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Main power supply wire size

Practical Motorsport Wiring - Club Level

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

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Hi,

I’ve got the battery in the nose of my car and the Ecu/fusebox fitted in the boot/trunk. The main power wire would need to be 5 meters in length to reach the fusebox.

1 What size power wire would I need? Would it need a fuse at the battery?

2 Also the ignition wire would need to be 4 meters. What size would you recommend for that one?

Its only an engine harness powering the ECU, 8off high impedance injectors. 2off Bosch wasted spark coils and an efi fuel pump running 3bar. All sensors are controlled and switched by the Ecu.

Regards

What is the total current draw to the fuse box? I would bet that an 8 or 10 ga wire would work.

I assume it's a rear mount engine, any reason not to take if off the starter solenoid terminal?

Assuming you have sized the starter cable, and the ground circuit(s) for minimal voltage drop, there should be negligible difference from an independent power supply from the battery.

David - I’m not sure the full current draw as I don’t know what the two Bosch 0221503407 wasted coils ( I don’t know how to work it out?) and original fuel pump draw?

Gord - yes, mid engine layout. I was considering taking the power from the starter motor, but as I’m keeping the original wiring in the car for now (43yrs old), I didn’t want to mix old and new in anyway and I’ve got to send wires to the dashboard for the switched ignition and Rev counter. Do you think I should review this and take of the starter?

regards

Ok, I’ve added up the current draw from help here, ohms law and similar items. I’m still learning. Can you check for me please?

ECU - 1amp - manual dictates it needs a 1amp +12v feed.

Fuel pump - 11amps - I don’t know the pump on the car. A Bosch 044 motorsport pump draws 11 amps, so I’ll go with that incase I upgrade.

Injectors - 8amps - 280150943 220cc/min 14.3 ohms resistance. Ohms law = 0.97amps(1amp) x 8 cylinders = 8amps.

Coils - 14amps - Bosch 0221503407 x 2off wasted spark coils. Another member on here through my other post says they draw 7amps each. So 14amps.

Total current draw - 34amps

Does that current draw sound about right and what size cable would I need? 8awg?

Thanks in advance, much appreciated.

10 ga is good for 35 amps, and your highest draw items are not constant duty cycle. 8 ga (50 A) gives you some headroom if you change fuel pumps or add some other load in the future.

Generally there are two things to consider when sizing wire - the current which affects the waste heat, and the voltage drop that affects the power transmission.

With signal wiring, especially simple on/off switching like pressure warning lights, the voltage drop may not be a concern - but with fans, fuel pumps, lights, etc, where the power potential is proportional to the square (even third power) I'd go one or two sizes larger. You can see what I mean if you look at, say, fuel pump ratings at different voltages. Don't forget to size the earths/grounds to suit.

Regarding your earlier question of whether I would recommend using the starter power terminal, I can't comment on what's best for you, but it seems like you are at this point looking at purely the high current 12V source for the ECU, and I would think it's much easier running from the starter than running a whole new run from the battery?

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