starter cable current

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Aug 29, 2021, 8:56:29 AM8/29/21


My H4 is fitted with a 2L silver top and I want to replace the main power cables (starter and ground). Can anyone advise what the current rating should be? 

I suspect the existing wiring was for the original Fiesta 1.6 Zetec so potentially is too lightweight. I have found a ~50Ahr battery that fits the original cradle (which was for a ~40Ahr battery). So I must have ~ 20% increase in cranking amps too. Some 50Ahr batteries are able to generate 450A crank current, but I assume (given the options available from suppliers) cables do not have to be rated at this current (and it is short duration). So what is used?

Any help appreciated.

Many thanks

Darren Siepka

Aug 29, 2021, 1:27:52 PM8/29/21
I use welding lead cable for my main lines, especially as I have the battery In the boot. 
Size wise I use 35 and 50 mm2 cross section.
Reason I use welding lead cable is it tends to be more flexible with thicker tougher plastic covering, and best of all usually cheaper too :-)


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Aug 29, 2021, 2:24:53 PM8/29/21
to, Darren Siepka

Hi Darren.

That seems logical given the cable lengths. Thanks. Fortunately my cable runs will be shorter.

I've done a fair bit of digging and like many other such questions the disagreement on the forums is fascinating with debate getting heated! 

The nearest I can get to any degree of certainty is that the expected current is likely to be under 200A initial peak, dropping rapidly to somewhat less - something in the range 100-150A. These numbers are taken from several threads about similar (and bigger, inc diesel) engines so the chances are they are on the high side but are in a decent ball park. Anyway, it is unlikely to be more and over-estimating current is preferred!

Clearly the voltage drop is what is most important, it must be small compared to battery V. I came across this site (which avoids doing calculations the hard way and maybe getting them wrong)   which has a calculator. If I calculate based on 1m each way this gives me a V drop of less than 0.25V at 150 A on 25mm2 cable (ie <2% of the voltage). So that looks like a good option. 

Unless anyone thinks that absurd, I'll give it a go and confirm by measurement.




Aug 29, 2021, 4:10:44 PM8/29/21

The Cold Cranking Amps is often given as a figure on the battery. To be sure, if you aren’t worried about the weight or cost, you could fit the largest available that will fit with an eyelet terminal that fits the starter post, going for bigger cable is preferable as you say.

Martin Scott


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Aug 29, 2021, 4:29:58 PM8/29/21
to, susanandmartin
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