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Auto Cable by Collyn Rivers

Auto Cable

There is increasing evidence that some RVs may have part or all 12/24 volt wiring that is substantially lighter than that intended by appliance makers. This will introduce a voltage drop up to 55% greater than intended - but in some circumstances it may be up to 70%. Where the original cable was over-specified this may not matter, but will cause a considerable loss of performance if that cable was originally specified to run close to its intended load.

The problem is caused by confusion between two cable rating systems that, in the smaller sizes, have apparently identical 'numbers', but are in fact specified in different ways. I earlier believed this to be a mainly (and recent) Australian phenomenon, but it increasingly appears to be a far wider problem than I suspected - and one that may have existed for many years. It does not appear to affect USA-built RVs as that country uses a different cable rating system (AWG).

It is caused as follows.

Manufacturers of 12/24 volt appliances specify the cable that must be used to connect an appliance, or supply that cable (where the cable is supplied and used there is no problem: unless it is lengthened).

This cable is almost always specified using the internationally recognised ISO (International Standards Organisation) rating system. The ISO system rates cables in terms of the cross-sectional area of the copper conductor - using the following increments. 1.0, 1.5, 2.5, 4.0, 6.0, 10, 16, 25, 35, 50, 70, 100 sq mm etc. Thus an ISO 4.0 sq mm cable is a cable that has a cross-sectional area of 4.00 sq mm. This is the cable used for mains wiring in Australia and is also fine for 12/24 volt RV use.

The auto-electrical industry however uses what is known as auto cable, and this is the cable that you will be sold if you buy it from almost anywhere except a general electric wholesaler. It is that sold by auto parts suppliers.

Unlike ISO-rated cable, auto cable is rated in terms of its overall diameter (ie. insulation and all). This insulation is typically 0.5-1.0 mm thick (but varies not only from maker to maker, but varies from one type of auto cable to another from the same maker). Thus the nominal auto cable size gives little indication of the amount of copper it contains (and hence current carrying capacity). The most commonly used auto cable sizes (in RVs) are 2-3 mm, 4 mm, and 6 mm.

The following Table shows the actual sizes (in the all-important square mm) of '4 mm auto cable' sold by various makers in March 2003.

Maker Marketed Size Cross-sectional Area

A 4 mm 1.25 sq mm

B 4 mm 1.80 sq mm

C 4 mm 1.85 sq mm

D 4 mm 2.00 sq mm

E 4 mm 4.00 sq mm

Cables C and D are made in Australia. Cable E is made in Italy.

Specified correctly, auto cable is a perfectly good product. The problem, from an RV point of view, is that auto cable, sold in some numerically identical sizes as ISO-rated cable, is being widely mistaken for the ISO rated cable usually intended.

In many instances auto cable's size in square mm is also marked on the drum (but in small print). But as very few people are aware of the situation, this marking generally has little relevance to them - and may only be noticed if one buys the whole drum.

There is also confusion about the current ratings usually attributed to most makes of auto cable.

These 'current ratings' are fire safety ratings. They relate only to acceptable temperature rise: what the cable can carry before the insulation becomes unsafe. Further, these 'ratings' often assume that current flow is not continuous.

One (imported) 4 mm auto cable of 1.85 sq mm is rated by its maker at an extraordinary 60 amps. Another 4 mm, 1.85 sq mm (Australian-made) auto cable is rated at a far more realistic 10 amps. But in no case are these ratings directly related to acceptable voltage drop.

Current ratings provided with 12/24 volt cable are always of this nature. To determine cable size you must know:

  1. The total conductor length,
  2. The current that the cable will carry,
  3. The permissible voltage drop (0.36 volts is fine for most 12 volt purposes, 0.72 for 24 volts).

From that it is possible to work out conductor size in sq mm as follows.

Length in metres x current in amps x 0.017/conductor size in sq mm = voltage drop.

In other words there is really no such thing as '5 amp' or '10 amp' cable. There is only cable that can carry 5 or 10 amps over a certain distance without exceeding 'n' volts drop. If that distance is one metre, 1 sq mm cable is fine. But if that distance were to be 100 metres, even starter cable is too thin!


This matter came to light during 2002. During that year I became increasingly aware that complaints of (mainly) gas/electric fridges working fine on 240 volts, but not on 12 volts, needed closer investigation.

There were simply too many to be random faults: there had to be some common cause.

I looked into about 50 instances of this (often via email/telephone) and found that that all but two were due to 4 mm auto cable having been used in place of that specified. This typically introduced about 0.8 volt drop between battery and fridge (the odd two were caused by corroded connectors) and were fixed completely by installing adequate cable (6 mm auto cable is usually fine - it is typically 4.9 sq mm).

Here are the typical calculations involved with gas/electric fridge wiring.

Some early fridges had low wattage 12-volt elements that were not particularly effective, but more recent models have generally similar performance on 12 volts and 240 volts.


Most 12-volt fridges typically draw 12.5 amps (current models draw 15 amps or more). Fridge maker usually specify a maximum voltage drop of 3% (0.36 volts). For this example assume a fridge that is three metres cable run from the battery (i.e. six metres of conductor).

