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Just wondering about vehicle electrical wire sizes.

 

If I was to rewire either the full harness or just one wire, is it better to use the same wire size as original or is there a benefit in using the next size up?

eg. using 1.5mm sq instead of 1.0 mm or even more such as 2.5mm.

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Same size if its the same or better material (normally copper on older vehicles)

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Modern vehicle wiring tends to be adequate / just adequate on £ cost basis , older is often heavier.  The problem arises with accessories and such as headlamp bulb wattage increases where a increase in copper prevents voltage drop that is easily arrived at with only 12 volt / 24 volt - over even a short distance of a few feet.   I hope you are identifying proper 'stranded'  coppe wire - the better stuff is tinned.

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Modern vehicle wiring, as mentioned, tends to be engineered down to a spec; as thin a wire as the manufacturer can get away with (Helps shave costs, weight, and makes it easier to fit the not-insignificant amounts of cable required for all the gubbins that manufacturers cram into a modern vehicle.) , and designing to avoid long runs that need to transmit high current (To allow the use of smaller cable, etc.)

Older vehicle wiring tends to be simpler, and do things that result in long runs that have to carry a high current; for example, headlights, powered directly via the switch...

And it's those types of cable runs that could potentially stand to benefit from thicker cable; others, like low-wattage lights, I'd say keeping it the original size is probably better simply from the standpoint of saving money on wire. (I say this, having a not insignificant collection of wire spools already. :$ Vehicle wiring is kinda my thing, in case you couldn't tell.)

So my 'simple' answer is: It depends. Headlight wiring, starter solenoid wiring, etc? Go to a slightly bigger cable. Something like sidelight wiring? Unless there's a measurable voltage drop or the lights are dimmer than they should be, using the same size cable will be fine.

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The main thing to do is look at the material it’s made from, and the cross sectional area which then drives the current it can handle.  Old cables will probably be thick copper.  Modern cables could be made from other metals or copper plated, which reduces it’s conductivity.

 

As long as you replace like-for-like rather than relying on modern descriptions you’ll be fine.

 

An example would be battery leads - a modern battery harness is much much lighter weight than an older one.  To make mine work properly, I went with copper welding cable of 70mm2 size, which is much larger than the modern norm, but was close to the original.  This was on 6volt, so much higher current than modern spec anyway, but the same seems to be true in general.

 

While modern cables can vary in type and quality a lot more than older ones, once you've cut through the rubbish you can find some good variation, for example modern high temperature insulation can be useful in an engine bay, and ultra flexible cables can help in tight spots for routing.

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