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Calculating cable size


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So my brother-in-law wants to temporarily hack a 12v indicator system into his 6v (non Land Rover) vehicle.

I've drawn up a circuit diagram with indicators and hazards. I'm trying to calculate the max cable size required.

I can't find (or is blocked from work :angry: ) a chart listing amps and cable sizes for 12v.

By my reckoning 4 bulbs @ 21W = 84W. These will draw 7A in total. How do I calculate the required cable size for a nominal length of say 2m?


Edited to add:

so VWP show amps for their cables, but I'd still like to know how to calculate it :)

Edited by Gromit
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This page should give you an idea.

My link

14/030 should do your job OK.

Cable size depends on amps, not voltage. For long runs (e.g. one end of a bus to the other) use the next size up.

It's worth mentioning that modern cables can handle a higher current per cross-sectional area than older ones because the newer types of insulation can operate at a higher temperature.

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Thanks for that. Didn't realise it was dependent on the insulation, which seems counter-intuitive. :rolleyes:

After a fair bit of hunting I came across this table, which is useful:

Strands mm2	Resistance
/diam			Amps	V drop over 5m

9/030	0.65	0.0294	5.75	0.845v	
14/030	1	0.0189 	8.75	0.827v	
21/030	1.5	0.0125	12.75	0.797v	
28/030	2	0.0094	17.50	0.823v	
35/030	2.5	0.0075	21.75	0.816v	
44/030	3	0.0060	25.5	0.765v	
206/030	16		110
194/040	25		170

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