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Injectors & Coils- Cable Size Vs Fuses

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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Sorry Guys, probably another dumb question... Just looking here at sizing wire and fuses for my M20, So in the worked example of the course, the guys are using 22AWG for the injector cable, and 18 Awg for the coil packs .

I've been recommended 22 AWG is the go to for injector wiring, and Coilpack wiring seems to vary between 18 or 20AWG.

However; going back to the worked example in the course, the 22gauge wire is then accompanied by a 10A Fuse, and the 18 Gauge by a 15A fuse... But according to the specs, the typical maximum current rating of the cable is much lower than these fuse ratings... what am i missing ?

P.S sorry for another stupid question.

I don't think those values are out of line. What specs are you using or are worried about? You can always start with a smaller fuse if you are concerned.

my thoughts...

There are two ratings for wiring.

Max rating for the wire's current will be at 100% duty, primarily to control the heat build-up from the resistance with a safe degree of headroom for when it's bundled, etc.

The second is the current for a specific length and voltage drop.

It would usually be good practice to size the wiring for continuous current of at least the fuse/circuit breaker rating but I can only assume they're working on the duty cycle lowering - on that, with injectors they have two current demands, opening which can be several times the other which is the 'hold open' current - the fuse has to be rated for the peak, opening current with some reserve, but the wiring will be exposed mostly to holding or no current, so limiting the heating.

Personally, I'd suggest up-sizing one or two sizes, as the pins/terminals allow, because power is normally proportional to the square of the voltage, so a 10% voltage drop means only 81% of potential power, which may make a noticeable difference with high draw injectors, especially when cranking and battery voltage is 9-10V compared to the 14-14.4V when running with the alternator charging. Same with the ignition system, as the greater the energy to the spark plug, the better the chance of igniting alcohol fuels and/or the plug resisting fuel fouling.

Thanks Guys,

So According to the table, the 22SWG Cable has a max load of 2.4 A and in the example is fused with a 10A Fuse, while the Coils are wired with 18SWG, Capable of 5.6A, but has a 15A fuse in place...

Are we saying that in reality, for short periods of time the wire will be able to take significantly more load and hence the larger fuse?

Well, I think 22 awg is good for about 7A, and 18 awg for about 16A, according to charts like the one found here:


I theory, yes, but it's not a practice I would endorse - you especially don't want to get into the range where the wiring becomes the "fusable link", because that lets the smoke out and can ultimately ignite a wiring fire.

Different tables may use different limiting values, and it should also be noted that different quality materials will also affect the values.

Some of the variables - for example, for the thermal limitations there's the quality of the conductor and the metal used, which affects how much heat is produced in the conductor; the insulation material used - the specific material's thermal limits (how hot it is safe to run it to) and conduction (how effectively it moves the heat from the wire to the surroundings), and whether it's a single wire exposed to ambient temperatures (which is another factor - engines and engine compartments can get very hot) or if it's buried in a mass of other wires which can make a big reduction in the heat rejection or, worse, contribute to it!.

For the voltage limitations, there are the acceptable voltage drop for given lengths of the wiring, some may be 10% over 10 feet, others may be 1% over the same distance, others may have the same drop but different lengths - the conductor can also make a huge difference in this - IIRC, silver is best (yes, some very high end applications do use it), followed by copper (higher the purity the better - I am aware of some 'cheap' Chinese wire/cable being very badly contaminated and causing high resistance/heat build up), and then aluminium (again the purity is important - main advantage is it's cheap and light - but it is also very susceptible to work hardening and breaking).

So, if looking at a chart, be sure you understand what it's saying, and remember it's for that specific wiring product - other lines or manufacturers may have different figures.

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