Cable Weight / AWG best practice?

[bmouthboyo]

I am new to auto electrics and so still trying to make sure my understanding is sound before attacking a new leisure system.

I just wanted to check, say I am going to power a 20A rated USB / cigarette socket, the fuse I will use is 20A but what gauge cable should I use? Is there a best practice? i.e 20% higher than fuse etc?

I have found charts that for instance state that at 3 foot and 20A I should use 18 gauge. Does this have the safety margin included so the 20A fuse would go before the wire?

Sorry if this is basic stuff, I just always value input from people in this forum and like to double check.

[syncroandy]

Here are some numbers lifted from a cable manufacturers spec, for ISO6722 thinwall copper stranded cable. CSA in sq. mm followed by nominal current rating in Amps:

0.5 11
1 16.5
1.5 21
2.5 29
4 39
6 50
10 70
16 110
25 170

BTW, the latest USB spec AFAIK provides for up to 3 amps only, nowhere near 20 amps. However for a dash mounted power socket with a 20A fuse, I would use 1.5mm cable.

[bmouthboyo]

Thanks for the advice. So basically a 20amp fuse will go before a 20amp wire? And if I plan to draw say 10 amp with a device I can use 10amp wire? No need to always use say double or 50% more?

[marlinowner]

Its more the voltage drop across the cable, the heavier the cable the lower the voltage drop for a given amperage. With 20A through 1.5sqmm you might drop around 0.2V per metre.

http://www.12voltplanet.co.uk/cable-siz ... ction.html" onclick="window.open(this.href);return false;

[California Dreamin]

There are a few factors involved in selecting the right wire rating but as a general rule I would select a wire rated 50% over the consumers maximum rating. If the cable lengths are over a couple of meters this over rating should be higher...the fuse should also be higher than the consumer but lower than the wire rating..even more reason for having a 'buffer' so the fuse can be rated a fair bit lower than the wire rating without it coming too close to the consumer rating.

20amp consumer/30amp wire/25amp fuse ... still close if the consumer was drawing a constant 20amps but acceptable if this max draw was only occasional.
And remember..a higher rating on the wiring is always better especially for voltage sensitive consumers...for instance, wiring to headlamps where a significant voltage drop can reduce light output massively.

Martin

[bmouthboyo]

Great tips thanks Martin.

My next question is how much I should fuse. Say as an example I have 5 LED spotlights and each was drawing an amp. Should I fuse all 5 lights with say 2amp fuse or can you fuse all 5? If so how do you calculate the cable needed then?

Just firing some basic questions as I will be onto leisure system in a week or so.

Thanks again

[CJH]

The fuse is there to protect the wiring. If there is a short between the fuse and the battery (say a seat runner slides over your wiring), then you would have the full power of the battery trying to flow through that small bit of wire, it would get hot, and you may have a fire on your hands. If the short happens 'after' the fuse, then in the same situation the fuse would just blow and break the circuit.

So two things follow from this. Firstly, the fuse needs to be as close to the battery as possible. Secondly, it needs to be big enough that it's not going to blow when all the consumers on that bit of wire are on at the same time. So in your example, you need a fuse that's bigger than 5 amps (but less than the wire rating), and you need to put it near the battery. If you were to fuse each light separately with a smaller fuse, then you'd be protecting that local bit of the circuit against, say, the LED electronics shorting internally, and in theory that would allow the remaining lights to continue working even if one shorted and blew it's local fuse. But you'd still need a bigger fuse near the battery, for the reason I described, so these extra fuses would be a bit redundant.

[bmouthboyo]

The fuse is there to protect the wiring. If there is a short between the fuse and the battery (say a seat runner slides over your wiring), then you would have the full power of the battery trying to flow through that small bit of wire, it would get hot, and you may have a fire on your hands. If the short happens 'after' the fuse, then in the same situation the fuse would just blow and break the circuit.

So two things follow from this. Firstly, the fuse needs to be as close to the battery as possible. Secondly, it needs to be big enough that it's not going to blow when all the consumers on that bit of wire are on at the same time. So in your example, you need a fuse that's bigger than 5 amps (but less than the wire rating), and you need to put it near the battery. If you were to fuse each light separately with a smaller fuse, then you'd be protecting that local bit of the circuit against, say, the LED electronics shorting internally, and in theory that would allow the remaining lights to continue working even if one shorted and blew it's local fuse. But you'd still need a bigger fuse near the battery, for the reason I described, so these extra fuses would be a bit redundant.

And with regard to the wiring I guess I would need to use over 5 amp wiring for all parts, even when the main cable splits off to each 1 amp light? Basically ensuring the fuse is always the week point?

[CJH]

Exactly. In your LED example you'd be hard pressed to find cable that's rated below 5A (e.g. see this 8.75A cable at only 1mm2), but the theory would apply for bigger consumers.

Incidentally, I've often used the two core thinwall cable that Vehicle Wiring Products sell (here) - it's double insulated, and it makes it easy to run the live and earth simultaneously. The 11A stuff is only 5mm OD, while the 16A is only 5.5mm OD and is plenty for a lighter socket fused at, say, 10A.

[bmouthboyo]

Exactly. In your LED example you'd be hard pressed to find cable that's rated below 5A (e.g. see this 8.75A cable at only 1mm2), but the theory would apply for bigger consumers.

Incidentally, I've often used the two core thinwall cable that Vehicle Wiring Products sell (here) - it's double insulated, and it makes it easy to run the live and earth simultaneously. The 11A stuff is only 5mm OD, while the 16A is only 5.5mm OD and is plenty for a lighter socket fused at, say, 10A.

Interesting. Why do you need to run an earth back to an earth point? Is it OK to just earth locally to chassis? Or is it a messy way of doing things?

[CJH]

Interesting. Why do you need to run an earth back to an earth point? Is it OK to just earth locally to chassis? Or is it a messy way of doing things?

Either way is fine, as long as you can get a good connection to the chassis. But it's not always convenient, such as in the poptop for instance, and it's not always desirable to make a new hole in the chassis if there isn't a screw or bolt nearby.