Using correct size cables for RVs is essential. Get it wrong and your fridge may not cool nor batteries fully charge for the entire life of your RV. Anyone sufficiently capable can legally cable RVs and cabins as long as no part involves grid voltage. This article outlines why.
Using correct size cables for RVs is essential – here’s why.
As with pumping water through a pipe, electricity dislikes flowing through cables. It reduces their effective pressure. This is particularly so of the low voltage (typically 12 volts) used in almost all RVs. Unless truly adequate, whatever is being powered will never work as intended. This particularly affects fridges and auxiliary battery charging.
This applies not only to ‘do-yourself’ cabling. The copper used in cables is very costly. As a result, many RVs have lighter cabling than desired. The major issues, however, are due to a general misunderstanding of how cable sizes are defined.
Using correct size cables for RVs is essential – the energy rating trap
It is not uncommon when in an auto-parts store, to overhear a buyer ask for (say) 10-amp cable. Few sales staff, however, know that cable rating is not a recommendation. It indicates only the current that cable carries without seriously overheating. It does not indicate voltage drop. ‘Fifteen amp’ cable powering a three-way fridge may thus be fine if a metre or two long. It may, however, drop a volt or more if several metres away.
Using correct size cables for RVs is essential – the auto cable trap
Vehicle and appliance makers specify cable sizes in two main ways. One is the USA’s BS (Browne and Sharpe) or the all-but-identical AWG (American Wire Gauge) sizes. The other is ISO (International Standards Organisation) sizes. Both relate to the cable’s current-carrying copper area. The latter shows it in mm2.
Rarely used in the USA, auto cable is what you are sold elsewhere by auto-parts and chain-stores. It is marketed in sizes numerically identical to ISO’s – e.g. 4.0 mm2. But 4 mm auto cable is not 4.0 mm2. It is the cable’s overall diameter: i.e. – it includes the insulation. That ‘rating’ is literally the size hole you can just push the cable through.
Even if you ask specifically for (say) 4.0 square millimetre cable it’s all but certain you will be sold 4 mm2 auto cable. Sales staff are rarely aware of this issue – let alone why it matters. Most reels of auto cable do however reveal the copper size in the fine print on the reel. It is usually shown also in the makers’ handbooks.
An auto cables’ copper content varies with its price. Most 4 mm auto cable is typically 1.8 – 2.0 mm2 – but maybe only 1.5 mm2. Typical 2.5 mm2 auto cable may have copper of a bare 0.5 mm2. The 6 mm auto cable is not too bad (typically 4.7 mm2).
Because of this wide misunderstanding, non-electricians are likely to use cabling that may be a mere 25% of that required. It is why so many RV electric fridges fail to cool in hot climates and batteries are far from fully charged.
There is nothing wrong technically with the better types of auto cable. But you must know its copper content’s area.
Using correct size cables for RVs is essential – read more
It is feasible to compare AWG/BS and ISO sizes (Solar Success, Solar That Really Works! and Caravan & Motorhome Electrics include such charts) but is not feasible to provide anything but a very guide for auto cable. All our books explain in detail that required to assess the cable sizes needed.