All electrical cable resists the flow of current, but the larger the diameter of the copper within the cable, and the shorter its length, the lower that resistance. We need to minimise resistance because it reduces the voltage that refrigerators, lights etc need to work properly.

Because all cables resist current, all cables will incur voltage loss. The highest acceptable voltage loss in 12/24-volt caravan wiring is 3% of the battery voltage. That is: 0.36 volts, at 12 volts. Professional installers specify cable for 5% loss because it cuts cost - at the expense of ‘van owners whose electric fridges never work as well as their makers’ claimed and, for years, wonder why.

Tables of cable sizes for various current flows (at 3% drop) are readily available. It would seem easy then to select the right cable. But there’s a HUGE trap.

Automobile cable utilises a rating system that appears to correspond with (but doesn’t) with appliance manufacturers recommendations and ratings. It does not correspond with other cable size standards either, nor is it possible to make meaningful comparisons. Which is a pity because otherwise it’s good cable.

Sold by auto parts stores, auto-electricians, etc, auto-cable’s rating is of its outside diameter, not the conductor size. That is it includes the thickness of the insulation - and this varies. All this absurd rating thus tells you is what size hole the cable can pass through.

Appliance manufacturers and non-auto electrical trades mostly use one or other of two systems  - ISO and AWG. The former rates cable directly in terms of the cross-sectional area (in sq.mm) of its copper conductor. Thus ISO 2.5 cable has a conductor area of 2.5 sq.mm.

The main alternative, AWG, uses a numerical system (of which AWG 0-18 is likely to be used in caravans). Comparison tables, that give cross sectional area, are available. Most AWG/ISO conversion tables (not mine) have ‘rounding-up’ discrepancies that work only in favour of AWG cable vendors. Use one AWG gauge size larger (ie. numerically lower) than the conversion indicates. For example, ISO 2.5 is AWG 13 (not 14 as usually shown) etc.

Compounding potential confusion, ISO and auto-cable ratings use similar numbers. For example, a typically recommended cable size is 4.00 mm.sq. This may be spelled out like that, but may often just be noted as 4 mm cable.

An unwary buyer asking for 4 mm cable at an auto-parts store will be sold cable marked 4 mm, but with a conductor area of (typically) 1.80 mm.sq. The voltage drop is 55% greater! A fridge so-wired will never work adequately.

This cable confusion is also the reason why some solar installations disappoint, and why batteries are often grossly undercharged. Countless vehicles are wired this way!

As if the above were not bad enough, auto-cable is often also rated in amps. But that rating does not indicate what the cable should carry. Instead, it warns of the highest current that cable can carry before its insulation melts (this rating is often known as ‘ampacity’). It does not take voltage drop into account.

Appliance makers usually specify the minimum-sized cable that may be used. Some supply the actual cable. Heavier cable must be used if the cable has to be longer than that specified. But this is of no help if wiring a complete vehicle. There, one must assess the maximum current that will be carried by each cable and specify the size accordingly.

Tables of cable sizes for various lengths of cable and current carrying capacity are obtainable from cable manufacturers. They are published in books such as my own ‘Motorhome Electrics’. There is a simplified version in this feature.

Using ISO gauges is easy. The actual gauge number (eg. 1.5, 2.5, 4.0, 6.0, 10 etc) is the actual cross sectional area of the conductor, in sq.mm.

Another easy way to work out the voltage drop for any given length of cable, current flow, and cable size is the simple formula shown below. I use this in conjunction with my published tables. This formula is simply:

Voltage drop equals (cable length (in metres) X current (in amps) X 0.017) divided by cable cross-section in mm.sq.

For example: 10 metres X 5 amps X 0.017 = 0.85. Divided by (say) 2.5, the voltage drop is 0.34 volt. This is just acceptable.

The above, and wire-tables indicate voltage drop across a single conductor. For chassis (earth) returns, the resistance of the return path can be ignored. Where there’s another conductor for the return path, the total cable length must be taken into account. In other words, if there’s a separate conductor for the earth return that’s ten metres of cable, so you do the sum as if it were ten.

Single-core ISO-rated cable in sizes from 1.5 sq.mm. - 16 sq.mm is obtainable from electrical wholesalers, and some marine electrical suppliers. The former is likely to be rated at 240 volts. This is fine for 12/24 volts but, because its insulation is thicker, it is bulkier than auto-cable of the same conducting capacity.

Auto-cable is fine but only if the conductor area is also shown (for some brands it is). Check this out yourself. The seller is not likely to understood your query (let alone the vital need to know).

The very best cable for caravan and motorhome wiring is so-called ‘tinned’ copper. It’s not actually tinned, but rather copper electroplated with a nickel alloy. It is obtainable from boating electrical equipment suppliers, but is hard to locate otherwise. It’s worth tracking down.

Collyn Rivers