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0.0164 x I x L
E = volt drop in volts
S = conductor in sq mm
I = load in amps
L = length in metres
So, in my case for the masthead light - 8m mast - approx 3m to panel, that's 22 m total run. Masthead ligth is 20w - 20 over 12, or more realistically 12.7 volts = 1.57, say 1.6 amps. S = 1.5 sq mm.
0.0164 x 1.6 = 0.026.
0.026 x 22 = 0.572
so E = 0.572 divided 1.5 ( S )
A volt drop of .38 volts is OK - considering the batteries are unlikely to be putting out much less than 12.4 or 5 volts. Also, my nasthead light is quartz halogen, so it's not to important. But if I re-new the masthead at any time it will certainly be a filament bulb which are affected more by less than ideal voltage.
I've just bought a cheap little unit from Maplins for a laptop I got cheap from a boot sale - it is a 12volt supply, varible voltage regulator - put out 14, 16 18 volts etc from a 12v supply. Got me thinking about running one to the vhf - they like good voltage, pref 13,7 or something - so 14 would always keep them well supplied when battery power was fading - just a thought.
Trouble is - the laptop didn't come with a mains lead, and the cheapest I can buy is 30.00. So I have to use it plugged in ti the boat battery which is sitting on my fireplace at the moment !