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Power supply, fusebox - relays ground, power wire sizing

Practical Motorsport Wiring - Club Level

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I'm just in the process of making my first standalone engine harness and I have a few questions regarding the power supply.

1. Where should relays be grounded? Should they head to the common ground on the engine block? Seems a long way.

Or maybe somewhere near on the chassis (but that could be violating star point earthing)

2. I have a fusebox containing 4 relays: Main Power, Enable Power and 2x Cooling Fan.

What size of wire should I use for relays Pin 30 and how to split them? Will 4mm (14 AWG) to 4x 2.5mm (12 AWG) wire be enough?


1/ With relays, you should be perfectly fine grounding them where they are, they're just an on/off device. it's sensors and other devices that need to know the relative voltages that need to be grounded to common points, because it in normal for the relative ground potentials (voltages) at different places to be a little different, which will throw off the values - especially when cranking the engine.

2/ Impossible to say from that information. It will depend on the current being drawn through the wire(s) and whether it's in the open or buried in the harness, and the acceptable voltage drop at the device being powered by the relay. There are several videos and guides to wire selection for you to look through - and bear in mind the type of wire will also affect the ratings for current and/or voltage drops. The 87 wire will normally be the same gauge as the 30, as it's the same circuit, as is usually the ground wire from the device.

My personal preference is to size up one or two gauges from the 'nominal', but I've a thing about voltage drops, rather than current per se.

Thanks for such a quick reply!

1. Excellent reply. I was a little bit worried about ground problems.

2. I'm running FLRY-B wires, which are rated:

* 2.5mm2 (12 AWG) for 30 amps,

* 4mm2 (12 AWG) for 41 amps,

* 6mm2 (10 AWG) 53 amps,

* and 10mm2 (8 AWG) for 68 amps

I calculated the fuses for given relays as:

* Main Relay powers the ECU via 3A fuse

* Enable Relay powers ignition coils (15A), injectors (5A), Solenoids + CAS (10A), WBO Heater (5A), AUX connector (5A)

* each Fan Relay powers a single fan via 20A fuse

I'm running 40A relays. I've been thinking about providing 2.5mm2 (12 AWG) wires per relay, merged into one 6mm2 (10 AWG) wire.

Any suggestions with these details?