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MG MGB Technical - Battery cable size and current rating

I'm replacing my main battery to starter cable with one I'll make up myself. The current standard one I have is only just long enough and I'd like to replace it with one a couple of inches longer.

I can't find any information on the current rating or the area of the cable that is used as standard.

Does any one know what the OEM spec is/was?

Many thanks,

Richard.
Richard Thompson

I'm almost sure Paul Hunt has covered the main battery cable fairly recently, have a look in the Archives here or have a look on Paul's excellent website. - http://www.mgb-stuff.org.uk/hammertext.htm
Nigel Atkins

You'll get a rough idea from the measured diameter of the individual strands and the bundle.

This http://www.offroaders.com/technical/12-volt-wiring-tech-gauge-to-amps/ has a chart plotting current capacity against length to determine the gauge required, and this https://www.autoelectricsupplies.co.uk/cms/files/awg_to_mm2_conversion_data.pdf converts the gauge to the diameter and area.

Be aware of the overall thickness of the cable when it comes to fitting it into the clamps and channels.

paulh4

Also very thick cable is hard to bend round the corners, but generally the thicker you can use the better for the main power cable. Less voltage drop, less heating of the cable. Operating the starter isn't too far away from a dead short across the battery.
Mike Howlett

Just did mine with 322/0.30, 25mm², 10mm, 170A flexible cable from Auto Electrical Supplies

I say 'did mine' but despite having borrowed an hydraulic crimper, struggled to be honest so got them made up and fitted at my local garage - cheaper than ruining cable and crimp and only an hours labour. It was the cable size they recommended.
Mike Dixon

More flexible versions of larger cables are available - basically they have many more thin strands in the make up.
They are often used in the marine industry where vibration / possible work hardening is an issue.

The attached link gives an idea + has useful info and links to sizing for load and voltage drop over length etc.

I first used it fitting out a boat about 30 years ago and I got the supplier to crimp the ends for me - not the one I’ve listed as I can’t find them anymore.

R.


https://www.12voltplanet.co.uk/plain-copper-battery-cable.html
richard b

I recently replaced a friend’s battery to starter cable on his B and used 32mm2, 448/0.30 cable, probably overkill but he’d complained of a slow starter and it was a bit short so needed replacing. The OD was just over 12mm and it fitted in the original rubber combs no problem. The buyer I got it from on ebay supplied it with lugs, which I soldered on. I know someone will say a B starter doesn’t need a cable this big but he’s running a tuned 2L Pinto (yes I know, sacrilege).
R.A Davis

The starter shouldn't be slow with the standard cable as long as you have a decent battery and connections throughout. You can easily lose three volts through bad connections but still have enough power reaching the battery and coil for it to start, albeit crank slower than it should.
paulh4

Hi all,

Many thanks for your replies and links.

I've been distracted from the original cable job.

When removing my old cable one of the captive bolts that holds the cable clamps under the floor came loose from the floor panel - one of those under the driver's seat - so seat out, fix with new bolt - seat back. I'd forgotten what a pain re-fitting the seat is!


When I got the captive bolt out it appeared to only be welded in with three spot welds each the size of a pin head. - I would not have minded, even expected this on an original body shell due to corosion, but this is on a BMHT shell where I'd have expected better welding quality - a bit annoying. I'll upload a photo when I'm next in the garage.

All back together now so I can think about the original cable task. Having researched I'll go for the thickest cable I can get in the underfloor clamps which seems to be ~10mm in diameter and which would be up to the current rating.

Thanks again,

Richard.
Richard Thompson

Richard,
the welding for that captive sounds bad but don't get your expectations too high on a Heritage shell.
Nigel Atkins

Just in case the engine has been out recently, it is easy enough to attach the earth strap on the engine to an engine side nut on the engine mount. My starter was sluggish after having the engine out and I discovered that it was earthed to itself, not the chassis. Various cables had been providing the necessary ground.
Glenn Mallory

CB cars had the earth strap from a bolt that attaches the engine mount mounting plate to the front plate, to one that attaches the mount to the chassis bracket.

On RB cars it's in a similar fashion round one of the gearbox mount rubbers.




paulh4

This thread was discussed between 25/05/2020 and 29/05/2020

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