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Wiring sizing

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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After looking at the table supplied in the video sizing the wire in the Wiring Fundamentals course.

It states a 16AWG wire will take a max of 8 AMPS

When purchasing some wire, I have been given a table also stating the AMP rating on the wire I have purchased. It stated a 17AWG wire can take 16.5 AMPS

I know these figures are a guide and it depends on application , condition etc. But there is quite a big difference between what the course table shows and the table that has come with my purchased wire.

The component in question is my wet nitrous system solenoids. The nitrous solenoid max amps rating is 10A and the fuel solenoid max rating is 4A. I am running both solenoids from the same relay. So the wire from the relay to the solenoids I have purchased is 17AWG (16.5 amp rated) so in my head this should be fine.

But now I’m a bit worried and think it may not be enough.

I would have to use 12AWG wire according to the table shown in the training course. This is massive in comparison to the wires coming out of the solenoids themselves around 20AWG

Can anyone shed so light for me. I’m still in the build stage so can easily change wire size now.



I noticed the same thing about the sample numbers provided. Those numbers are VERY conservative.

I don't think they specify what kind of wire they are rating there. But I assume it's TXL at best. Tefzel wire can absolutely flow more current than shown in those examples.

Also note: those current ratings are for continuous current. Something like a solenoid that is only powered for a short burst at a time, it's perfectly fine to overrate the wire (within reason) in that scenario.

Broadly speaking, there are two important ratings, which are quite different in practice.

One is the rated current that the wire can accept without getting too hot, and risking the insulation failing, this is going to vary with the insulation material and the diameter - the linear-square law, as the insulator circumference that is rejecting the heat increases in proportion to the diameter, but the cross-sectional area that is producing the heat goes up as the square.

The second is the voltage drop along a specified length of the wire, for example, it may specify a current rating for a 10% voltage drop over 10 feet of wire.

In this instance, the more important factor may be the voltage drop, as insufficient voltage across the solinoids may cause erratic operation even a disasterous condition where the nitrous triggers but the fuel doesn't.

As I have an aversion to voltage drops, especially on power circuits, I'd be looking at over-rating the wiring to, and from, the relay, but using their OEM from the relay (that should be as close as practical) to the solinoids, as they should know what is required.

I could be wrong, though, so take it as something to consider, rather than a 'rule'.

You're spot on Matt, the supplied numbers are conservative. There isn't a complete wrote solution for this as it differs in all applications as Gord has pointed out. The table we provided is safe, as we didn't want anyone starting any fires in their projects, but there is absolutely room for optimisation!