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Old 17-12-2015, 16:06   #1
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Cable size for 3 pair 6V batteries ??

I have seen the charts for the 3% voltage drop but nothing for my new battery bank.
I have six 6v batteries, 3 on a shelf above the other 3. Obviously set up in pairs to give me 12v. From the Positive buss to the upper pos terminal is 10" and then 24" from the neg to the lower pos terminal and finally 27" back to the negative buss bar. These lengths will keep everything exactly the same distance. The Heart Freedom 20 is very close to the batteries and has some massive cables between the buss bars and the inverter. And the other side of the buss bar goes to the windlass (130 A) and house panel and finally all the way back to the Pathmaker power switch/starting batteries.

So the round trip for each pair of batteries ( 215 Ahr) is 5 ft. What size should these cables be? The inverter has the largest potential load or 167A /3 = 55A which suggests #6 would be plenty. using only #6, i could probably even knock a couple inches off the length and would be easier to route in the tight space. Is #6 correct?
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Old 17-12-2015, 16:16   #2
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Re: Cable size for 3 pair 6V batteries ??

That is not what I came up with. #2 would allow you to carry 170a in a closed space and #4 out in the open. It's not just a length thing, it's the amp capacity of a wire also.
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Old 17-12-2015, 16:19   #3
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Re: Cable size for 3 pair 6V batteries ??

NO, not sufficient. I'd use AWG 1/0 for interconnections.

That may sound like overkill, but it's really not given the need to reduce resistances between the batteries, corrosion potential in the marine environment, etc., etc.

Bill
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Old 17-12-2015, 17:24   #4
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Re: Cable size for 3 pair 6V batteries ??

Quote:
#2 would allow you to carry 170a in a closed space and #4 out in the open
I do not understand. Max inverter load is 167A. This is split amongst 3 pair so only 55A per battery pair. Outside of the engine compartment, #6 is rated for 120A. At a 5 ft round trip, #6 has a voltage drop of 2% at 60A. Remember, each pair connects to a buss bar before the inverter connection.

Quote:
NO, not sufficient. I'd use AWG 1/0 for interconnections.
That may sound like overkill, but it's really not given the need to reduce resistances between the batteries, corrosion potential in the marine environment, etc., etc.
1/0 reduces the voltage drop to 0.5% total but between batteries in a pair, the distance is only 2 ft, so #6 is 0.8%. how much more is required and remember, this is at max inverter load which is unlikely. The batteries are in the forward cabin with reduced corrosion potential. And bending 1/0 wire would be a bear! I would have to double the cable length.

Not to be argumentative but what am I missing?
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Old 17-12-2015, 17:56   #5
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Re: Cable size for 3 pair 6V batteries ??

1/0 is about right--you want minimum voltage drop between the batteries.
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Old 18-12-2015, 10:07   #6
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Re: Cable size for 3 pair 6V batteries ??

I'm going to go off the conservative deep in and say 2/0 as the minimum. You are only talking short runs so the cost differential over 1/0 is not much. Anything below 1/0 would be an absolute non-starter for me for that size bank.
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Old 18-12-2015, 10:12   #7
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Re: Cable size for 3 pair 6V batteries ??

The inverter/charger manual recommends that all cables between the batteries be the same size as the wiring between the bank and the I/C. We have a Freedom 15. RTFM.
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Old 18-12-2015, 10:45   #8
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Re: Cable size for 3 pair 6V batteries ??

Thanks to all with the suggestions and Stu, I did go back and you are correct, the manual did call out the interconnect cable to be 00 AWG.
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Old 18-12-2015, 23:16   #9
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Re: Cable size for 3 pair 6V batteries ??

Quote:
Originally Posted by btrayfors View Post
NO, not sufficient. I'd use AWG 1/0 for interconnections.

That may sound like overkill, but it's really not given the need to reduce resistances between the batteries, corrosion potential in the marine environment, etc., etc.

Bill
I agree with Bill - except I often use 2/0.

Keep in mind that these cables interconnecting the batteries are a vital part of your charging system. Looking at 12 volt pairs lets say your alternator (or shore charger or solar controller) have a setting of 14.4 volts. With 3% drop that means only 13.97 volts is getting to the batteries. That doesn't include and drop due to connections. I try to keep all battery cabling that is part of the charging system to as little voltage drop as possible.

Also the rule of thumb I use for inverter current is 10% - 2000 watts = 200 amp draw at max.
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Old 19-12-2015, 19:34   #10
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Re: Cable size for 3 pair 6V batteries ??

I've installed many inverters. The biggest mistake made is too small of cable. You need minimum 1/0 cable. 2/0 preferably. S/o cable is what you want. Many very fine wires. As flexible as your dock lines.
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Old 19-12-2015, 20:02   #11
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Re: Cable size for 3 pair 6V batteries ??

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You need minimum 1/0 cable. 2/0 preferably. S/o cable is what you want. Many very fine wires. As flexible as your dock lines.
Not sure what you mean by "S/o" cable? Tinned marine wire such as Ancor is the best choice as it has little chance of corroding over time.

I have never had any great issue with flexibility, including the times I have used 4/0 cables.
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Old 19-12-2015, 20:58   #12
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Re: Cable size for 3 pair 6V batteries ??

Do it right. This is my 6V interconnect, for example, the cable with the blue heat shrink:
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Old 19-12-2015, 20:59   #13
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Re: Cable size for 3 pair 6V batteries ??

S/o is the electricians slang. Sorry same stuff you are talking about. Very flexible.
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Old 19-12-2015, 21:04   #14
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Re: Cable size for 3 pair 6V batteries ??

Remember that this conductor needs to be fused as well, something that often gets overlooked.

And as for bending heavy cable, the better quality cables are comprised of many more, but smaller, individual wire strands. They will bend much more readily than cables with fewer but thicker wire strands.

I have used cable as large as 350 MCM (about the size of a large garden hose) in some applications. Bigger is always better!
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Old 19-12-2015, 22:29   #15
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Re: Cable size for 3 pair 6V batteries ??

The thinner the individual strands in the cable, the faster they turn to dust when moisture wicks in. Stay away from the S/o stuff on boats.
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