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Old 16-11-2016, 05:37   #1
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Starter Cable Size Question

What size negative cable should I be running from my starter back to my house bank if the run from the house bank requires 2/0 on the positive side? Can it be 1/0? Thanks
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Old 16-11-2016, 05:40   #2
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Re: Starter Cable Size Question

Its the same amps, presumably the same distance. Why would you use any different cable size?
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Old 16-11-2016, 05:52   #3
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Re: Starter Cable Size Question

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Originally Posted by bobofthenorth View Post
Its the same amps, presumably the same distance. Why would you use any different cable size?
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Old 16-11-2016, 05:54   #4
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Re: Starter Cable Size Question

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What size negative cable should I be running from my starter back to my house bank if the run from the house bank requires 2/0 on the positive side? Can it be 1/0? Thanks
If you are powering your starter from the "house bank", it's not a house bank.

Regardless, it's a series circuit and the current is the same at any point in a series circuit.
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Old 16-11-2016, 06:17   #5
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Re: Starter Cable Size Question

Well I guess I should have mentioned the distance from the batteries to the starter is about 4 feet shorter on the negative side than it is from the batteries to the battery switch and back to the starter on the positive side.

Yes the current will be the same on both sides but the length of the run it not.
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Old 16-11-2016, 07:26   #6
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Re: Starter Cable Size Question

The current carrying capacity is a function of conductor size and material (assume to be copper), the temperature rating of the insulating material, and the temperature of the environment it lives in. Duty cycle is also a factor but not usually included in ampacity tables.

The voltage drop is a function of the resistance of the conductor (also a function of conductor size and material) and the length (including both the conductor on the positive leg and the negative leg).

Obviously both of these parameters figure in the optimum selection of wire size. Often in 12 Volt circuits the voltage drop will be the real short pole in the tent (IE the condition on which drives the selection).

To answer your question directly, they don't have to be the same size but they do carry the same current. We need some numbers to provide a more detailed answer.
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Old 16-11-2016, 07:38   #7
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Re: Starter Cable Size Question

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...I should have mentioned the distance from the batteries to the starter is about 4 feet shorter on the negative side than it is from the batteries to the battery switch and back to the starter on the positive side...
No. You should have mentioned the TOTAL distance from battery + all the way back to battery -. That is what determines cable size.
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Old 16-11-2016, 08:35   #8
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Re: Starter Cable Size Question

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No. You should have mentioned the TOTAL distance from battery + all the way back to battery -. That is what determines cable size.
Correct. There is some logic in keeping the cable sizes identical and of equal length, as well. You will probably find much to ponder in Calder and in this thread: Interconnecting/Bus Bar Battery Cable Lengths - Some Closure
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Old 16-11-2016, 08:58   #9
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Re: Starter Cable Size Question

I use the same size cable for battery interconnects, batteries to switches, and switches to starter. Usually 1/0 or 2/0. I see no reason for one to be smaller gauge.
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Old 16-11-2016, 16:46   #10
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Re: Starter Cable Size Question

However......

Your starter is bolted to a cast iron engine block which should also be grounded. If one ground wire is serving both starter and engine, often attached at one of the starter mounting bolts or other point close to the starter, it should be the same size as the positive wire. If there is a second separate engine ground it will act in parallel to your starter ground and the starter ground can be reduced accordingly.

Good practice is to provide a beefy copper ground bus at or close to the starter,
and bring the larger ground cables to that point - house bus, alternator, ac panel, inverter, etc. - and then run #1 or 1/0, starter to bus to batteries.
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Old 16-11-2016, 17:48   #11
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Re: Starter Cable Size Question

I like Boatwright's advice.

From a theoretical perspective, using a longer cable for the positive link vs negative link to the starter motor, has the same effect as using a thinner cable for one of the links - the voltage drop on that link will be greater than on the other.

As long as the total voltage drop during engine start is within your tolerance, you can adjust the length and/or thickness of each link to suit your requirements (it will involve calculation using max DC current and copper cross section of each conductor).

Usually, it's quicker to just calculate copper thickness using the overall path length, but there is absolutely nothing stopping you from calculating voltage drops on two segments of the current path, and just adding them.
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Old 16-11-2016, 18:01   #12
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Re: Starter Cable Size Question

The information about the unequal length runs is interesting but not particularly relevant in the real world. Bigger is always better and the cost difference will be insignificant.
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Old 16-11-2016, 18:09   #13
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Re: Starter Cable Size Question

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Originally Posted by Delancey View Post
What size negative cable should I be running from my starter back to my house bank if the run from the house bank requires 2/0 on the positive side? Can it be 1/0? Thanks
It's the same current draw.

House bank? Do you mean starting battery or bank?

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Old 17-11-2016, 04:19   #14
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Re: Starter Cable Size Question

Thanks for the feedback. Reconsidering my switch location.
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Old 17-11-2016, 04:51   #15
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Re: Starter Cable Size Question

Only savings is cost, and the wire is as good as it will ever be the day you buy it, from that day on resistance will slowly increase. One day having cables that are "too big" may be what saves the day when you have to start with a low battery or they merely get old.
I see wires like anchors, what does "too big" hurt?
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