battery cable

[jerry f]

Does anyone have a good "standard to go by" for sizing battery cable for gas engines up to 7.4 liter. ABYC guidelines dont cover the amps drawn by a starter and they would be unrealistic anyway do to the short duration for starters. I have crimping equipement for up to 1/0 and I feel that should be sufficient for starting requirements but would appreciate any input (including what size are factory cable's?? I know west marine pre-assembled cables are only 4 gauge-seems small to me)

[Alan Wheeler]

Sorry, I am not familiar with the gauge sizing you guy's use. I would consider the use of 50mm cross section size for the cables. Hopefully you can recognise that size, or convert it. Anyways, It also depends on length. The longer the cable the heavier it needs to be. One important point. It's not so much about the short duration of cranking and the cable heating, blah blah, it's the fact that if the cable is heating, then the power that should have been supplied to the starter, is now being disappated into the cable as heat. When you are working with low voltages like 12/24 volt, even a very low cable resistance ends up disappating an enormouse amount of power and this results in a voltage drop at the starter. The result is a starter that does not turn the engine over fast enough and makes it harder to start. Harder to start means turning the engine over longer which results in the battery running out of snuff earlier. Especially if the battery is down a bit to begin with.
My suggestion, go with the heaviest cable you can work with and keep it as short as possible.

[GordMay]

The larger the cable, the less voltage drop, and the faster the starter will turn. Use the largest size starter cable that’s practicable.

A 1/0 AWG (53.5 mm2) cable is rated at between 242 A & 285 Amps* maximum, or about 3500A (at 12V nom.) per foot of circuit length (Pos. + Neg.)
Most engine starters (gas or diesel) will draw less than 100A, so I would expect a 1/0 copper cable to be an adequate starter cable size up to a distance of 15 feet (voltage drop**). 1/0 cable would suffice for a larger starter rated 150 Amps, located withing about 10 feet.

* Depends upon ambient temperature & insulation temperature rating.
* The longer the cable run, the larger cable size required***.
*** See ‘Ohm’s Law & Boats’ at: http://cruisersforum.com/showthread.php?s=&threadid=372

HTH,
Gord

[jerry f]

Thanks guys. I tried an experiment on my auto with my clamp meter set to record max amps and it maxed out my 400amp meter but i forgot that LRA amps (maybe 1/10th of a second "locked rotor amps")was what i was reading not true current. Pulling the coil wire and having someone crank while watching meter would be a fool proof way. Anyway i feel confident now in the 1/0 cable.