RV Wiring Basics

In addition to vehicle wiring (in a motorhome), there are usually two separate electrical systems inside the coach portion of an RV:  a 12-volt system and a 120-volt system which are separate from vehicle wiring (even trailers have vehicle wiring for running lights, etc).  The 12-volt coach system usually runs the lights, water pump, fans, and the electronic components of furnaces, water heaters, and refrigerators.   The 120-volt system powers the refrigerator when connected to 120-volt power, runs the roof air conditioner, provides power to the convertor to create 12-volt power without drawing down the batteries, and often powers TVs, VCRs, DVDs, and (if so equipped) washers and dryers etc.  Some RVs have water heaters with electrical heating elements.   Optional 120 volt "heat strips" on roof air conditioners sometimes supplement propane furnaces.  Winterized RVs may have heating pads or heat tape on holding tanks and exterior plumbing.  These heating elements are often designed with dual voltage capabilities so they can be run off either 12 DC or 120 volt AC electrical sources.  120-volt power is provided from a shore connection, on board generator, solar panels, or from batteries via an inverter.

12-volt wiring is usually fairly small gauge, mostly #12 or smaller.  The red wire is usually the 'hot' wire and the black wire is the ground.  Larger #10 wire may be used for special, high-amperage appliances, charging circuits, and main lines.   120-volt wiring usually follows standard configurations for residential and commercial wiring.  The black wire is the 'hot' wire, the white wire is the neutral and the green wire is the ground.   Yes, it is confusing that the black wire is ground for 12 volt and hot for 120-volt, but just remember that whenever you are working on your RV electrical systems.  I like to carry a few feet of #12 and #10 automotive wire, some extra crimp terminals, a roll of electrical tape, and some spare fuses in case I have to make repairs in camp or on the road.

12-volt systems are fairly safe and simple to work on.   There is no danger of getting electrocuted or even shocked by a 12-volt wire.  The biggest problem you might see is shorting out a circuit which will blow the fuse, or, if there is no fuse, melt the wiring. You can get arcing when you ground a 12-volt hot wire and that might burn you or start a fire.  You should never wire circuits that are not fuse or circuit breaker protected.  The whole point of a fuse or circuit breaker is to fail BEFORE the wiring gets hot enough to start melting insulation.   A short in an unprotected circuit can cause a fire. Normally the red wire will be 12-volt positive and the black wire the negative ground. Try to adhere to this convention whenever you add 12-volt wiring to your rig and always make sure you use a wire of a least sufficient size for the expected load and protect it with a fuse or circuit breaker of the proper rating.   Using a heavier wire (lower #) will never hurt, but using a lighter wire (higher gauge) risks overheating and possibly a fire.  Avoid working with live circuits.  Connecting and disconnecting live wires can create sparks, which could blow a fuse, cause a fire, or burn your hands.  If there are volatile fumes present (propane or gasoline vapor) sparks could cause an explosion.

120-volt systems are capable of delivering a nasty, potentially lethal, shock.   If you aren't familiar or comfortable with working on residential wiring, have the work done by a qualified electrician or RV technician.  Always turn off the circuit breaker or unplug the power to the RV and shut down the generator when working on 120-volt systems.   Professional electricians sometimes change outlets or switches without turning the power off, but the practice is not recommended, even for them.  If you're going to be working with wires, switches, outlets, or other 120-volt connections, turn off the breakers and/or unplug the shore power cord and turn off the generator and any inverters before you do anything and don't turn them back on until you have completed your repairs.  Normally the black wire in 120-volt applications is the hot wire, the white wire is the neutral, and the green or un-insulated copper wire is the ground.  It is important to conform to these standards when working with 120-volt wiring.   Electricity doesn't know or care what color wiring you're using, but it will matter to the next person who works on it -- which might be you!  It is critical that all 120-volt wiring is of sufficient size to handle the load on each circuit.  15 amp circuits need minimum 14 gauge wire; 20 amp circuits need 12 gauge wire; 30 amp circuits need 10 gauge wire.  There shouldn't ever be a problem using a heavier wire than necessary, but using a lighter gauge could prove disastrous.  Running higher amperage than the wire is rated for can result in heat, melting insulation and causing a fire.  Most RVs with 30 amp shore power will have mostly 15 amp breakers for the outlets plus a heavier breaker (and heavier wiring) to the roof AC.  At least one of my motorhomes used 12 gauge wiring on the 15 amp circuits (which is overkill) but you may find yours uses 14 gauge wire.

