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Author: Subject: Wire amperage
JC

posted on 29/7/15 at 04:34 PM Reply With Quote
Wire amperage

Hi there,

I need to amend my wiring loom on my midi to take the headlights and sidelights from the back of the car to the front! The lights run off a 10A fuse, but I presume that this covers the total load in that circuit. For the individual lights, what amperage wire should I use - CBS and most suppliers list 5A and 8A wire.

Oh, and what amperage for the fog lights too!

Thanks

JC

[Edited on 29/7/15 by JC]

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gremlin1234

posted on 29/7/15 at 04:55 PM Reply With Quote
the fuse is (partly) to protect the wire, hence the wire should be at least 10A rating
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cliftyhanger

posted on 29/7/15 at 04:56 PM Reply With Quote
Divide the wattage of the bulb (or whatever) by 12 and that will give you the current. However, always best to make an allowance.

So headlights, 55Watts/12= approx 5amps. However, go the next size up. So in this case 8A Per headlight.

Tail lights, 5W, so 1A plenty per lamp, stop lamps and foglights 21W, so allow 3A each.

This is assuming normal (not LED) lamps. However, best to wire to allow for normal lamps.......LED's take much less.

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blakep82

posted on 29/7/15 at 05:05 PM Reply With Quote
I'm doing all mine with 1mm square thinwall wire, 16a max
I got mine from polevolt

[Edited on 29/7/15 by blakep82]





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dilley

posted on 29/7/15 at 07:11 PM Reply With Quote
quote:
Originally posted by cliftyhanger
Divide the wattage of the bulb (or whatever) by 12 and that will give you the current. However, always best to make an allowance.

So headlights, 55Watts/12= approx 5amps. However, go the next size up. So in this case 8A Per headlight.

Tail lights, 5W, so 1A plenty per lamp, stop lamps and foglights 21W, so allow 3A each.

This is assuming normal (not LED) lamps. However, best to wire to allow for normal lamps.......LED's take much less.


I'd divide by 14v, charging rate....

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gremlin1234

posted on 29/7/15 at 07:38 PM Reply With Quote
quote:
I'd divide by 14v, charging rate....
they are nominally 12v, so are overrunning at 14volts, (with a significantly higher output and current too! )
ps use thicker wires where possible, since you get a lower voltage drop.

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Smoking Frog

posted on 29/7/15 at 08:25 PM Reply With Quote
5A or 8A cable does not seem enough to me, unless you're using LED lights. It's not just cable amperage that needs to be considered, it's also the length of cable and load. The calculations here may help.
http://www.locostbuilders.co.uk/viewthread.php?tid=197694
If you get this wrong best case scenery is the lights will be dimmer, worst case FIRE!

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peter030371

posted on 29/7/15 at 08:49 PM Reply With Quote
The wire should always have a higher capacity than the fuse otherwise if something shorts out your loom will melt before the fuse
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cliftyhanger

posted on 29/7/15 at 08:54 PM Reply With Quote
quote:
Originally posted by dilley
quote:
Originally posted by cliftyhanger
Divide the wattage of the bulb (or whatever) by 12 and that will give you the current. However, always best to make an allowance.

So headlights, 55Watts/12= approx 5amps. However, go the next size up. So in this case 8A Per headlight.

Tail lights, 5W, so 1A plenty per lamp, stop lamps and foglights 21W, so allow 3A each.

This is assuming normal (not LED) lamps. However, best to wire to allow for normal lamps.......LED's take much less.


I'd divide by 14v, charging rate....


I "think" the power ratings are generally given at 12V.
Besides, divide by 14 will give you a lower answer, here you want to build in a nice safety margin.

Re fuses, it may be that the original loom at 10a fed both headlights. I would be using 16-20A wiring for the pair. It really is an area you do not want to cut it to the bone.

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rodgling

posted on 29/7/15 at 09:09 PM Reply With Quote
Bear in mind the wire's max rating will drop significantly when it's wrapped in a bundle of wires (because it heats up more easily).
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JC

posted on 29/7/15 at 09:21 PM Reply With Quote
Thanks all - so a couple of reels of 8A should see me right then
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gremlin1234

posted on 29/7/15 at 11:26 PM Reply With Quote
quote:
Originally posted by JC
Thanks all - so a couple of reels of 8A should see me right then
nope, use 16A
or ideally, a 36A cable fused at 30A, and locally fused relays to 10A feeders

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David Jenkins

posted on 30/7/15 at 07:31 AM Reply With Quote
Don't forget that it's not just about current-carrying capacity - a thin wire will have more resistance than a thick one, so when taking significant current there will be voltage drop (Ohm's law). This could mean that your lights are dimmer than they should be, even though the wiring is within capacity limits.





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Stot

posted on 30/7/15 at 09:03 AM Reply With Quote
Worth remembering that if you are sharing ground wires that they also need to be sufficient to support all of the feeds they are grounding. So If you have 2 15a feeds sharing a ground it should be 30a ground from where they join back to ground etc.

Cheers
Stot

[Edited on 30/7/15 by Stot]

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craig1410

posted on 30/7/15 at 10:50 AM Reply With Quote
I got all my wire from these guys: http://www.vehicle-wiring-products.eu/section.php/198/1/single-core-pvc-thin-wall-cable
The thin wall cable is excellent and available in every colour you can imagine. It's very compact and crimps nicely.

For your application I'd go with a least the 16.5A 1mm wire because your wire must be rated higher than the fuse protecting it as some others have mentioned. Actually, personally I'd go with the 25A 2mm wire to minimise voltage drop over the length of the vehicle and because 10A is, in my opinion marginal for headlights and you might need to upgrade your fuse. Typically headlights are around the 60w range so that is 5A per headlight. That means your 10A fuse will be running at 100% of rated current and will eventually blow due to thermal cycling. In theory a 10A continuous rated fuse *should* run happily at 10A but a lot depends on how high the surge is when switching the lights on.

HTH,
Craig.

[Edited on 30/7/2015 by craig1410]

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snowy2

posted on 30/7/15 at 07:51 PM Reply With Quote
higher volts = lower amps so always use 12v as a calculation it will give you a margin of safety

think of it this way.....vacuum cleaner 1000w and 3 thin wires to the plug and 240v with a 5A fuse......... your starter motor on the car 1000w and a wire 10mm thick!
A starter can draw 1000A at peak cranking (especially when its cold) which is why they are not normally fused....the fuse would be like a nail!





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craig1410

posted on 30/7/15 at 07:56 PM Reply With Quote
Forget calculating watts and amps, it's short circuits you need to protect against unless you like smoke and flames!

The wire must be rated at least as high as the fuse protecting that circuit.

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