Battery interconnection wire size

[foo1bar]

NEC doesn't allow parallel wires in residential, but does in commercial. Not that I really care if you do it. But paralleling wires like that looks like a hack job.

NEC allows parallel in residential. (If you think I'm wrong, point me to what code section says I'm wrong)

But in practicality - there isn't much need in residential, as parallel wires is at least 1/0, and they must be same electrical characteristics though (same length, cross section, etc) (NEC 310.4 and NEC 310.10)

So why bother with parallel in residential if you're just doing 200A service anyhow. Just get 2/0 and be done. (or 4/0 aluminum)

In general, I'd say if you can meet your needs with 4/0, I would use 4/0 rather than do parallel.

Or in the case of batteries, I would probably look at doing appropriately sized copper bus bars that were
But that's me.

[bob-n]

I don't know NEC well enough to comment there. I'm not an electrician and am not giving electrical advice.

foo1bar is certainly right that there isn't much need in residential for parallel.

SunEagle's point made me think. So many things can go wrong. Connections can fail. Stranded wire can have a broken strand. Solid wire can have a dent or crimp. Installation can be defective. I guess that those issues are already addressed by testing and statistics. Hundreds of millions of homes are built to the NEC and UL/CSA approved devices are submitted to rigorous testing. For everyone's sake, we hope that all of that data is incorporated into the code. For example, the NEC includes a restriction on the number of bends in conduit to minimize the risk to the wire.

Parallel wiring situations are certainly not as rigorously tested. The issues that can arrise in parallel wiring are more complex because there are more variables. Not all connectors are engineered and tested for more than one wire.

So if it isn't in the code, don't do it. There is no cost benefit to using two smaller wires instead of one larger wire. Do it right. SunEagle's original advice ("I would never run parallel wires...") is very good, as with all of his advice. End of discussion.

But as an engineer, I don't see the risk. SunEagle's point of one wire being weaker than the other seems to be the same risk as part of a larger wire being damaged. We could argue that the odds of one of two connections failing are higher than the odds of one of one connection failing because there are more points to fail. Mike90250 makes the point that one single failure is easier to detect than one of two failing. The most common analogy is that many people drive with one headlight burned out and don't know it, but when both fail, you surely know it.

I could also argue that having a second connection is more fault tolerant. If one connection is defective and becomes resistive, the other will take more current and reduce the burden on the bad connection. If a single connection fails intermittently, it may arc, leading to a fire. If one of two connections fails intermittently, the other will take the load and there won't be any arc at all. The other may not be reliable to handle the load continuously for a long time, but it would be reliable for short intervals of overload. Again with the headlight analogy, you can't see as well with only one headlight compared to two, but you can see much better with one than with none.

Ampster's point about current sharing being based on resistance is also valid. If things are built correctly and there are no defects, two wires in parallel will be just as good as one of twice the size, even if the two wires are slightly different length. A small difference in length will result in a small difference in current balance, exceeding the rating of the wire, but by an insignificant amount. Nothing is perfect. NEC has margin built in for this very reason. A 15A circuit is only allowed to conduct 12A continuously.

But back to the original point: Don't do it.

[Sunsun]

Thanks for all the replies. Mike's 'if one wire fails' point is good practical advice and has convinced me to go ahead and buy the 4/0 cables even though parallel 2/0 wires would have more load carrying capacity than one 4/0 wire.


Of course except for a short situation, its unlikely that much current would flow though those interconnect wires. But may as well do the "safety first" thing.

This does bring up another question. Does the main breaker to the inverter from the battery protect the interconnect cables when a cell shorts out?

Chris

[NewBostonConst]

Thanks for all the replies. Mike's 'if one wire fails' point is good practical advice and has convinced me to go ahead and buy the 4/0 cables even though parallel 2/0 wires would have more load carrying capacity than one 4/0 wire.


Of course except for a short situation, its unlikely that much current would flow though those interconnect wires. But may as well do the "safety first" thing.

This does bring up another question. Does the main breaker to the inverter from the battery protect the interconnect cables when a cell shorts out?

Chris

Your question doesn't make sense. So I will try my best.

The cable from the battery to the inverter has a circuit breaker. It normally protects the batteries from the invertor pulling to much current and damaging them. Shorted cells usually will cause the battery to heat up and have low voltage which eventually cause the inverter to shut down. Shorted cells can cause a fire but I don't think the breaker will trip. The interconnect cables are the ones that go between AC breaker and the inverter? I don't they will trip either if the battery shorts.

[Sunsun]

Perhaps I should have been clearer. I'm wondering what, if anything, protects the battery to battery interconnects cables if a cell shorts out. I assume a large amount of current will flow through the interconnect cables to that shorted cell.

That did happen to me while we were away on vacation and had to spend an extra week out waiting for a used replacement rear end for an 87 Dodge camper van.

You are correct, when a cell shorts the battery heats up as the electrolyte boils off. Then a Cascade effect happens where the two adjacent cells boil off their electrolyte.

The inverter did not shut down due to low voltage and the 250 Amp DC panel breaker did not trip (Breaker between the battery bank and inverter).

And that was just one shorted cell in a large 1380 amp battery bank. A 4/0 cable can handle 302 amps and a 2/0 cable 190 amps. I don't think they are protected if a cell shorts.

I consider myself lucky that I Dodged a bullet on that one and got home before any fire started.



Chris