Most 120-volt wiring these days uses copper wire.  You might encounter aluminum wire in some older units.  Mixing different types of metals in the same circuit is not a good idea.   Aluminum and copper are both fairly good electrical conductors, but they have slightly different electrical properties and respond differently to various loads.   Connections between copper and aluminum can promote a electro-chemical reaction that accelerates corrosion.   If you MUST join copper and aluminum wires, treat the joint a special grease made for the purpose.  Do NOT use dielectric grease, which has insulating properties and should only be used on non-conducting portions of connectors.

RVs have a limited number of outlets.  Each circuit is protected by a circuit breaker similar to the ones in your home.  If you overload a circuit, the breaker will trip.  Make sure you know where the breaker panel is so you can reset breakers as needed.  Since RVs have limited power available, either from the shore cable or a generator, adding outlets or circuits may or may not be possible without exceeding the available amperage.  Be sure to check the total amperage so you don't exceed the input power available.  My big diesel pusher actually had some empty slots in the breaker panel where I was able to add a dedicated 20 amp circuit for an outside outlet to power the enclosed motorcycle trailer I towed behind it.  The 6000 watt generator also had some excess capacity.  Smaller rigs I've had have not had any empty slots and an analysis of the amperage indicated adding more circuits would exceed the shore power rating.  In that case you have to find a way to work with existing outlets.  If you have to splice into any existing wires, be sure to enclose the splice in an approved junction box.  DO NOT just twist wires together and wrap them with electrical tape like you do with 12-volt wiring.  You might install extra outlets on existing circuits for convenience, but take care to monitor the total load so you don't exceed the breaker rating.  Adding outlets may add convenience but it won;t add capacity!

The vehicle wiring on a motorhome or trailer should follow conventional wiring colors used on cars and trucks.  Normally, the positive battery cable or terminal should be red and the negative side black.  But it isn't always so be sure to check the polarity at the battery.   If your battery cables are not properly color coded, wrap each one with the appropriate color electrical tape (red for positive(+), black for ground(-)) for future reference.   Interior 12 volt lighting then follows the same color scheme:  red for hot, black for ground.  Trailer connector wiring usually uses yellow for the left turn signal, green for the right turn signal, brown for the tail lights, and white for ground.   On a 6 prong or bigger trailer connector, blue is usually used for the electric brakes and red for a battery-charging circuit.  You may encounter additions or repairs someone has done previously using different colors so always test the wiring using a test light or voltmeter before making any modifications.  Compare the connections to the plug to the appropriate wiring diagrams for each style of trailer connector.  You sometimes find that a vehicle and trailer have both been mis-wired and are compatible with each other but not with other units with standard wiring.  If you find any of your vehicles is mis-wired, it is always a good idea to bring it into compliance with accepted standards so your vehicle is compatible with other trailers and your trailer is compatible with other vehicles.

Trailer connector adapters are available to convert almost any trailer plug to fit almost any receptacle on the vehicle.  Small, light weight trailers without electric brakes often use a simple flat 4- connector plug or a round 4-connector plug.  Trailers with electric brake will have a round 6 or 7 connector plug to accommodate the brake wiring.  These plugs also have an extra terminal than can be wired to the tow vehicle battery/charging system so the two vehicle charges the trailer batteries in route.  Any time you have a charging circuit connected to the trailer the vehicle should use a battery isolator to keep the trailer from drawing down the vehicle battery in camp.  Battery isolators may be either electronic or solenoid types.  Electronic units use diodes to limit flow of current in one direction; solenoids use magnetically controlled switches to connect and disconnect isolated batteries depending on whether there is charging voltage available.

