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.
Read on!

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.

Conclusion

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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28 Responses to “Wiring a motorhome – 12 volts or 24 volts?”

  1. greg Says:

    hi i am thankfull for your site the info you have supplyed has been very helpfull i am building a motorhome from scrach and i need all the healp i can get and i thankyou for it greg

  2. Roger Watts Says:

    We bought a motorhome with a 24v chassis and 12v house system. I hate this mix; any earthing problems in the 12v system will find a welcome solution with the 24v system (everything in the 24v can earth via the chassis) causing any appliance earthed to the chassis to get anywhere between 12 & 24v which wreaks havoc with fridges, stereos & other sensitive equipment. I would never ever have this mix again and am considering changing to 24v only. This will simplify the charging and maintenance of all systems.
    Cheers,
    Roger

  3. Hobo Says:

    Hi Roger,
    I agree that this is perhaps the worst possible configuration. It is a really bad mix and can result is some very strange faults that are nearly impossible to find. While not a simple task to switch to 24v across the entire motorhome – it would be my recommendation. Should we look for some cheap second-hand 12v lights on eBay sometime soon?
    Gavin

  4. Dave Says:

    Hello Gavin.

    I’ve got a ’61 Leyland bus, and the generator (before the days of the alternator, believe the Brits called it a dynamo) is shaft driven from the engine, which is a mid-chassis mount. I was wondering what you might suggest in this case. Is it conceivable to have the dynamo charge both sets of batts while driving? As of yet, no solar although it is planned down the track.

    Thanks.

    Dave

  5. Hobo Says:

    Hi Dave,

    There are a number of ways to charge both sets of batteries from one generator (or alternator). The simplest involves using a large relay to connect the house batteries to the generator when the key is turned on – then disconnect them when the vehicle stops. This is a $50 solution. Other smarter options are available from most 4WD and caravan shops. I would recommend the RedArc brand “Smart Relay”.

    Cheers
    Gavin

  6. Andrea Perris Says:

    Hi Gavin, We are building a motorhome, using a Scania 112 coach (24 volts), and are in dire need of some advice regarding our electrical set up. We are planning a 3-5 year trip. We”d like to know is an 18,000watt start up and 6,000watt continuous invertor overkill? We are planning to use solar just to trickle charge the house batteries. Would a 5kva generator be suitable because we maybe running 240 volt fridge, miicrowave, kettle, Tv etc. Or would it be better to run two invertors- one for the fridge and one for every thing else??? Your feed back would be greatly appreciated as we don’t want to be outlaying huge dollars to find out we have brought the inappropiate stuff. Regards Andrea

  7. Hobo Says:

    Hi Andrea,
    Size of inverter, Generator etc is closely related to the size of the proposed solar array and even more importantly the size of the battery bank. For example a 6000w inverter running at full load will completely flatten (and possibly kill) a 250ah battery bank in less than 30 minutes – clearly this would be a mis-match.
    A 500w solar system would take 12 hours of continuous full sun to put that power back (with nothing using power) – again, a mis-match. Let me know what size battery bank and what size solar array and I can provide more info.
    6000w is a very large inverter – to make use of an inverter of this size you are going to need a massive battery bank and an even bigger solar array. To correctly size an inverter, firstly get the wattage rating of all of the appliances you plan to run from it. Now figure out which of these must run simultaneously. Add the wattages of these appliance together (add 10% for safety) and that is the correct size inverter (assuming there are no motors involved). Larger inverters are very inefficient at low loads – however running two inverters would probably be even less efficient. 5kva Generator is fine (when we must use a generator we use a 2kva – but there are times when I wish it was a little larger).

  8. Andrea Says:

    Hey Gavin, Thanks for your advice, we can do some calculations concerning size of invertor etc. Col was told by someone working at an electrical outlet that solar power would be inadequate and panels only last a short time???? This has turned him away from seriously considering solar as a means of power. Is there any truth in this advice??? Regards Andrea

  9. Hobo Says:

    Hi Andrea,
    That could not be further from the truth…. Most solar panels have a 25year output guarantee (try getting that on a generator). We have lived almost 100% on solar power for over 8 years now – it has been very adequate for us. As our power needs have increased, we have just added more panels. It is far more economical now than it has ever been in the past – the first panels we purchased we at $12/watt – you can buy panels now for as little as $3/watt. It is very important to get the planning and installation correct – but it is very practical to live in a bus powered 100% by solar.
    Cheers

  10. mick Says:

    HOBO this is a great site, im in the middle of our built at present , was wiring it up for 12v but after reading here im thinking 24v ,bus is a 91 hino 24v , looking on ebay it even seem cheaper to buy 240watt 24v compared to 180 watt 12v
    regards mick
    ps if any 1 is traveling in west vic pop into the edenhope pub n say hi 🙂

  11. Hobo Says:

    Gidday Mick,
    Yep, anything bus sized really should be 24v. It will work out cheaper for sure. You will have problems getting a 12v solar regulator big enough for a larger solar setup – go 24v.
    Will call in when down there 4 sure.

    Gavin (Hobo)

  12. Glad Doherty Says:

    Hi, I have ,bought a japanese import motorhome 5 metres X 2 metres. The compliance place disconnected the gas and electrics to make it compliant,6 and now I need to have them reconnected to Aussie standards. As am completely ignorant of these things and could get taken advantage of, can you give me an idea of what a reasonable cost would be and what power base to go for.
    regards,
    Glad

  13. Hobo Says:

    Unfortunaly I have no understanding of how the Japanese wire their motor homes. I know they use 100volts (where we use 240v).
    Worst case, you would need to replace all of the wiring (unless you can prove it is rated for 240v).
    All switches and outlets will need to be replaced with double pole Australian standard units and any lights and appliances (air con, fridge etc) will have to be replaced. I have seen American RVs that used a large transformer to supply 110v to the vehicle from the 240v mains. While this may be cheaper, it is a major nightmare to deal with. I would advise against this approach.

