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Heatshrink size vs. number of wires inside

EFI Wiring Fundamentals

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Hello everyone,

I've already searched all over the internet about it and couldn't find anythink much usefull. The best info about it I've found in one of RBR tutorials (rbracing-rsr.com/wiring_ecu.html)

But what about bigger or smaller wires?

Thanks in advance.

Another thing is.... I'm thinking about running 2x 18 AWG wires to each COP (coil on plug). It's a proper wire gauge for it? Should I go bigger or smaller?

For high impedance fuel injectors I should be fine with 20 AWG (+- .50mm) and for low impedance fuel injectors 18 AWG would be enough? Or 16 AWG?

I've made a simple design for the COPs:

Which one should work better? Make the multi-wire splice after (top design) or before (bottom design) the connector?

And which AWG I should run from the splice to the power source? 4x 18 AWG wire into 1x ?? AWG wire.

I don't get too technical on the heatshrink size I'm going to use. You tend to get a feel for what will shrink nicely over a certain bundle size. If you're unsure you can always perform a small test shrink with a piece to see how it's going to work out. If you're concentric twisting your harness you can usually get away with a small size DR25 as it will still be easy to slide over the harness. If you aren't concentric twisting then you may have a little more trouble installing the DR25 if it's a tight fit.

18 AWG would be fine for your coils. Since the coils only charge briefly during the dwell period, the average current draw is actually not that high. I've used 20 AWG with no trouble on stock LS inductive coils for an example.

20 AWG will be fine for injectors. Again the average current draw is quite low due to the way the injector is driven. A high impedance injector is also going to have a lower current draw which helps your case.

I usually go by the rule of thumb for 22awg for high impedance injectors. 20awg for smart coils, 18 awg for dumb coils, and 18awg shielded for CDI. This is for standard normal wire lengths (less than 6 feet). If your ecu and power distribution is in the front, and the engine is in the rear of the car then you need to account for resistance in the wires.

In regards to your second question, in that scenario, it doesn't matter where you put splice before or after. I would look at what's more convenient and neater. Normally, I keep all splices under the main plug boot with maybe one or two splices elsewhere in the harness for serviceability.

Keep in mind that when you go up in a awg size, you are theoretically going up 150% in capacity. You can splice a 16awg -> 4 20 awg wires and that will satisfy more than enough juice. I have spliced with 18awg for coils and never had an issue but nowadays I splice it with 16awg since Ive adjusted my concentric layouts. I don't usually use wasted spark so Andre would know much more on awg requirements then I would for that setup :)

I've just made a basic layout using MS Visio, it's better than my drawing:

I will try to run the following setup:


4x 20 AWG (+12v) -> 1x 16 AWG

4x 20 AWG (signal)


4x 20 AWG (+12v) -> 1x 16 AWG

4x 20 AWG (signal)

Either wiring configuration will work on the coils.

On high impedance injectors, you definitely do not need 16awg to feed them. Take a 14ohm injector, well in a 12v system (14v with alternator) your MAX draw would be 1amp-ish per injector so 4amp-ish total max. I feed 4 injectors with a single 20awg to 4x 22awg. Just my 2 cents to reduce bulk wire in your loom. a 16awg to 4x20awg will work but not necessary to go that big

Agree 100% with what Ryaano19 is saying. We have been making wiring looms with the sort of wire sizes Ryan is recommending for the past 11 years.

Keep in mind that a lot of suppliers for OEM style connectors (like what you use on fuel injectors) supply terminals for wire which is way too big for the job. A common mistake we see is people using these terminals on 22awg wire and wondering why the wire pulls out or the terminal fails (because it has been over crimped).

Be careful also with the size of rubber seals they supply as well. Once again, some suppliers ship with seals for the maximum sized thickly insulated automotive or marine wire.

What sort of size would you recommend for fuel pumps etc?


Going by that guide above, for a Walbro 416 (F90000267) for example, a 12ga is the correct choice, yet speaking to a local supplier, they recommended 16ga for that pump, saying they've run up to 30A on the 16ga.

Any sort of definitive standard to use ?

Current rating is dependent of the ambient temperature and the maximum heat the insulation can withstand. (see attached file)

30A on a 16ga wire is just stupid -> it will burn or at least heat like crazy !

You should also consider the Vdrop


Attached Files

I understand the relationship of heat, distance and voltage drop as I am in the electrical trade prior to tuning.

We have a wiring standards book which lists all that for AC, just wasn't sure if that existed for DC applications.

Thanks for the replies.

What about the 044 fuel pump?


This datasheet says 14A +- 1 @ 5 bar.

I'm checking the correct wire gauge for the radiator fan, the original one is something like 10 to 11 AWG. I will be safe using a smaller wire? Or I should use the same size?

Haruki, for the wire size you have 2 (3) things to consider: current and length (heat).

For one 044 fuel pump, 12 AWG should be fine for most application

For the fan, you would have to check the specs. But most fan have a high current drow (~20A) so a "big" wire make sense.

For a single coil (coil pack) with 3 wires like the ford 1S7G-12029-AC:

1- sparks cyl 1 and 4

2- 12v power supply

3- sparks cyl 2 and 3

The 12v feed should be 16 AWG or 18 will handle? What about the other two wires, 20 AWG will do?

I couldn`t find any useful info about this coils. There is people saying it just draws about 4 amps, another places says 15 amps, so I don`t know.

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