Loose connections are one of the most common electrical problems in an RV.  Connections to appliances and the fuse panel may use screw connectors or spade terminals.  If they use screw connectors, make sure the screws are tight, but take care not to strip them by over tightening.   Spade terminals may get loose over time, especially if they are removed and reinstalled a number of times. Sometimes you can crimp the female terminal gently with pliers to restore a tight fit.   If that doesn't work, you may have to replace the terminal.  It is a pretty simple task.  Sometimes you can pull them off the wire. If they are too tight you may have to cut the wire and strip about 1/4" of insulation to install a new terminal.  New terminals are best installed using special crimping tools.  You might try to crimp them with ordinary pliers in an emergency but that usually flattens out the connection and doesn't grip properly.  Heavy duty crimping pliers have a nub that presses into the terminal to ensure a good connection by tightly pinching the wire inside the sleeve of the terminal. They also have a channel that holds the sleeve of the terminal so it doesn't just flatten out when crimped.  When adding or replacing fixtures you may need to extend the wiring.  To ensure a good connection either use proper connections, such as butt connectors or wire nuts or twist the wires together and solder them, then wrap the joint with electrical tape or use heat-shrink insulation.  For some light duty applications, like speaker wires and some low amperage 12-volt connections you might get away with just twisting the wires together and wrapping the joint with electrical tape but using a connector or soldering the joint is always a better permanent solution.  Twisted joints that are not soldered may come loose if there is any tension on the wires and sometimes can come loose from vibration alone while traveling.   A loose speaker wire is annoying.  A loose hot wire can blow a fuse or even cause  a fire.

Ground connections are just as important as the hot wires.  Without a good ground, the circuit is not complete.  Bad grounds are a very common problem on RVs, in both the coach and chassis wiring.  If your clearance lights, park lights, turn signals or brake lights are dim, intermittent, or otherwise don't work properly, the problem is often a bad ground.  When diagnosing electrical problems always check the ground connections as well as the hot wires.  Ground connections seem to be particularly susceptible to corrosion.  Sometimes merely tightening a loose screw will solve the problem but often you may have to remove it and clean the surface and the connector and reinstall it to get a good ground. Protecting connections with some kind of corrosion block or battery terminal protectorant can reduce corrosion.  I've seen a bad ground cause the turn signals to blink all four park lights and all the clearance lights.  Lacking a good ground at one fixture, the circuit was completed through the ground on other fixtures, after passing through and activating the bulbs in those fixtures.  If you experience strange or unexplained electrical problems, check the ground connections.  When you have problems with lights, be sure to check the bulbs.  Sometimes a burned filament will short out inside a bulb and cause strange symptoms.   Corrosion on bulb bases and sockets, especially in exterior lights, is fairly common problem and can usually be solved by cleaning the socket and the bulb or cleaning the socket and replacing the bulb.  Sometimes corrosion causes the spring-loaded base in he socket to stick.  You can sometimes free it up by pressing on the base with a non-conducting tool such as a wooden dowel or a plastic shaft.  If that doesn't work, you may have to replace the socket.   Bad grounds can usually be fixed by removing the connection and cleaning both the terminal and the metal surface to which it is attached with a wire brush or emery cloth.  Both surfaces should be shiny before you put it back together.  Spraying the restored connection with battery terminal protector will help prevent (but not completely eliminate) a re-occurrence.  Even starting problems can often be traced to a bad grounding strap between the engine and the frame.  Many RVs have a fairly small grounding strap.  Enterprising owners have cured grounding problems by replacing the original grounding straps with much large battery grounding straps.

Motor vehicles, like motorhomes and tow vehicles usually have a big ground strap between the engine and the frame.  It is usually an un-insulated braided metal strap.  The connections often get corroded or may come loose over time, causing many electrical problems in the vehicle.  For example, a bad ground often causes symptoms similar to a faulty starter.  Simply removing the mounting bolts and cleaning the terminals and the mating surfaces with a wire brush until they are both shiny, then reinstalling the bolt tightly will usually solve these problems.  The terminal on the engine side may be greasy and would then benefit from cleaning with solvent as well.  The frame end is usually only corroded but if it is greasy, it should also be cleaned with solvent.  If the strap has deteriorated, it may have to be replaced.  The ground strap is often on the left (driver's side) of the engine.  Some folks add a second strap on the passenger side for extra reliability; it shouldn't be necessary, but it won't hurt anything.  Sometimes the factory grounding straps are pretty light weight, only about 3/8" wide and pretty thin.  I prefer to use one the size used as a ground strap on automotive batteries, about 1" wide and 1/8" thick.  They are less likely to be weakened by rust, corrosion, or vibration.

