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Wiring sizing help

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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I've just finished watching the wiring fundamentals course to try and assist with a wiring job that I am doing but I'm still stuck trying to work out what wire size I should use as I keep finding conflicting information online.

My car has an existing relayed circuit for an ECU which I am going to use to power a Link but it does not have an ignition circuit so I need to make my own.

I'd planned to run 18AWG power feed wiring to 4x injectors as from what I can find this should be good for 10A and injectors should pull 1-2A each totaling 8A maximum.

I'd also planned to use an 18AWG power feed to a VW wasted spark ignition coil (part #032905106) as I believe this would also only need a 10A maximum as well.

From what I can find 18AWG should be good for 1.5M for 15A. Do my calculations seem correct or should I be using a large wire size?


This is the closest i have found to being accurate.

However, when in doubt go up a size or two.

That chart you've linked says that 18AWG will be good for 14A which lines up with my calculation but it doesn't say how long the wire is for the rated amps. I've based my calculations on an aviation wiring chart that I found here:


What really confuses me is that in the wiring fundamentals videos, there is a chart that says that 18AWG is only good for 5.6A which seems really low. They also mention that as a general rule of thumb they use 18AWG (for power and signal) for coils and 22AWG for injectors but it never mentions if it splits them off one power feed or if they all have their own separate power feeds.

The main issue here is all the different wire charts available online generally make a lot of assumptions as there is so many variables, they each make different assumptions based on the context. That aviation one is a prime example where it is based on 20 degrees ambient temp and also a single wire in free air and is very simplified focusing on voltage drop. So any numbers off that chart need to be de-rated based on other factors.

I base my wire size on the charts in SAE AS50881 (you can download older versions online for free, the current charts haven't changed. )

Start with the wire temp rating and max ambient temp to work out a temp delta (I plan for 40c temp delta, which for spec 55 wire allows upto 110C ambient). Then with the size of wire you get the max current capability of that wire.

For example 18g at 40c temp delta can carry 18amps

Then you de-rate that by the bundle size/capacity

For example 10 wires at 40% is 0.62, so now max current of 18awg is reduced to 11.2amps

Then finally de-rate for altitude.

For example 2000ft is about 0.98, so now max current of 18awg is reduced further to 10.9amps

Once you have that you check voltage drop for the length is within acceptable limits. 2-5% generally. Based on the specifications of resistance per foot/meter

Yes it is a bit complicated, but once you understand the concept and all the different parameters you will see how why there is so many different answers to wire size.

Then you can work out a wire size chart that works for your wire type, application and environment.

Hope that helps.



As Aaron said, it depends... Different applications - acceptable voltage drops*, wire lengths, ambient temperatures, packaging (affects heat rejection), insulation tolerances, etc, will all be factors.

For non-power circuits, such as warning lights, relay switching or simple on/off senders a 10%, or higher, drop may be acceptable.

For power circuits 5%, or even 2%, may be what one would aim for - remember, P= V^2/R so a 10% voltage drop equates to a 19% power loss.

In your case, you should be able to find, on-line, the hold current for the injectors and the current draw for opening them which can be several times higher - either way, unless you have a 100% duty cycle assuming all are drawing will give a 'safety margin'. Same with the coils.

Like Jonathan, I tend to upsize.

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