way, there is a good article about Anderson Powerpole devices in the March 2006 issue of QST starting on page 31.
The big three all publish wiring guides for mobile radio installations. These guides give you a basic over view. Here are the guides from Ford, and GM. Ford's guide lines do not contain wiring schematics, but are otherwise correct. GM's guide does contain wiring schematics which illustrate the correct way to do it.
Chrysler, on the other hand, recommends connecting to the battery's accessory frame ground, rather than directly to the battery. If you're running high power, it is best to connect directly to the battery to avoid damaging the factory battery ground.
The one place you don't want to connect wiring to, is the engine block. The reason is, during starting the voltage differential creates a ground loop through your transceiver's chassis and/or coax connection. If you don't understand what a ground loop is, they're covered in detail here.
There are a lot of after market battery connectors designed to replace the existing factory ones. The requisite wiring is then clamped into the new connectors with set screws; a less than ideal scenario. It has also been suggested to replace the stock battery with an after market one which has both side and top terminals, and use which ever isn't being used by the vehicle's wiring. This isn't a very economical solution for a new or near new vehicle owner. The left photo shows a better solution.
Nowadays, most battery clamp bolts are metric, and with few exceptions, 5 mm. This is slightly smaller than the previous 1/4 x 20 standard, so 1/4 inch holed lugs will fit easily. The nuts are usually 10 mm, but maybe 9 mm as shown here. This is a good solution whether your using a RigRunner or any similar wiring termination.
What's not shown in the photo is the fuse blocks protecting the wiring from shorts. That's covered here.
A lot of amateurs bypass proper wiring practices (use vehicle wiring for example) because their radio doesn't have an automatic shut off feature. The PowerWerx APO3 is the best solution to this problem. The APO3 is more than just a timed (0, 5, 10, 20 minutes), auto-off device.
For example, there are four pre-programmed voltages (11.8, 12.1, 12.7, 13.05 volts) settings. Properly programmed, the APO3 will turn off, and on (with some exceptions), your radio just by monitoring the battery voltage! Remembering that static battery voltage is about 12.4 volts, and at least 13.5 volts when the engine is running, so it's easy to see how it does works. There is a 10 minute off delay which avoids unnecessary operation (cold weather, heavy accessory load, etc.).
It switches 20 amps, and can handle up to 30 amps, making it compatible with almost any radio. It comes equipped with Anderson Powerpole connectors, making wiring simple, and quick! At $60 (MSRP) it is also affordable. Visit their web site for more information.
There are a few other items to consider. One is fuse holders. Maxi fuse holders like the littelfuse MAB1 (shown at left) are not easy to find, so some folks opt to go to their local auto parts store, and buy ones with pigtails attached. I don't like these holders because they require splicing, and the wire size doesn't meet the aforementioned criteria. Here's an alternative to the MAB1.
The PP75 Anderson Powerpole connectors, fit a Maxifuse perfectly, and is available with lugs for up to AWG #6. The PP120 will also work, and they're available with lugs for up to awg #2. Incidentally, the PP15/45 connectors fit ATO fuses. West Mountain Radio carries the complete Anderson Power Pole line. Hy the way, if you run an amplifier, the PP180 is a good choice for a quick disconnect for wire sizes up to 2/0.
There is also a mini size fuse, but I don't recommend them if for no other reason than their minimally sized contacts. And I don't recommend import fuses, and for good reason. Some are so poorly made, that they will carry a 200% load for many hours before they open. I've seen cases where the plastic melted off the fuse, and it still hadn't opened. Calling these a fuse is a misnomer.
If you want to get critical, you should take the internal resistance of the fuse (and the contact resistance) into account. In the case of a 60 amp Maxi fuse, that's .00089 ohms, and the holder will account for about .0004 ohms. In our case, we needn't be this critical especially if you follow the suggestions outlined here.
Whatever you do, make sure you carry spares. I have a small plastic box affixed to the top of my amplifier which contains two or more spares for every fuse type and size I use.