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Wire Sizing with cumulative loads

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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Good day,

I am currently attempting to design a fairly simple circuit but can not figure out what the best practice for sizing the wire is;

For example lets say you have;

4 x Injectors each with a maximum draw of 2A at 13.8V DC.

Now the simple method is to simply calculate that you need 8A capacity for a given length of wire lets say 3m (or roughly 10 ft) with a 2% voltage drop. = You would end up needing a 14GA wire to safely supply these 4 injectors.

Now the real question is that these injectors will not be loaded at the max continuously at the same time. Is there a best practice to use in order to determine this, and can it be scaled to more complex wiring systems using various components on a circuit?



Im only going to partially answer your question as the answer varies depending on what component you are driving...

For injector wiring I would make my calculation based on that maximum current being continuous. The reason being that it is very common to see injector duty cycles up near the 80% mark. Even though in many cases they wont be running at 80% for a long period of time it could still long enough to generate considerable heat and melt insulation if for instance you done your calcs based on 50% DC. Having said that, 14AWG sounds big for 4 x hi Z injectors. normally a 16 or 18awg will do 4 (im not calculating nothing here, just quoting what I typically do).

For things like direct spark ignition coils where even at redline they are rarely running at more than say 20%DC and there will unlikely ever be more than one charging at the same time then you can use wiring much reduced over the total maximum calculated current required for all 4 coils.


Thank you for your response, I do understand you reasoning, and it is in line with my own. I know the safest method is always to size the wire for maximum continuous draw at 100%. but in some cases this would result in a wire being much to large for the actual draw that the devices in the circuit will see in real time.

I see that in most cases people have their rules of thumb much like the one you mentioned above. This makes me wonder how you ended up there, experience, trial and error, or a very in depth calculation of the circuit?

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