Which Cable for Battery Leads

[Jeff Taylor]

Hi Howard,

I'm sure you're correct in saying that 180amp is probably the largest size of armoured cable available. If The Seven Workshop recommend it then it must be fine - I wouldn't want to disagree as I'm no electrical expert. My suggestion was based on the cable spec recommended for use on my 1936 Morris 8 Tourer which is also 6v. I was intending using the same 300amp spec on my 1934 Austin 7 AVH Van - that's if I ever get round to sourcing the missing parts and to begin reassembling it.

Cheers,
Jeff.

[Howard Wright]

Hi Jeff

I’m no electrician either.  It may be that the Morris starter is higher powered and therefore draws more current.  

Good luck with the build!

Cheers

Howard

[Colin Wilks]

Having rewired our narrow boat a few years ago I am sure this conundrum is all about voltage drop (if we have any sort of technical issue my wife now states brightly "It's probably voltage drop!". Current and cable run are critical with a low voltage system. Bigger section cable will enable a less than pristine battery to do the business. Bear in mind that it is the volume of copper between A and B that matters, so cutting back a fat cable to make it fit the terminals is not wasted effort, but routing it may be problematic.

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Thinking further, the cable runs on a Seven are so short, I doubt voltage drop really matters. I would just take the current draw for your starter and multiply by 1.5 to specify the battery cable.

[Stuart Giles]

Ultimately, IMO WRT to cable size,  it depends on which end of the telescope you're looking through, if you set up your car from the perspective of  the CCA figure of a typical large 6V battery - like the MGB one which is the biggest 6V battery I've managed to squeeze into a non-special A7 floorpan -the CCA (Cold Cranking Amperage) figure on one of these  is about 600A. So, if you designed your setup from the cable  requirement for 600A, because that is the maximum current, that would require  70mm sq cable (i.e.) 17mm+ Outer Diameter. Counsel of perfection on this would say we need to minimise voltage drop, so why use 70 sq mm. 95 sq mm would be the thing to have for 600A load - the resulting cable would be the best part of an inch diameter of course.  The reality is that the battery's 600A CCA figure is deliverable for  moments at best; the  MGB 65 ampere Hour battery in this example won't deliver that 600A (3600W) for long, Certainly not long enough to overheat a reasonable sized battery cable. 

I have set up several 6V cars with 25 sq mm cable -the exact same stuff that has been good for extended use @ 180A+ on one of my welders without any undue heating- in an A7 they've all cranked the motor at a reasonable (6V) speed and started the car OK; if the battery happens to be  right at the rear of the car, it may be worth going up to 35 sq mm cable, to be sure.

[Colin Swinbourne]

I have been designing and sorting out cable specs for my Delage - there is some useful information, and a handy voltage drop calculator, on this site (all be it using voltages greater than 6v though it’s a simple matter to recalculate for 6v)

https://www.12voltplanet.co.uk/cable-siz...ction.html

[spannerman]

Try putting clamp on ampmeter on starter
And see what it is pulling in amps use next
Size cable job done!