  14. Glad Doherty Says:

    Hi Hobo,
    Thanks for your prompt response. Gives me food for thought altho a bit depressing.
    regards,
    Glad

  15. Charlie Says:

    Hi there, we are in the process of converting our Coaster into a camper.

    We previously had a hiace with a 12v 120 A/H gel deep cycle house battery and a 12v Redarc dual battery isolator.

    we wrecked the hiace and pulled the electronics planning to install it in our new coaster.

    After getting the coaster last week and finding out its 24v I planned to just wire our battery (which is still quite new) to some solar panels and keep the systems completely seperate, as all of our lights and appliances are 12v and we are on a tight budget, but after reading your articles I am having second thoughts.

    Whats are your thoughts? I want to get it right as we plans to live in her for a while.

    Thanks, Charlie

  16. Hobo Says:

    Hi Charlie,
    While this is not the ideal situation – it will work. The major downside is that charging the house system from the vehicle alternator becomes more difficult (although there are now a number of devices that will do this).
    If budget was less of an issue I would always recommend 24v for both the house and crank system with a 24v to 12v converter for the items that MUST run from 12v (ie phone chargers etc).

    Cheers

  17. les k Says:

    hi mate , I have a 24v to 12 v 14AMP invertor for my 24v bus to run a 12V navman ,12v dash cam and a small 12v 4amp cooler and a 12v CB …Total amps 8-9. can I EARTH each item to bus chassis via a bus bar as invertor has 24v pos & neg IN WIRES and only ONE 12v pos OUT. no earth . is it safe to earth a 12v appliance to a 24volt chassis system . ……..many thanks les…….

  18. Hobo Says:

    Yes, this is fine. It is the normal way of doing this. I have the same setup in my 24v motorhome. You just need to take great care that the -ive is never removed from the DC/DC converter while power is on it (doing so will put 24v on the 12v side of the converter).

  19. Allan Says:

    Hi, we have spoken before about 12v and 24v setups. I am starting to lean the 24v way and are looking at buying a charge/inverter 6000w/18000w peak at 24v as I would like to run my A/C while driving. Also I am looking at get more batteries to boost up that side of thing as I already have two 12v 210AH so by making it 24v I will only have 210AH instead of 420. Anyway what I would like to know is could you say put 120Ah batteries with the 210AH or they need to be the same AH’s. And also would you run the starter batteries with the house or keep them apart if so what would be the best way to charge them.

  20. Hobo Says:

    This is a complex topic, it would be best for me to talk with you by phone about this. I see some major issues with what you have planned.

  21. Benen Says:

    Hi,
    I am looking to convert a hiace commuter with my wife and have always assumed I will use 12v. I have even already purchased a 12v pure sine inverter and 12v ctek charger.
    We will only be running a waeco fridge (5A), lights, an exhaust fan when cooking, a 12v Flojet diaphragm pump (6A) and basic needs like phone charging. Charging the batteries from 160W solar when stopped, alternator when driving and mains if available.
    Surely 12v would be the better option for us here due to the low power needs?

    Thanks

    Benen

  22. Hobo Says:

    Hi Benen,

    For a smaller motorhome (ie the one you have purchased), 12v is the correct choice. My comments regarding the use of 24v are in relation to larger motorhomes where the cable runs are long and voltage drop is a major issue.

    Cheers

  23. Taz Says:

    Hi Hobo,

    My canter truck is currently 24v, will i run into problems using my 12v solar setup – which includes 2×120 AH batteries, 400W 12v panels, 12v 300W puresine wave inverter, 30 AMP MPPT controller? especially with earthing to the chasis? if the truck 24v is earthing to chasis already? will it cause dramas with my 12V appliances?

  24. Hobo Says:

    This is a fairly common setup and while not ideal, will work fine. Simply connect both battery sets negative to the chassis. It should not give you any dramas.
    RedArc make devices that will charge 12v house batteries from a 24v alternator. I have used and they work well (just in case you need extra charge while driving).

  25. Chen lien shing Says:

    Hi all,
    Just finished building a SB33 Bedford into a motor home. It’s chassis electrical system and house power are 12v but solar, batteries and inverter are 24. Each system has its own earth loop and are isolated from one another. A small AGM 150Ah powers lights and water pumps. Solar trickles into it via a 24vdc/12vdc transformer. Solar and 24v system 600Ah Lipo4 are for inverter only.
    Isolating the systems from each other and the chassis obviates any mixed voltage issues, is little extra to fit, and ensures an adequate earth at each load.
    Cheers.

  26. Hobo Says:

    Interesting approach. I wonder why you decided to use 12v for the lights and pumps – both are easily obtained in 24v?

  27. Sam Says:

    Hey Gavin l really enjoy reading your articles they make technical subjects easy . My problem is my bus is 24v I would like to take your advice and run 24v however my 3 pumps ,2 fans, 3 tv,s ,lighting outdoor radio and dometic 1180 3way fridge are all 12v would reducers make all these less effective these are all new.

  28. Hobo Says:

    Hi,
    This is a bit of a problem. Every time you convert from one voltage to another there are losses. It may not be too bad if you can run all of the 12v items from one converter – but it would be very inefficient to run one converter for each device.
    Cheers

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