Electrics Explained

Malcolm Street — 7 May 2020
Over the last ten years, power systems in RVs have become much more sophisticated, so having a basic understanding is helpful

There was a time when recreational vehicle (RV) electrics were very simple indeed, way back when they didn't have 240V mains power at all — anyone remember LP gas lights? Many of our readers will recall when caravans and motorhomes had a few 240V power points and dome lights that came with both 240V AC and 12V DC incandescent light globes. There were no 12V batteries of any sort and the 12V supply came from a ‘hot wire’ to the tow vehicle, while fridges were mostly 2-way, 240V AC and LP gas. Very simple, hey?

TECHNOLOGY PLUS

Things have changed, have they not? Even basic vans now have both 240V AC and 12V DC systems, and a 12V DC battery is very much an expected feature. Then there are solar panels, inverters, DC to DC chargers, battery management systems, sophisticated LED lighting and generators, all of which are not uncommon. Additionally, there are RVs which have 12V DC power systems with enough capacity to be used for extended periods without being connected (aka off the grid) to 240V AC (aka shore power) at all.



SEPARATION IS GOOD

Having two different voltage systems isn’t a problem, but the cabling for each should be kept physically separate from the other in the most practical way possible. Apart from anything else, insulation levels are different, and a problem in a 240V AC mains power circuit should not cause a problem on the 12V DC side and vice versa. 

Although it’s tempting to DIY it, all work on 240V AC systems should be done by a licensed person. In many ways an RV is just like a small mobile house. Unlike a house, however, an RV is constantly mobile, so it’s more vulnerable to making any loose/bad connections or dodgy cabling worse. 

It is legal to work on a 12V DC system; however, anyone working on a 12V DC system should know what they are doing and be a competent handyman. Given the sophistication of 12V DC power systems, a mistake could be expensive and perhaps life threatening — for example, short circuits because of incorrectly fused and wired circuits can cause excessive heat and burning. 

240V AC POWER

12V DC power systems are increasingly being used in RVs but generally speaking 240V AC power is still required for high-power devices like air conditioners, microwave ovens and 3-way fridges. In the case of a 3-way fridge, it operates best off either 240V AC or LP gas but is somewhat less efficient when running off 12V DC. 240V AC can of course also be used for battery chargers of all sizes — everything from the house battery to personal devices, fry pans, kettles and coffee makers. 

All 240V AC power systems in RVs are governed by Australian/New Zealand standards AS/NZS 3000-2018 & 3001-2008. Apart from anything else, it means all power sockets have to be double pole switched. This means both the active and neutral lines have to be switched, not just the active as in normal domestic power points. There are also prohibited location requirements in bathrooms and kitchens, which is why power points sometimes end up in funny places like cupboards. It’s not mandatory but all electrical extension cords should also be tagged and tested on a regular basis — extension cords are responsible for more electrocution issues than just about anything else.

Compliance issues with imported RVs can be caused by 240V AC mains power. An RV from Europe is generally okay because the domestic power supply is either 220V AC or 230V AC, but anything from North America is problematic because everything is wired for 110V AC. Neither cables, wall fittings nor devices are compatible. In all cases, except New Zealand, all fittings have to be changed. The odd exception for NZ built RVs is the external power connection — the Kiwis use a much more solid looking plug and socket. 

240V POWER FOR TECH HEADS

There is something I need to explain. Although I have used the term 240V AC for mains power, it is not strictly correct. Back in 1980, the International Electrotechnical Commission (IEC) rationalised the 220V, 230V and 240V nominal domestic voltage levels to 230V, mostly for economic reasons. Since then, in Australia and New Zealand there has been a slow shift towards that, but it hasn’t been universal across the states despite the fact we all use the same standards. It’s all governed by this mouthful of a standard: AS 61000.3.100: 2011. There are allowable tolerances (-6 per cent to +10 per cent) which is why if you carefully stick your multimeter probes into a powerpoint, you might get a slightly different result depending on the state you are in. It should be somewhere between 230V AC and 240V AC — in my case I did a quick test and it was 236V AC. For most people it doesn’t matter, but it’s why electrical devices built for use in Australia can also be used in Britain, Europe and a considerable number of other countries. The notable exceptions to this are Canada, the USA and Japan, which use 100V–120V AC systems. 

12V DC POWER

Although there are some 24V DC power systems around, 12V DC is the voltage of choice for a low voltage system, helped mostly by the fact most tow vehicles/cab chassis are also 12V DC. The power supply can either be derived from an onboard house battery or from the tow vehicle/cab chassis itself. Some caravans have their own 240V AC/12V DC transformer on board but that, of course, still requires a 240V AC connection. For the most part, 12V DC is derived from house batteries which are self-contained in the case of a caravan and in a motorhome, apart from a charging connection totally separate from the base vehicle’s 12V DC electrics. There are two good reasons for that. Starter batteries are designed differently to house batteries, given their different load cycles. Plus, should the house battery run flat overnight the vehicle starter battery won’t be affected. 

Up until a few years ago, deep cycle batteries, usually rated at around 100Ah, were usual in a basic 12V DC setup, but in recent times high powered lithium (LifePO4) batteries are becoming more common. Currently they are still relatively expensive, and although nice to have, a usage versus cost check before purchase isn’t a bad idea. The benefits of lithium batteries are that they have a better charging characteristic and can deliver high currents for items like 12V DC–240V AC inverters. 

BATTERY MANAGEMENT SYSTEMS

Charging sources for 12V DC batteries have multiplied in a relatively short time. Mains chargers, generators, solar panels, DC to DC chargers and wind generators are now all in the mix. In addition to that, different types of batteries (lead acid, gel cell, AGM and lithium) have different charging characteristics. It’s for these reasons that battery management systems are being used instead of just ‘simple’ multi-stage chargers. In effect, the battery management system is connected to all the batteries, various charging sources and loads, and operates the entire system more efficiently, giving, if nothing else, a longer battery life. 

TREAT WITH RESPECT

That’s a brief rundown on the main power systems found in any RV. On that note, any power system and items like extension cables should be well maintained and, certainly in the case of 240V AC power, treated with care and respect. 

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Feature RV Electrics 240V AC 12V DC

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Malcolm Street