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Wire Gauge advise

Practical Motorsport Wiring - Club Level

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Hello Andre and guys

Im designing a new wiring harness for my BMW e30 M3 race car, this time using Tefzel wire M22759/32.

Have a couple of doubts about what AWG to use for some components, considering the distance they are from the source.

Also, where can I use shielded wires, I have a 2 twisted + shielded wire, which I can use for a ATL Fuel lever probe, which needs 2 signals and 1 ground, will this cable be OK for this use? and I then just splice the shielding into a Tefzel wire and ground it?

All the engine harness is BMW OEM, so I will not be touching that.

Main power and ground is done using a Cartek GT battery Isolator, going to the car roll cage and I have also wired a 10 Gauge ground to a front ground splitting, where I am grounding every component signal too, as well as the engine block and cylinder head main grounds.

Im considering using 20 AWG on the low side of the relays, since its only a triggering signal for the coil to operate.

The IGN Relay K1, is the one that give IGN signal to several components, such as Pre fuel pump, windshield wash pump, Coolshirt helmet fan, Coolshirt cool system, ECU Ign signal, reverse lights, etc. all summed up around 20 Amp Current draw.

Please if you can take a look at my Relay section, and components, too see if the wire gauge im suggesting is the correct one

Attached Files

Hi Said, you can use a shielded cable for convenience if you wish for your fuel level sender, however that particular sensor doesn't strictly need to be shielded. I generally reserve shielding for high frequency inputs such as engine speed and position, cam position and knock sensors. On this basis if you're using it due to a convenient form factor then go for it, however there's no need to actually grounding the shield.

While there's nothing wrong with what you're intending to use for the different wire gauges, I'd say in general you're probably a little heavier on some of the wire gauges than necessary. The only downside to stepping up in wire gauge is size and weight of the harness but if you're ok with that then go for it. For an example generally a starter solenoid will draw perhaps 10 amps during cranking which could be comfortably supported by a 18 AWG wire, or 16 if you want a little additional headroom.

Thanks for the reply, will see if I use the cable or not.

Regarding the other cables, can you make suggestions on what sizes to use? I believe Tefzel have another amp rating?

thanks for the help!

Hello Andre

Any more help Will be appreciated


Most of what you have specified is probably fine. I noticed however you're actually a little light on the wire gauge supplying power to the diff cooler relay since the combined current draw will be 18 amps but you're running a 16 AWG power supply to it that will only support around 14 amps continuous. I'd suggest that you run a 14 AWG power feed to the relay and the pump could be 16 AWG given the longer run, while the fan could be 18 AWG.

For your Fan 2 relay you're using a 16 AWG supply but then a 14 AWG feed to the fan which is overkill for the distance. 8 amps can adequately be supported by 18 AWG with headroom.

Thanks Andre


For the Coil Side of the relay, can i use a 22 AWG? since its only a "trigger" signal?

thanks Andre

Yes absolutely. There's minimal current draw on the trigger side.


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