There are a few important things to know about electricity when wiring up an RV or solar electric system.
- Electrical energy (Watts) is is determined by multiplying voltage (Volts) by current (Amps). This means that as you decrease the voltage on a wire, the current required to send the same amount of power increases. For example – to send 120 watts of power over a 120 volt electrical system (like typical home wiring) requires just 1 amp of current. But to send that same 120 watts of power over a 12 volt electrical system (like in your car or RV) requires 10 amps of current.
- Electrical wires resist current flow. The higher the current (Amps) being pushed over the wire, the more the wire resists. The longer the wire is, the more the wire resists. And the thinner the wire is (the wire’s gauge), the more the wire resists.
Here, it is helpful to think of electricity like water. Say that it takes one minute to fill a gallon jug with a hose. Now say you wanted to fill up a bathtub (more Watts) in the same one minute. To do so, you can either vastly increase the water pressure (Voltage), or you can use a much bigger hose (same Voltage, but allowing for more current flow).
When wires are resisting current, they actually waste energy by getting hot. And if you send too much current down too small of a wire, your wires can get so hot that they melt. This is generally considered to be a BAD thing, and this is why every circuit needs a circuit breaker or fuse to prevent too much current from overwhelming the wires, melting them, and starting a fire in the event of a short circuit.
High Voltage & Small Wires or Low Voltage & Big Wires:
What all of this means is that if you want to send more power over a wire, you generally have two choices – you can increase the voltage, or you can use a shorter and/or thicker wire.
Thick wires are cumbersome and expensive – so if you can it is better to increase the voltage. This is one of the reasons why your household electrical system runs at 120V and not 12V – it is much more efficient this way, and even large appliances can be connected by affordable small gauge house wiring. Major regional power lines take this to an extreme – running hundreds of thousands of volts over long distances with wires the same thickness that you would use to hook up a large inverter to the 12V system in an RV.
Unfortunately, in an RV we are pretty much limited to a low voltage 12V systems, though some large RV’s and solar installations go with 24V or even 48V for vastly improved efficiency and to avoid the need for thick cables. But for a small trailer like the Oliver, 12V is the standard we have to work with, since batteries, tow vehicle charge voltage, mobile electrical appliances, and low voltage lights have been standardized on 12V since the dawn of the automobile.
Without the option of running at a higher voltage, this means that adequate wire gauge becomes a critically important part of a safe and efficient electrical system.
To see just how much power is lost running over various sized wires, I found a nice simple online wire voltage drop calculator here.
Some examples of energy loss in 12V systems:
- 23.6% – 15 amps over 30ft of 14 AWG wire.
- 9.3% – 15 amps over 30ft of 10 AWG wire.
- 18.6% – 30 amps over 30ft of 10 AWG wire.
- 4.7% – 30 amps over 30ft of 4 AWG wire.
Wire Gauge – Bigger equals Smaller:
Non-intuitively, the larger the wire’s gauge number – the smaller the wire is. Typical 120V home wiring uses 12 or 14 gauge conductor, while the tiny wires within a telephone cable are typically AWG 24. Really thick wires are indicated by multiple zeros – AWG 0 (or written as 1/0), AWG 00 (2/0), and so on. AWG 0000 (4/0) wire is nearly half an inch across – and it is expensive, stiff, and very hard to work with.
Most RV loads are relatively low power and despite the low voltage, they can operate just fine and without too much efficiency loss with 12 AWG or 14 AWG wire. A typical RV incandescent light burns 12W, and thus only 1 amp of of current is required. A standard 25W halogen bulbs only requires 2 amps. And the LED equivalent lighting to a 25W halogen uses just 3.3W, a barely perceptible 0.25 amps.
There are however a few places in an RV where the currents can get high, and where wire gauge really starts to matter:
- Aux 12V Outlets: People tend to use 12V auxiliary outlets for either low power gadgets (like cell phone chargers) or high powered appliances (like small inverters, blenders, or even coffee pots – which can use 17 amps!). Yet many RV’s come pre-wired with inadequate wiring to these jacks, using the same gauge wire that would be suitable to power a single 12W lightbulb. This was certainly the case in my Tab clamshell, where the 12V aux jack in the kitchen proved to be a horribly inefficient power source for even a small inverter.
If a 12V jack is inadequately wired, a significant amount of your battery power will be going towards heating up wires and not powering your inverter or appliance. Many higher powered loads may fail to work at all. This is why it is always better to connect even a small inverter directly to your RV battery, if possible.
The Oliver uses 10 gauge wiring for all the aux 12V outlets, which is a reasonable amount for this job. The power distribution panel near the door is connected upstream with 4 gauge wire, which is fabulous.
- Refrigerator: The 3-way refrigerator in most RV’s like the Oliver can run off of 12V DC, 120V AC, or propane. While running off of DC, the Oliver’s refrigerator requires a massive 15 amps of current (contrast that with the less than 2 amps required with 120V AC). To handle a 15 amp load, the refrigerator specifies 10 AWG DC wiring and maximum wire length of 20 feet.
