# Wire sizing for battery interconnecting cables - 2/0 or 4/0

Solar Expert Posts: 225 ✭✭✭
My battery bank will be 12VDC. Six, 6-volt 225ah batteries wired for 12 volts @ 675ah. My inverter is a 2000W Prosine 2.0.

The formula I'm using is 2000w / 12VDC = 166A. 2/0 should be able to handle that. That said, should I use 2/0 or 4/0 for wiring the batteries together? Am I missing something?

FYI, I'll be using 4/0 wire from the battery bank to the inverter.

thanks,

For one or two parallel strings, I would use full size cable for batteries. For three our more parallel strings you could reduce by 1/2 cross sectional area (+3 awg gauges smaller).

Again, my personal opinion.

Going to fuse each string? (Three or more parallel strings).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 225 ✭✭✭
Bill, I'll be using three parallel strings.
For three our more parallel strings you could reduce by 1/2 cross sectional area (+3 awg gauges smaller).
Are you saying that 2/0 will be acceptable for three or more parallel strings? Keep in mind you're dealing with first-rate rookie here un-versed in PV vernacular

I hadn't considered fusing the battery cables, but it wouldn't be a bad idea.
Well--If you want to put me on the hot seat... First the inverter calculation:
• 2,000 Watt continuous * 1/010.5 volts cutoff * 1/0.85 AC Inverter eff * 1.25 NEC derating for wire/breakers = 280 Amp branch circuit rating for full load continuous
Using the NEC table (without the rest of the NEC temperature, conduit fill, etc. derating calculations):

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

Between 250 and 400 cmil wire (or paralleled smaller cables).

If you use marine ratings:

http://www.boatus.com/boattech/articles/choosing-cables-and-terminals.asp

1/0 or 2/0 cable minimum...

So--If you use Marine wiring standards, 2/0 would be OK for main or battery string wiring... You should double check the voltage drop using a voltage drop calculator... Recommend you try for 0.5 volt drop maximum for Inverter and 0.1 to 0.05 for charging.

http://www.calculator.net/voltage-drop-calculator.html

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 225 ✭✭✭

Thanks for your great info Bill. I will keep this in mind when setting up my battery system.
• Solar Expert Posts: 5,987 ✭✭✭✭✭
I think in a previous thread you spoke of this.

What might be missing is voltage drop over distance. I believe Prosine will have proper sizing in there manual for different distances. at higher amps voltage drop can be substantial particularly for low voltage systems.
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
• Solar Expert Posts: 9,583 ✭✭✭✭✭
Hairfarm wrote: »
......166A. 2/0 should be able to handle that.....,

But can a string of 225A batteries (x3) handle that for very long ? 55A per string ? IF it's evenly split amongst the batteries. More than a couple minutes, and the batteries are going to start to sag under a 50a load.

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