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# splitloom + battery cable sizing

### Tech Articles

Discussion and questions related to the course Practical Motorsport Wiring - Club Level

Hi

My car: 1993 E36 M3 euro/JDM

car is stripped engine is basically stock with stock ECU. ABS, headlights, taillights, indicators retained. no body module,heater core ect..

I have 2 questions.

1) what is the cleanest way to finish split loom? braided tape, SCL, or other. I am going to cover some of the longer runs in the cabin like the fuel pump and the taillights with split loom. whats the best way to finish it for a tidy appearance?

2) how do I size the battery cable for relocation? am I basing my calculation on starter + injectors, ignition, and fuel pump + safety %. I would like to save some weigh (every oz adds up) and cost. I will be retaining the size of the second wire as there is a parallel smaller wire that I guess provides stable power for electronics like ecu . I may even move battery into the engine bay if i can find some room.

thanks

Edit*

Added OEM wiring diagram. You can see 6mm^2 power to ECM and the 70mm^2 that I would like to downsize. 70mm probably a little overkill for charging circuit, starter, abs, lights and cooling fan.

Starter is 1.4kw, spal fan 9amps,

Edit** 70mm for long run 25mm from battery + distribution in engine bay.

so 1.4kw at 10amps (I read something about calculating this based on lower voltage due to voltage drop. cant find it. I will use this as it adds safety factor) 1.4kw/10 = 140a

looking at prowire their 2awg is rated at 200a @ 50' and 1awg does not have a rating listed but based on some rough math and assumptions 250a @ 50'

the wire size from the bat+ to charging system is 25mm^2 I could probably use 2awg for my shortened wire to battery behind passenger seat about 5' long but I think I will use 1awg for safety factor.

now the question becomes is it better to stack the battery ring terminals of the 6mm^2 and the 1awg on the battery or try to crimp them into one large terminal, I feel like this will be difficult to do with ring terminal due to the difference in cross section of the wires.

If someone could check my math on the battery cable size and confirm I am not overlooking anything I would really appreciate it.

I might be overthinking this a little.

In most instances you'll be absolutely fine with 2 AWG for battery relocation and that's what I'd personally use in your situation. Your main consideration is really during cold cranking where the current draw to the starter motor will spike. Once the engine is up and running your 2 AWG will be more than sufficient for your anticipated loads. If you want added peace of mind then stepping up to 1 AWG is going to only cost you a touch more and add a little additional bulk. In terms of termination at the battery I'd recommend a separate ring terminal for each wire and stack these on the positive stud.

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