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Old 08-13-2019, 06:16 AM   #1
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Battery bank wire size questions

I'm upgrading my house bank from 4, 6Vlead acid golf cart to 4, 12V lithium batteries in parallel .
My inverter has a max charge rate of 150 amps and each battery has a max continuous discharge rate of 150A.
According to the various cable size charts I have looked at 4 gauge is good for 200A at 4 ft with 2% loss. My cables will be less that 2 ft.
I can get a really good deal on some 4 gauge wire. Does that wire size sound right with a safety margin?
Is there any good electrical reason why I can't simply connect all four batteries to the inverter at one stud instead of using some kind of buss bar? All connections from stud to each battery will be the same length.
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Old 08-13-2019, 06:38 AM   #2
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#4 for 200 amp doesnít sound right. Iím running double #2 for the same load. My run is a little longer but not long. Seems like different charts give different results. Most here will say just go with 2/0 welding cable and sleep easy/call it a day. This is what I would do if starting again. I started with #2 and rather than replacing everything when feeling undersized I saved money by just adding another set of #2. It works but makes for a lot of cable termination.

If your run really is that short, it will not be very expensive anyway.
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Old 08-13-2019, 07:28 AM   #3
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I'm not talking about the wire to the inverter it is 4/0 I'm talking about the battery connections from the stud to the individual batteries wired in parallel. Inverter is 3000 W.
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Old 08-13-2019, 08:07 AM   #4
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There are a bunch of charts for sizing available. The chart says 4 is ok for that length, but it's always better to over size because although your batteries can produce 200 amps they're capable of producing much larger currents under the load of your 250 amp inverter. I have 8ga and switched to 4ga on mine and it doesn't get warm at all. You could use also copper bus bars, or make them out of copper pipe.
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Old 08-13-2019, 09:14 AM   #5
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Nevermind. Really long post when I thought you were talking about something entirely different.
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Old 08-13-2019, 10:32 AM   #6
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Nevermind. Really long post when I thought you were talking about something entirely different.
I got your whole original post in the email notification and it is all good info.
Iíve had no trouble using cheap copper coated aluminum wires btw at loads up to 250 amps.
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Old 08-13-2019, 11:10 AM   #7
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I got your whole original post in the email notification and it is all good info.
I’ve had no trouble using cheap copper coated aluminum wires btw at loads up to 250 amps.
Actually it included a couple errors I was happy to be rid of, and was editing out before I read the OP's correction & realized he was talking about different wire runs But formulating answers to questions is as much a learning experience as it is a teaching one at times, at least for me.

As for CCA, the problem as I see it lies more in people using ampacity charts designed for pure copper when CCA would be better assumed to be aluminum for determining current carrying ability. I'm sure it's somewhere in the middle, but since most references don't offer a third column...
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Old 08-13-2019, 11:31 AM   #8
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Originally Posted by somewhereinusa View Post
I'm not talking about the wire to the inverter it is 4/0 I'm talking about the battery connections from the stud to the individual batteries wired in parallel. Inverter is 3000 W.
Keep in mind 4g and 4/0 are completely different size wires.
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Old 08-13-2019, 01:39 PM   #9
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Take another stab here now that I get the question, have my head screwed on right, and learned a couple things.

Use the inverter's maximum continuous DC input current specification as the basis for your calculations. In the case of a 3000W inverter I picked at random, that's 373A (yours likely differs, I'd look up your specs. Might save you some amps and some cable size). Multiply that by 1.25 for an NEC-compliant fudge-factor to get the total ampacity requirement for the leads from the bank to the inverter: 466A in this example. Using the following chart, assuming 90-degree copper cable, and room temperature (no temp derating), that would be 2 x 4/0 (ouch). So if you're running single 4/0 now, your cabling is bigly undersized. Assuming your over-current protection is sized for 4/0, your wiring is likely protected (so long as it functions correctly). But if your numbers are even close to my examples (and they should be real close) you can't safely supply the current your inverter is capable of pulling. At 3000W off a 12V bank, though, you may never have had the opportunity to try.

https://www.usawire-cable.com/pdfs/nec%20ampacities.pdf

As for each parallel run, ideally, that would be A/n (n being the number of parallel runs). So again using the example above, 466/4 == ~117. 2awg in 90-degree copper. BUT...

The problem with parallel runs, particularly with batteries like yours with low internal resistance, is that even a tiny difference in resistance can cause rather large differences in current. That's why multiple parallel strings are frowned on... one or more more batteries with ever-so-slightly reduced resistance in their current paths end up supplying a disproportionate amount of current compared to the rest, discharging deeper than they should have to, which eventually leads to premature battery/bank failure. So you could potentially have a significant difference in current between each run, which could mean 2awg might not be enough in the highest-demand applications.

I guess if it were me, I'd stick w/ 2awg. But I'd fuse every single parallel lead right off the battery. And it's critical every parallel lead is exactly the same length, with the same terminations, torqued to the same inch/lbs, in order to do everything possible to minimize any differences in resistance. Actually, if it was me, I wouldn't run any more than 2 parallel strings max, and I wouldn't use a 12V bank w/ an inverter > 1500W or so, but that's another discussion.

Keep in mind I'm speaking from a book-learning perspective. What I'd do is not necessarily what you should do. I think it is, but I could be wrong


Edit: changed stuff again. I meant 2awg not 2/0 (now edited to be correct above). I'm giving up thinking for the day. It's not working out.
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Old 08-13-2019, 02:30 PM   #10
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More thoughts on how to (or not to) interconnect batteries: SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank
There's lots of other good technical info on SmartGauge's website - it's well worth reading.

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