Wiring a motorhome – 12 volts or 24 volts?
If you are building a motorhome or even buying one, have you considered what voltage the house system will run?
Most people automatically assume that 12 volts is best – because … well cars use 12 volts and it never causes issues for them.
If you are building or buying a motorhome, I urge you to carefully consider 24 volts as a superior option. In reality, I strongly feel that if the motorhome is larger than about 6 or 7 meters, 24 volts is the only really sensible option.
Let’s firstly look at why most people choose a 12 volt system…
1. The vehicle is already 12 volts.
Fine – this does make things a little more difficult, but assuming that after you have read this article in its entirety, you agree that 24 volts is better, there are two options available. You could convert the vehicle to 24v – but this is not very practical for modern (all electronic) vehicles. The second option is to keep the two systems (the vehicle and house systems) 100% separate. To do this we simply fit a second (24v) alternator for charging the house system when in motion.
2. Car radios, phone chargers etc all use 12v.
This is very true, however you will be surprised to know that most 12v appliances are also available in 24v. For the things that must use 12v, we simply use a 24v to 12v converter.
3. I can’t see any good reason to deviate from what cars use.
A touch of electrical theory
Before I get into why 24 volts is a better option, let’s revisit some basic electrical theory (don’t worry it is very simple stuff).
Every item that uses electricity does some form of “useful work”. No matter whether we are talking about a light or a microwave oven – they all consume power while doing work. The amount of power they consume depends on how much work they do. This power is measured in watts. Have a look at any household appliance, somewhere near the power cord you will find a nameplate with electrical information. This information will include the power consumption expressed in watts.
If you have read my article, basic electrics, you will remember that watts is the product of volts times amps (V x A = W). Now if I want 100 watts worth of work done, I could do this in many ways;
- with 10 amps and 10 volts (10v x 10a = 100w)
- I could use 100 volts and I would then only need 1 amp (100v x 1a = 100w)
- Or, I could use just 1 volt – but then I would need 100 amps (1v x 100v = 100w).
All three of these options are perfectly valid and will all do the same amount of work – so then why choose one over the other?
Here is why – as we increase the voltage (remember voltage is electrical pressure) we have to increase the quality and/or thickness of the insulation on all of the wiring and appliances. We do this to make sure that the electrons (now under greater pressure) do not leak out and cause a short circuit. This is just like increasing the wall thickness of a pipe so it can withstand more water pressure.
As we increase the current (remember current or amps is flow of electricity) we have to increase the amount of copper in the wire. This is just like increasing the diameter of a pipe to allow it to carry more water.
Let’s talk briefly about proportions – how much we have to increase the insulation when we increase the voltage OR increase the diameter of copper when we increase the current.
As we increase the voltage, we increase the thickness of the insulation on a wire “just a bit”. Take a look at a 240v cable and compare it to a 12v cable. The insulation on the 240v cable is only slightly thicker than the 12v cable.
Now have a look at a welding cable designed to carry the large currents (amps) needed to weld. Compare this to the cable currently powering your computer monitor. The welding cable is massive. As we increase the current, we must increase the copper in the cable proportionally.
There is one other factor that we must consider when deciding on the right cable for a given task – the length of the cable. The longer the cable, the thicker we need to make it.
From this discussion it should be very clear that we need to keep the current as low as we can AND the length of the cable as short as we can.
Back to motorhome wiring
When wiring your motorhome there is not much you can do about cable lengths (apart from making the runs as direct as practical) – but you can keep the current down by increasing the voltage.
Take a look at the huge pylons that carry power for hundreds of kilometres across the country. You might wonder how they can power entire towns with these seemingly tiny wires. The answer is the voltage that they carry – 220,000 volts in some cases. Just 100 amps at this voltage is 22,000,000 watts or enough to power nearly 1,600 family homes.
Let’s take a look at some practical (motorhome) examples:
A large motorhome has 6 x 200w 12v solar panels on the roof. If wired in parallel these would produce a theoretical maximum of 100amps. This would require massive cables and a huge solar regulator to control the charge. If these same panels were wired in series/parallel giving 24v, the current would be halved to 50amps well within the capacity of a PL60 solar regulator.
A motorhome has a 4000w inverter fitted (not uncommon these days). If this were fed from a 12v battery bank you would need to use cables able to carry a massive 333 amps (this is well beyond welding cable size). At 24v you are still going to need a good sized cable to carry the 166 amps, but this is far more manageable. Remember, it is not only the cable that has to handle the current – it is every fuse and connection along the way.
If your motorhome is large and you plan to install anything more than a very basic electrical system, I strongly recommend that consider a 24v electrical system. It is safer, more robust and probably much cheaper than the 12v alternative.
Do you agree or disagree? Do you have a large motorhome currently running on 12v? Does it work ok? Why not let us know what you think. Use the comments box at the bottom of the page and tell us what you think.
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