Amps Volts and Cable size

sperko · 14-08-2010, 23:56

Hi all I hope I’m not treading on anyone’s toes with this post. But after looking at some of the threads is seems some of you lads have problems with selecting cable sizes for some of your projects. So I hope the following helps Ho forgot to say I’m an Electrical Engineer.
Now once you have worked out how much current the circuit is going to use you can work out what size cable you will need. To do this you have to take into account the length of the cable because all cables have a resistance to electricity its very small but it does exist and this turns to heat when current is flowing, and reduces the Volts. Below you will see a table of cable size and amps carrying capacity and the amount of voltage drop per meter of cable run these are for flexible cables only (they are all available from any electrical wholesale outlet including your B&Q type DIY stores). And a word of warning do not use solid core cable on vehicles or caravans and trailers that type of cable is for wiring your house only. I read on one of the threads somebody using twin lawn mower cable this is a good type of cable to use and will work for some of the projects you may be doing.
Cable size mm2 Amps carrying capacity Volt drop per meter per Amp
1.0 11 0.044 44mV
1.5 14.5 0.029 29mV
2.5 20 0.018 18mV
4.0 26 0.011 11Mv
Say you want to install a bit of kit in your caravan that consumes 8 amps, the 1.0mm2 cable will carry 11 amps so that’s ok but the length of the cable from the battery is 7mts
So 8x0.044═ 0.352 7x 0.352═ 2.464 Volt Drop so 12 Volts – 2.464 ═ 9.536 Volts I think we would all agree that this is to low. So let’s try the 1.5mm2 cable
8x0.029═0.232 7x0.232═1.624 Volt drop so 12 Volts – 1.624═ 10.376 Volts still low but better so let’s try the 2.5mm cable
8x0.018═0.144 7x0.144═1.008 Volt drop so 12 Volts – 1.008═10.992 Volts this is ok to use but let’s see what the 4.0mm2 cable does
8x0.011═0.088 7x0.088═0.616 Volt drop so 12 Volts – 0.616═11.384 Volts so as you can see the bigger the cable the less volt drop along the cable now if the cable was only 2mts long the 1.5mm2 cable would be the one to use. So the calculation is
(Amps x Volt Drop per meter) then this figure is multiplied by the cable length in metres this is the volt drop along the cable. Then deduct this from 12 to give you the Voltage at the equipment
If you’re unsure PM me the amps and the cable length and I’ll work it out for you any time. Hope this helps some one

Brian

solarman216 · 15-08-2010, 00:43

Second that, I had a narrow boat for years and the cable sizes needed to run even low amp stuff over a long run was at first mind boggling, air horn for instance at the bow battery at the stern 6 mm2 and even then the volt drop was enough, Rick

14-08-2010, 23:56	#1
sperko Member Join Date: Aug 2009 Location: West Midlands Vehicle: Nissan Terrano 2 1999 2.7 Posts: 65	Amps Volts and Cable size Hi all I hope I’m not treading on anyone’s toes with this post. But after looking at some of the threads is seems some of you lads have problems with selecting cable sizes for some of your projects. So I hope the following helps Ho forgot to say I’m an Electrical Engineer. Now once you have worked out how much current the circuit is going to use you can work out what size cable you will need. To do this you have to take into account the length of the cable because all cables have a resistance to electricity its very small but it does exist and this turns to heat when current is flowing, and reduces the Volts. Below you will see a table of cable size and amps carrying capacity and the amount of voltage drop per meter of cable run these are for flexible cables only (they are all available from any electrical wholesale outlet including your B&Q type DIY stores). And a word of warning do not use solid core cable on vehicles or caravans and trailers that type of cable is for wiring your house only. I read on one of the threads somebody using twin lawn mower cable this is a good type of cable to use and will work for some of the projects you may be doing. Cable size mm2 Amps carrying capacity Volt drop per meter per Amp 1.0 11 0.044 44mV 1.5 14.5 0.029 29mV 2.5 20 0.018 18mV 4.0 26 0.011 11Mv Say you want to install a bit of kit in your caravan that consumes 8 amps, the 1.0mm2 cable will carry 11 amps so that’s ok but the length of the cable from the battery is 7mts So 8x0.044═ 0.352 7x 0.352═ 2.464 Volt Drop so 12 Volts – 2.464 ═ 9.536 Volts I think we would all agree that this is to low. So let’s try the 1.5mm2 cable 8x0.029═0.232 7x0.232═1.624 Volt drop so 12 Volts – 1.624═ 10.376 Volts still low but better so let’s try the 2.5mm cable 8x0.018═0.144 7x0.144═1.008 Volt drop so 12 Volts – 1.008═10.992 Volts this is ok to use but let’s see what the 4.0mm2 cable does 8x0.011═0.088 7x0.088═0.616 Volt drop so 12 Volts – 0.616═11.384 Volts so as you can see the bigger the cable the less volt drop along the cable now if the cable was only 2mts long the 1.5mm2 cable would be the one to use. So the calculation is (Amps x Volt Drop per meter) then this figure is multiplied by the cable length in metres this is the volt drop along the cable. Then deduct this from 12 to give you the Voltage at the equipment If you’re unsure PM me the amps and the cable length and I’ll work it out for you any time. Hope this helps some one Brian

15-08-2010, 00:43	#2
solarman216 Off road maniac Join Date: Dec 2009 Location: Bexhill on Sea Vehicle: Y60 Patrol Me, 3 ltr Mrs Posts: 17,416	Second that, I had a narrow boat for years and the cable sizes needed to run even low amp stuff over a long run was at first mind boggling, air horn for instance at the bow battery at the stern 6 mm2 and even then the volt drop was enough, Rick