This heavy current draw is why you should never leave a 3-way refrigerator running on DC power while stopped. The fridge can drain your trailer battery in just a matter of a few hours – and if your tow vehicle lacks a battery isolator circuit you could find that battery dead as well after a long lunch!
- Tow Vehicle Charge Line: When you are towing, the battery charge line from your tow vehicle provides charging current to your trailer to top off the batteries while underway. This wire though has a long distance to cover – from your engine’s alternator way up front to your trailers batteries, which in the Oliver are located in the back corner.
If your trailer battery is at all run down and needing charging, or you are running the refrigerator on DC power – this line also needs to carry a lot of current. The current demands are even higher if you have BOTH a low battery and the fridge set to run on DC.
Unfortunately, most trailers use undersized wiring for the tow vehicle charge lines – and you can actually easily end up with a situation where your battery discharges and drains while driving instead of charges. This has been a well discussed design flaw in the Casita world – you can read posts on the subject here, here, here, and particularly here. To get around this problem, some Casita owners have even resorted to using an inverter in their tow vehicle and running a shore power extension cord back to their trailer. (High voltage 120V AC power is so much more efficient over distance that this crazy sounding scheme actually makes electrical sense!)
The Oliver presently uses 10 gauge wire for the charge line, except for the final 4 foot cable that plugs into your tow vehicle, which uses 14 gauge wire.
This is probably adequate for the job, but based upon the experiences of many Casita owners, AWG 4 or AWG 8 wire would be even better.
Even if you do upgrade the charging wire in the trailer, most tow vehicles have AWG 10 or smaller wires running from the trailer hitch to the engine. For optimal charging while on the go, this charge line should be upgraded as well.
Warning: Until you know for certain how your own particular charging system performs, you should not count on getting a good full charge of your trailer battery from your tow vehicle. And if you are running your refrigerator on 12V power while underway, you may actually arrive with your batteries dead or lower than you started. Be on guard!
- Inverter Wiring: A 1000W or 2000W inverter needs to be able to handle momentary power surges of twice the maximum load, which means there is the potential for a huge amp draw from the battery. Because of this, any large inverter is going to require some LARGE cables, and you will want to place the inverter as close to the battery bank as physically possible.
Some examples – a basic Xantrex 1500W inverter recommends AWG 0 (1/0) cable when placing an inverter within 6 feet of the batteries, and 00 (2/0) AWG wire for six to ten feet distance. The Xantrex Prosine 2.0 2000W inverter / 100A charger suggests 4/0 AWG wire, and that the inverter be located no more than 12 feet from the batteries.
- Charger / Converter: If you are not using a combination inverter / charger, you will still want a reasonably sizable wire between your converter and the batteries. Most RV chargers are capable of putting out 30 – 90 amps of charge current. The stock power converter in the Oliver is the Progressive Dynamics PD9145A, which can put out 45 amps of current. Based on how close the converter is located to the Oliver’s batteries – AWG 8 or AWG 10 wire should suffice.
- Ground Wire: Finally – it is important that the grounding wire from the battery to the frame be able to handle ALL of the current that the power system will put out. If you install a large inverter into an RV that has inadequate grounding, you are at risk of overwhelming the ground wire when the inverter handles the surge of large loads starting up. You should make certain that your ground wire is at least as as thick as the largest wires hooked up to the battery.
If you are using solar panels and you have invested in an MPPT controller and efficient AGM batteries, it would be a shame to build a system that throws that efficiency away by using small and less efficient wires. 14 gauge wires may “work” just fine, but if a significant percentage of the energy you are collecting from the sun is wasted before it every gets to your battery, you are certainly missing the point.
When running with 20 amps of input, the BlueSky 2512iX MPPT solar charge controller recommends a maximum wire distance of just 10.2 feet of AWG 10 wire between the solar panels and the batteries (passing through the charge controller), or only 16.2 feet with AWG 8 wire. One of the advantages of an MPPT controller however is that the wires between the solar panel and the charge controller can run at a higher voltage (and thus with lower current), reducing the amount of power lost by wire resistance. The MorningStar SunSaver MPPT controller can even handle a 75 V input!
For the 200W solar system with 21.5V panels that I am constructing – AWG 10 wire should suffice.
The Oliver uses AWG 10 wire for all the high current DC wiring internally, which is great – and way better than most RV’s. The wiring forward to the trailer hitch is mostly AWG 10, with the final 4 feet being AWG 14. This is probably adequate for both the charge and ground lines.
I have asked the Oliver factory to perform some tests on their current wiring setup to confirm that Oliver owners will not face the same mysterious “dead on arrival” batteries that plague some Casita owners who run their refrigerator on 12V while in transit. I will report back here once I get the results.
NOTE: This article was updated on June 12th with confirmed wire gauge details from the Oliver factory.