The subject of this month's installment was brought to the fore by an experience two nights ago while overnighting in a friend's commercially manufactured motor home. A slight burning odor was detected, but the source was not discovered until morning, an extension cord left on the ground under the coach had developed a short circuit and slowly burned the ends of the cord and the appliance plugged into it to vapor over the course of the night. The only thing that saved me was the fact that the cord had been lying on a graveled drive. If this fault had occurred inside the RV, or on any combustible material, I would not be writing this to you now.
Like the rest of your mobile electrical installation, there is nothing magic about doing wiring, and in most cases, simply following applicable codes and applying good engineering practice will result in a quite satisfactory installation. There are, however, a few caveats to installing wiring in a moving vehicle, as well as precautions to observe when working with low voltage, high current circuits. These mostly have to do with wire types and sizes, and termination connectors.
For nearly all of the wiring in your coach, you will want to use multi-stranded, flexible wiring whenever possible. Solid conductor wire, which is suitable for stationary usage in buildings, is susceptible to fatigue and failure when subjected to the flexing and vibration encountered in house truck or bus systems. Additionally, connections to the ends of this wiring will remain tighter and be more secure when stranded wire is used.
Non-metallic flexible cable is commonly used as supply wiring for AC circuits inside buildings, and is the most obvious choice for wiring the high voltage appliances in your bus. This cable is comprised of several solid conductor wires inside a single plastic sheath. This is probably the most likely wire to use in your AC wiring system, and although it is solid conductor, it is the only choice that will meet electrical codes. Unless you plan on installing your wiring inside conduit, there is no officially accepted way to use stranded wiring for AC circuits. When NM cable is installed using code-approved enclosures which have proper strain-relief clamps, you will probably experience no problems with your AC installation.
A few weeks ago, someone mentioned to me that they were considering using heavy-duty extension cord wire as AC wiring inside their bus. Although this wire is not code-approved for this application, I can see no reason why it wouldn't result in an acceptable installation. Of course, all other aspects of a code installation should be observed, and the cable should be rated for the full current of the installed circuit. This may be a way for you to install flexible stranded cables in your AC supply circuits without using NM type cable
Low voltage DC circuits are another matter. Using type NM cable is not preferred, and the solid conductors are more difficult to make a proper connection at the ends, where smaller-gauge wiring will be more common at appliances, etc. Low voltage wiring will need to be protected from abrasion and damage, but otherwise it is usually acceptable to run wiring inside of walls and other spaces without the use of conduits. In many cases, you may choose to not install an enclosure behind the device (light fixture, switch, etc.). This means that you also will not have the benefit of proper strain relief on the wiring. The possible outcome of this is that movement of the wire while you are on the road will cause flexing of the cable, which results in fatigue of the wiring and eventual breakage and failure of the wire. Stranded wiring is better able to cope with this flexing, and is easier to work with. Solid conductor wire is notoriously difficult to stuff into confined spaces and to run around obstructions.
All wiring should be of the proper gauge to carry the full-rated current of the circuit which it supplies. Low voltage wiring actually should be oversized to minimise voltage drop in the circuit. This is particularly true in high current circuits such as inverter supply, battery, and starter motor cables. In low voltage circuits where the current exceeds 20 or 30 amps, special fine-strand wire is best used, such as electric welding cable. These cables are fairly expensive, but are better able to carry high currents with less loss. They are super flexible and have a very tough rubberized insulation which will stand up well to abrasion, punctures, and acid from the batteries.
Next time we will investigate the various methods of installing terminating connectors to our wiring.