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Ralph Mowery

In article ,
says...

Sizing it for 100 amps should work fine. If you don't like that,
just use my other method, look at the gauge size of existing jumper
cables. Even you could do that. And no, you don't need to know
why the car won't start, though that's certainly helpful. For
example, we wouldn't want a moron like you to waste time trying
to start a car with no gas or no engine.

Most wire size tables are useless for this. They are mainly for
continious use where heat build up can cause a fire or melt insulation.
Jumpers sized by those wire charts will be around size 0 or larger.
Jumper cables are in open air and are only used for a few seconds to
start most cars.

It takes from about 300 amps or more to start the small engines and much
more for the large engines.

Number 6 wire is usually good enough to get most engines running if a
quick start is needed because of a dead battery and a good engine.

It often helps if the battery is dead because the lights were left on
for a while to have the jumpers hooked up for a short period of time to
give the 'dead' battery a slight charge before starting the car.