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Mil Spec Wiring - Question of wire size

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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I am currently doing a mil spec wiring for a car and I'd like to know what size should the wires be from:

1) Relay to Fuel Pump (044 bosch)

2) Relay to Electric fan

3) Relay to Wiper motor

4) Relay to the lights


the very short (and half accurate) answer:

The long answer:


Thank you for the reply Ludo, the distance from relay to fuel pump/ wipers/ lights etc is about 6-7 feet and I was thinking of using 16awg mil spec tefzel wire. From your experience would that be ok? What if I run 2 x 16awg for each output? Or do I need to consider a higher awg wire?

additionally, i'm assuming there's a formula to determining multiple wires of smaller gauge vs a single wire of bigger gauge. would anyone have any insight into this? I found this link below, but it only states the answer based on the input calculator, and not the formula..


Mitch, the formula is the same than the resistors in parallel.

or, when it's the same wire size Rt= R/n ("n" being the number of wires)

(look at the link I posted above for the actual values.)

Two 16awg wires, 15.8Ω/km each -> 15.8/2 = 7.9 -> 7.9Ω/km

So ≈ one 14awg (10Ω/km) since 13awg doesn't exist

chrisqy, two 16awg should do it for one 044 pump (personally I still would use one 12awg wire)

For the rest, you have to look at the actual current draw.

thank you, that was exactly what I wanted to know

I have always compared wire sizes in terms of cross-sectional area. Maybe that is not correct as I have heard that most current travels along the outside of the wire?


Apologies for the zombie revival, but Gustave seems to have been mis-informed.

Extremely high frequencies seem to act that way, but when I say extremely it's well into what would be radio frequencies - megahertz, through gigahertz, and above - and the wire acts more as a wave guide.