Problems with grounding of 12-volt circuits on RVs are very common.   If a light or appliance doesn't work or has intermittent problems, it is likely a problem with the ground connection.  Many fixtures on an RV are mounted on wood paneling (interior) or fiberglass (exterior) and neither conducts electricity like the metal boy on most cars and trucks does.   Grounding for exterior lights is often done through a ground wire connected to the fixture and the other end connected somewhere to the vehicle metal frame.  The connection to either the fixture or the frame may get loose or corroded over time and will cease to function.  Repair normally consists of unfastening the connection and cleaning away any rust, dirt, or corrosion, then reconnecting the terminal.  Sometimes you will  need to replace a bad screw to get a good connection and you should always polish the terminal and any surface (such as the frame) that it comes in contact with.  You might want to spray some battery protectant on the new connection to help reduce future problems with corrosion.  Often the terminals are made of aluminum or brass and the frame is steel so it creates a bi-metal reaction, also known as a galvanic corrosion when exposed to an electrolyte and an electric current.  Coating the connection can help protect it from electrolytes.

Occasionally broken bulbs can cause strange problems with lighting.   When a bulb burns out, the filament breaks so no electricity can flow through the bulb.  Sometimes the broken filament may come in contact with the base of the bulb, creating an unexpected ground within the bulb.  I've seen situations where such a broken turn signal bulb caused the taillights and all the clearance lights to blink when the turn signal was activated.   Such symptoms are usually short-lived because the extra load quickly burns out the broken filament again.

What often appears to be a bad bulb, may turn out to be a corroded base rather than a burned out filament.   Or it may just be a loose bulb.  A loose bulb might be fixed by carefully twisting it back into place.  If the socket appears to be too large you might be able to gently squeeze it tighter with a pair of pliers.   Take care not to deform the socket.  If that doesn't work, it is best to replace the socket.  If the base of the bulb is corroded, it won't have make good ground contact with the socket or a good hot connection with the "button" at the bottom of the bulb.  Try cleaning the bulb and the parts of the socket.  Be careful not to short between the sleeve of the socket and the "hot" button in the middle.  For added safety, turn off the switch to the fixture or disconnect the battery before working on the damaged socket.  Sometimes simply replacing a corroded bulb will solve the problem, but if the socket itself is corroded, it should be cleaned or replaced for best and longest lasting results.

Ultimately you are going to find having certain wiring tools will make your job easier and give you better results.  It is possible to strip insulation from wires using a knife, but a wire stripper will make it a lot faster, safer, and easier and will generally allow you to make better connections.  Wire crimpers are needed for installing crimp terminals.  Trying to clamp them down with pliers just doesn't work.  Pliers won't provide the indentation that crimpers do.  They just squeeze two sides of the terminal together and the wire will usually just slip out when there is even a little tension on it.  Manual wire strippers are sometimes standalone units and sometimes part of a stripper/crimper design.  With manual strippers you position the jaws where you want to start stripping and pull the wire so the tool removes the insulation from the end.   You can also get automatic wire strippers that grip the wire, cut the insulation and pull if off all in one move. There are also at least two styles of crimpers.  Some have different notches along the handles of a stripper/crimper tool with each notch designed for a specific size of wire/terminal.  Another, heavier duty style looks more like pair of pliers,  It has a notch on one of the jaws to hold the terminal and a nub on the other side to crimp it deeply.  The plier's style crimpers usually let you make a stronger connection and can be used on just about any size of  wire/terminal.  Electrician's screwdrivers are really handy when working on 120 volt outlets and switches.  There are two different special features for electrician's screwdrivers.  Some have insulated shafts to prevent accidental shorting.  Some have bent shafts that gives them a kind of crank-like shape that speeds installation of the screws that secure outlets and switches.  Both styles are readily available and not terribly expensive.

Wire up!