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MG MGB Technical - Battery Cable Sizes

Hi

My battery challenges continue on my recently acquired 73 BGT. I have been lent a hydraulic crimper for the holidays and propose to make up my own leads to get the length I want - it seems to fall between the made up ones, and the shorter the better?


I have looked in the archives , looked at Astleys British car wiring book and spoken to suppliers....


Consensus for the earth wires( I am fitting an isolator switch ) seems to be 8mm/ 110A - Astley specifically says go no larger.


I can find nothing on the battery to solenoid one. The existing one is give or take 10mm - I assume I need 170A for that?

Any thoughts? Thanks

Mike
Mike Dixon

The same current passes through the solenoid cable as the battery cable, the difference is the length as you say earlier - the longer the cable the more volt-drop there will be, so it needs to be thicker.

This chart http://www.offroaders.com/technical/12-volt-wiring-tech-gauge-to-amps/ indicates 8AWG for 110 amps of less than 4ft, which goes up to 6 or 4AWG for 7 to 10ft.

This chart from AES converts AWG to diameter and area. 8AWG converts to 3.3mm dia and 8.4mm area which seems a bit small, I'd have said my earth cable conductors are getting on for double that diameter. 10mm conductor diameter is off the scale on the AES chart, I wouldn't say mine are that thick.

AES have 105A 15 sq mm which equates to 5/6 AWG on their chart which is a heavier gauge than the first chart, and 170A 25 sq mm which equates to between 3 and 4AWG which is closer.

It'll be governed more by what you can get than anything else I'd have thought, and whilst going thicker may not be necessary and would cost more I don't know why Astley specifically says not to go higher.

But fitting an isolator usually only involves one of the cables - battery or earth, not both. It's easier to fit one in the earth and you can get suitable cables already made up from Halfords, for example. I chose to put mine in the battery cable so I could run a separately fused supply just to the clocks and radios.
paulh4

I used an earth lead from an MGF - similar to this one.

Dave O'Neill 2

Thanks sorry to confuse the issue- the solenoid cable is not part of the isolator fitting. The solenoid cable I have is a bit past its best so I thought while I have the kit, it would make sense to make one up as well.

I think I may be confusing wire diameter and overall cable diameter - I will read it up and have a think
Mike Dixon

If you are going to fit the switch in the earth cable just get a ready made battery cable from one of the usual suspects.

It did subsequently occur to me that going thicker for that cable might cause problems with the combs and channels under the floor. Hopefully a ready-made would be the right thickness as well as already terminated.
paulh4

I managed to misinterpret Astley - it was no longer than 18in. I have have gone for 170A and ordered everything from Autoelectrical supplies - they agreed that was the most appropriate cable.
Mike Dixon

This thread was discussed between 18/12/2019 and 20/12/2019

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