Pic: Original source currently unknown.
Q. My caravan electric fridge works fine whilst on 230 volts, but not on 12 volts when we free camp. Is there any I can tell for sure if the fridge is faulty?
A. The only way to be 100% sure is to remove the fridge. Then see how it performs standing alone in a garage at much the same temperature.
Q. A friend says you emphasise in your book (Caravan & Motorhome Electrics ) that most caravan and motor home fridges faults are due to faulty installation. How has this come about?
A: Almost all (a probable 95%) of ‘fridge problems’ are really are due to faulty installation. There are two main types of caravan/motor home fridges: 12/230 volt compressor, and 12/230 volt/ LP gas (so-called) ‘three-way’ fridges – mainly from Dometic.
Both types tend to suffer from installation problems. One cause is that some RV makers and self installers appear to misunderstand how fridges actually work.
A fridge does not make cold. It’s simply a pump that moves heat from where it is not wanted – to where it does not matter. This necessitates a cool air inlet at the base of the fridge. Some way of guiding that cool air through the fridges cooling fins. And for the rising hot air to easily exit the caravan. This is fully explained and illustrated in my book.
Such ventilation is totally vital for three-way fridges – but is only too often not provided sufficiently. And sometimes not at all. Unless ventilation is provided as specified (and illustrated in Caravan & Motorhome Electrics) they have no chance of working correctly on either electricity or gas. If the fridge works on gas and 230 volts, but not 12 volts – see below.
The second fault is that because most caravans now have access to 230 volts, they use this most of the time. Here, cable size can be relied on to be fine. If however run from 12 volt solar/battery power much heavier cable is essential. But the required cable is costly – so is rarely used.
Q. I am aware that caravan fridge 12 volt fridge cable is usually too small. How heavy should it be?
A: This depends on the distance from the battery (that should not exceed two to three metres). Errors are caused by their being several ways of specifying cable size.
The dimension concerned is that of the copper conductor. When recommending cable size, makers of electrical stuff either quote the cross-sectional area in square millimetres, in AWG or B&S (these two are identical for all practical purposes). For most electric compressor fridge installation the minimum is 4 mm2 (AWG 10), but 6 mm2 ( AWG 8) is preferable. Three-way fridges draw from 12-30 amps. These really need 8 mm2 (AWG 7) unless the distance is less than two metres (when 6 mm2 is fine).
Q. I have heard there is a problem with the auto cable sold in auto parts and hardware stores.
A. Auto cable is usually just fine. But, for reasons that defy sanity, auto cable makers use similar ‘numbers’ as above (e.g., 4 mm, 6 mm) to imply something totally different.
Auto cable ‘4 mm’ is not 4.0 mm2 – but the overall diameter of the cable including its insulation. That rating is simply the size hole you can push the cable through!
Worse, insulation thickness and type varies from maker to maker. In practice most 4 mm auto cable is 1.8-2.0 mm2. Most 6 mm auto cable is 4.6 mm2. The reason why so many fridges are affected is because that 4 mm and 6 mm cable are the two sizes most commonly used.
With the former voltage drop is more than doubled (often to a volt or more). With the latter, the excess voltage drop is still far from good.
Be aware that even if you ask for (say) 4 mm2 cable what you almost always sold is 1.8-2.0 mm2 auto cable. Many people fall into this trap as the vendors are rarely aware there is a difference. (The square mm size is however usually shown in the cable’s specification.)
Q. My three-way fridge draws about 25 amps. It is connected to the battery (n the tow vehicle) via ten metres (total for twin conductor) of 35-amp cable – yet barely works at all. A friend has one of your books and says that the cable is not remotely heavy enough. How can this possibly be – it’s already three and a half times the necessary current rating!
A. This is yet another problem. Current ‘ratings’ are often misunderstood. The ‘current rating’ of 12 volt cable is almost meaningless. It relates only to the current the cable can carry before the insulation begins to melt: i.e. it is a fire rating, and has absolutely nothing to with voltage drop. It also varies hugely with insulation type. The most commonly used ’35-amp’ cable can be as small as 4 mm auto cable (1.8 sq. mm). Ten metres of this introduces no less than three volts drop. That fridge is attempting work on less than 10 volts. The minimum size cable you need is 13.5 sq. mm, over seven times the size.
A far better solution is to have that battery in the caravan and charged from the alternator by a caravan located dc-dc charger. See: dc-dc charging. And proper size cable. (- – Caravan & Motorhome Books has a thick skin – but you owe your friend, and that fridge, one considerable apology!)
Caravan & Motorhome Books founder, Collyn Rivers is an ex motor industry research engineer who switched careers in mid-life to write and publish technically correct books in plain English. They cover the caravan, motor home and solar areas.
All of the above are covered in depth in my various books. The ‘overall’ ones are the all-new Caravan & Motorhome Book, and the recently updated second edition Camper Trailer Book. Electrical issues are covered in Caravan & Motorhome Electrics, solar in Solar That Really Works (for cabins and RVs) and Solar Success (for home and property systems).
The next in this ongoing series – ‘Best answers to caravan battery charging problems’ will be posted here in June 2016.