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Wire Current Rating

EFI Wiring Fundamentals

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I tried a search for this, but struggled, so if it has been covered before then please go easy and just point me in the right direction.

The table in the course suggests that 20AWG/0.5mm2 wire is suitable for 4a. This seems to contradict much of the literature I've seen previously, that tells me 20awg should be good for up to 12a?

I understand this is newb level in terms of questions, and doesn't really deserve it's own thread, but I'm a little confused.


It is a darn good question, not just for "newbs", but for everyone as "standards" seem to be anything but. Anyway, my thoughts, which may need correcting or elaborating.

Off hand, it may come down to the voltage drop in the wire - the smaller the cross sectional area, the greater the voltage drop. Some standards allow 10% per 10 feet (or was it metres - been quite some time) - but the document giving the rating should state how the rating was decided.

Close behind that is the heat produced by the wire at the current rating - voltage drop * current will give the power heating the section of wire and that has to be dissipated to prevent it hesoftening, even melting, the insulation. Obviously, a single exposed wire will pass this heat off to the surroundings much, much easier than if it is grouped with a number of other wires and enclosed in a sleeve.

Could also be the quality of the conductor and insulation - some of the lower quality copper conductor may have a higher resistance due to impurities, so the rating is lower, and some insulation will tolerate higher temperatures and/or have better heat ejecting qualities. I have seen some real shit from cheap "Chinese" suppliers with undersize conductors* and thick insulation to make it look OK.

*Poor quality "copper" is one of the primary reasons for solder joint failure, BTW, so spend the extra to make sure you have a good quality product.

The table in the course is most definitely on the conservative side, as we need to keep the information we put out completely safe. We do this to account for long wire runs that could be bundled in a harness, subjected to engine bay heat also.