Which size ca­ble?

Canal Boat - - Back Cabin - TONY REPLIES… KEITH SEABRIDGE, via email

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I as­sume you have two items draw­ing 6 amps each, if you mean two items draw­ing three amps each, just change 12 to 6 in the cal­cu­la­tions below. This is not as straight­for­ward as you might think be­cause the length of ca­ble run, out and back, comes into it as well. Al­most any ca­ble sup­plier’s cat­a­logue will give you the cur­rent rat­ing for a ca­ble, but that is only the max­i­mum cur­rent that it can carry with­out dan­ger of over­heat­ing and fire. For in­stance, ve­hi­cleprod­ucts.co.uk ( Ve­hi­cle Wiring Prod­ucts) say that for 12 amps 32/ 0.20, 1sq mm con­duc­tor cross sec­tional area in thin wall or 20/ 0.30 2sq mm con­duc­tor cross sec­tional area in stan­dard PVC will be

AIs there a chart of what I can safely use for two items on a ca­ble car­ry­ing 6 amps? safe. If the to­tal run of both the pos­i­tive and neg­a­tive ca­ble is more than a me­tre or two, you are likely to get volt-drop and that gives symp­toms of a flat bat­tery. To cal­cu­late volt-drop use this for­mula: 0.0164 X cur­rent (amps) X to­tal ca­ble run length

The CSA of the ca­ble Try to get below 0.3-volt for start­ing and charg­ing cir­cuits while oth­ers can be up to 0.5-volt, but the lower the bet­ter.

Let’s say your items are 8 me­tres from the fuse­box, that gives a 16-me­tre run us­ing 2 sq mm ca­ble so: 0.0164 X (2 x 6) X16

2 sq mm This gives a volt-drop of about 1.6 volts and that is too high. 4sq mm ca­ble gives 0.8-volt while 6sq mm ca­ble gives 0.52 volts of volt-drop so, for most cir­cuits, this will do.

There are ta­bles on the In­ter­net if you Google ‘12v ca­ble size chart’.

Canal Boat Mag­a­zine, Archant Spe­cial­ist, Evo­lu­tion House, 2-6 Easthamp­stead Road, Wok­ing­ham, RG40 2EG

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