Voltage drop = Length of conductor in metres x Current in amps x 0.017 divided by conductor cross section is sq. mm. For early gas fridges this is thus: 6 x 12.5 x 0.017 = 1.275. Here 4.0 sq mm cable results in a voltage drop of 0.318 volts. With 15 amps draw, the drop with 4.0 sq mm cable is 0.382 volts. This is a bit high, but marginally acceptable.

Looking at the above but substituting 4 mm auto cable (of 1.85 sq mm) we have:

6 x 12.5 x 0.017 = 1.275 divided by 1.85. This is a voltage drop of 0.69 volts. Or, at 15 amps, of 0.83 volts.

The thing to bear in mind is that, voltage loss is also power loss. Thus if you lose (say) 0.8 volt along a cable carrying 15 amps - the loss is 15 x 0.8 watts = 12 watts. If the fridge cycles on 10 hours every 24 hours - that's 120 watt/hours/day lost in heating up the RV.

Circuits running a few globes at 12/24 volts are often specified to run over 1.5 sq mm cable. Most auto cable of the closest apparent size (2 mm) has 1 sq mm of conductor, but some has as little as 0.5 sq mm. The latter will typically introduce a drop of a volt or so (or about 8%). Globe brightness of incandescent and halogen globes is proportional to the fourth power of the voltage so the effect of voltage drop is profound: e.g. 5% voltage drop causes 20% loss of brilliance. (For people who care about this stuff, the relationship is defined by the Stefan-Boltzman equation).

The larger 6 mm auto cable is less of a worry. This cable is typically 4.9 sq mm - and is a good substitute for 4 mm cable where 4.00 sq mm was intended. But it still pays to read the small print on the drum: plastic is much cheaper than copper.

In most instances the incorrect use of auto cable will degrade performance but is unlikely to present as a general safety risk (fridge cables may run hot - but not to the extent of being a fire risk. There may however be a specific safety risk.

There have been various magazine and website articles (and website correspondence) actually recommending 4 mm auto cable for electric brake circuits. Anyone following this advice is likely to have brake wiring with totally unacceptable voltage loss. Electric brakes are conservatively rated, but not to that extent.

This is a field that's a long way from rocket science and I seek no credit for apparently uncovering it. Any electrical engineer would have done the same (and probably much quicker because I'm a research engineer). But there appear to be hardly any electrical engineers working in the practical side of the RV industry. It might also have been spotted years ago if users' complaints re fridge performance had been taken more seriously. Seemingly they were not.

The extent of the problem is beginning to be realised following my original writing about it in the CMCA's 'The Wanderer' magazine Tech News column. This column has been picked up by global Internet search engines and recently became 'number one on the Google hit parade' (search for 'auto cable' + 'sizes'). It is too early to comment but the issue is definitely not confined to Australia.

I alerted the Australian auto electrical industry about this problem (via an article in Automotive Electrical & Air Conditioning News in January 2003). Feedback from that initial article confirms that few auto-electricians are aware of ISO cable ratings - and several rightly point out appliance makers often specify 4 sq mm cable simply as 4 mm cable (i.e. the square millimetre bit is implied!).

When faced with the 12-volt fridge problem etc, auto electricians quickly pick the cause and fit heavier cable (usually 6 mm auto cable) but dealing as they are with isolated cases, were not aware that the problem may be so common.

Electricians (ie. other than auto electricians) seem well aware that with appliance cabling, cross-sectional area is implied (ISO is the Australian mains-wiring standard). Several I've discussed this with are well aware that auto cable is quite different - but all assumed that this was common knowledge.

Since the publication of my original articles on this matter I have been commissioned to write a complete series on electrical installation in RVs. This will be published in Automotive Electrical & Air Conditioning News, and may eventually be expanded and published as a specialist RV book for the auto electrical industry late this year or early 2004.

The generality of this Update Note will be incorporated into future editions of Motorhome Electrics but it may be a year or two before the full story is known.

There no easy fix for the fridge cable. It simply has to be replaced. For lighting circuits that use incandescent globes, a relatively simple fix is to replace the fittings or globes with halogen equivalents of half the wattage. These give the same light for half the current and thus half the voltage drop.

If you have the problem and are already using halogens you can either replace the cable - or wait for the shortly forthcoming LED (Light Emitting Diode) cluster globes, a few of which are already on the market (but costly). These will draw so little current that the original voltage drop will be no problem.

I should stress that I am attributing no blame whatever to RV manufacturers. It is an unfortunate but understandable problem that remained unsuspected for years. Some people are unlikely to thank me for uncovering it - but there's no reason to perpetuate it. If blame is to be attributed it should perhaps be directed at whoever thought up the auto cable method of rating without considering its possible repercussions.

Collyn Rivers (March 2003).

Do please note that the above column is copyright but may be reproduced subject to the following:

Copyright, 2003 - reproduced by permission of Collyn Rivers,  https://rvbooks.com.au/

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