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Old 01-28-2014, 08:28 PM   #1
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Battery Bank Wire Sizing

I'm in the process of upgrading our house battery bank to (6) 6V Golf Cart batteries.

My intention is to wire in my inverter (150 Amps), my windlass (80 Amps), and my house panel (80 Amps).

I was going to fuse the entire battery bank with a 400 amp fuse.

The supplier of my batteries has told me to wire up the entire battery bank to the main negative/positive buss with 4/0 cabling.

Another supplier told me to wire it all up with 2/0, but I don't think I believe him. His argument was that the wiring run was all very short and voltage drop wouldn't be much of an issue.

I am not talking about any of the wiring running from the battery bank to the house panel, inverter or windlass - that is already installed and sized appropriately.

Any comments or feedback based on your experience?

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Old 01-28-2014, 08:34 PM   #2
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Usually 2/0 to 4/0 is inverter size wiring...maybe to huge starters....

Without knowing exactly what you are doing and exact wire lengths...I would be just why 400 amp fuse?

Doesn't sound like a "designed" system...just pieces and parts you hope are all OK...

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Old 01-28-2014, 08:39 PM   #3
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Voltage drop has got nothing to do with this. 2/0 wire is rated for 200amps there is a + rating of approx. 20% if used in open air. Go with the 4/0 cable if your fusing at 400amps

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Old 01-28-2014, 09:34 PM   #4
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Mbevins is correct as far as it goes. But why such a big fuse. 200 amps should be fine.

You will never use the inverter at any where near maximum while you are pulling up the anchor, and powering your house loads.

In other words don't size the wire and fuse by adding up ALL of the possible loads connected to the batteries. Size it for what is practical.

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Old 01-28-2014, 11:16 PM   #5
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There are plenty of wire size calculators on the internet. You'll need to plug in the load in amps you expect the wire to carry and the length of the circuit.

You want to shoot for a 5% or less drop in voltage. Less drop is better.

I was just playing with this calculator.
Marine Wire and Cable: Voltage Drop Calculator
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Old 01-28-2014, 11:43 PM   #6
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Ok, let me frame the question a bit better now that I have had a nice glass of wine.

I have attached an image showing my original plan on wiring up the battery bank. It shows fuses for the following:

  • 250 Amp for the inverter (as per manufacturers recommendation)
  • 100 Amp for the windlass (which is what it has right now)
  • 100 Amp for my house panel (which is what it has right now)
  • 250 Amp for the Alternator (as per supplier recommendation)
In practice the actual current draw for each item is as follows:

  • 150 Amp MAXIMUM for the inverter (but we never gotten there, usually 80 amps for short periods).
  • 60-80 Amp for the windlass (observed on my battery monitor)
  • 30 Amps for the house panel with typical mix of loads - max observed for short durations.
So at worst case that works out to 170 amps with all loads running which would be pretty impractical.

My battery supplier just looked at my plan and said put a main fuse in between the battery and the switch of 400A and size your wire accordingly - which to me is 4/0. It just seemed wrong.

If I look at a more suitable maximum current draw of say 200A as suggested by djmarchand, then things start to look more reasonable and I should be able to get by with 1/0 or 2/0 if I wish.
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Old 01-28-2014, 11:52 PM   #7
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You will not go wrong if you use the larger size cables for linking your batteries. I would recommend you use the higher rating to ensure minimal voltage drop as this will also ensure that each battery is charged at the same voltage and hence more likely last the same period of time.

It also minimises the risk that in case where the charger is not operational, that you have one more more batterys charging others in the bank.

In the case of the fuse, you should size it to cope with your expected load + a margin, so that should something go wrong it will will fail first.

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Old 01-29-2014, 06:21 AM   #8
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However you hook it up remember IF a fuse does blow with the engine ON , and the alt does not see a battery , you may be purchasing a set of diodes for the alt.

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