Battery interconnection wire size

[Takis]

Hello friends,

I've just bought new battery bank. 4xRolls s6 fs 305 for my 24v system. C20 is 305ah.
I'm having difficulty in determining the wire size of the interconnection between batteries so ive chosen the thicker I could find, which is 50mm2 (about 1/0awg).

Two questions:
Is that wire size appropriate ?
If not, can I add custom wires in parallel so as to increase the overal size ?

[SunEagle]

The wire size for your batteries is usually determined by the inverter or charge controller. Based on your 4 6V Rolls wired in series for a 24V system you probably will not be charging them with more than 35amps.

But If your inverter is bigger than 3000watts you would want to use that #1/0 wire which is rated about 150amps.

So to answer your first question, it is more than appropriate for your battery wiring. And to answer your second question, I would never run parallel wires unless they are exactly the same length and terminated properly.

[Takis]

Thank you

[Sunsun]

I was wondering about running parallel wires also. Would it be acceptable to run two 16" 2/0 wires together for the series battery interconnects as opposed to one 4/0 wire?

The reason I ask is because I already have enough of the 2/0 cables to do the job.

Chris

[Ampster]

It is possible. There are tables to look that up. It has to do with the cross section of the two 2/0 wires equaling or being greater than the cross section of the 4/0.

[bob-n]

I'm confused by the comment about not running parallel wires. Why?

By my way of thinking, more copper is always better and will result in less voltage drop. Even if one wire is skinny and one is fat, they will still carry current in the same direction. If the fat wire was enough, the skinny additional wire will give safety margin.

Current capacity is roughly set by the amount of copper. Surface area to dissipate heat is another variable, but if that is the constraint, two thin wires may actually be better than one fat one because they have more surface area and are spaced apart.

If you need #10 and have two #12 conductors in parallel, I think you will still be OK. They may be slightly different diameter or slightly different length, so they won't share the current precisely equally, but the current mismatch will be no worse than the wire mismatch. I=V/R where V is the voltage drop from one end to the other and R is the resistance of the wire under consideration.

In my work, we do wires in parallel of different size all the time, but that's in a different field, not high-power solar.

[Takis]

Because the current always chooses the shortest way.
Difference in wire length would be devastated.

[Ampster]

Because the current always chooses the shortest way.
Difference in wire length would be devastated.

Can you explain that in terms of Ohm's law or physics for the benefit of future readers. Can we assume those wires are terminated at the same place? As a hint, the resistance of a thinner longer wire would be greater than that of a shorter fatter wire.

[Takis]

Can you explain that in terms of Ohm's law or physics for the benefit of future readers.

To be honest, no.
I've just posted as a simplified answer.

[bob-n]

SunEagle: What is the reason why you don't recommend parallel wires?

Takis: Warning: Ampster is baiting you.

Current doesn't take the shortest path. Just like water flow, current flow will split between parallel paths. Most will take the easiest path (fattest wire), but in this sense, many parallel wires are no different from wide wire.

Takis does have a point, however. If there is a very short, skinny wire and a very long fat wire, the skinny wire might wind up carring too much current. That's not what I call parallel paths, because the lengths are very different.

As Ampster implies, it all comes down to algebra and Ohm's law. If the wire lengths are within 10% of each other, then the current will share to a similar tolerance.

[Mike90250]

I'm confused by the comment about not running parallel wires. Why?..........

Because there is the chance a wire comes loose or goes bad, or a crimp falls apart. Then the remaining wire has to be sized to carry the load, so when sized right, using a single larger gauge is better than using 2 just acceptable gauges. A single wire failure is noticed right away, if a wire in a double run fails, nobody notices until the fire starts.

[NewBostonConst]

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.

[SunEagle]

I'm confused by the comment about not running parallel wires. Why?

By my way of thinking, more copper is always better and will result in less voltage drop. Even if one wire is skinny and one is fat, they will still carry current in the same direction. If the fat wire was enough, the skinny additional wire will give safety margin.

Current capacity is roughly set by the amount of copper. Surface area to dissipate heat is another variable, but if that is the constraint, two thin wires may actually be better than one fat one because they have more surface area and are spaced apart.

If you need #10 and have two #12 conductors in parallel, I think you will still be OK. They may be slightly different diameter or slightly different length, so they won't share the current precisely equally, but the current mismatch will be no worse than the wire mismatch. I=V/R where V is the voltage drop from one end to the other and R is the resistance of the wire under consideration.

In my work, we do wires in parallel of different size all the time, but that's in a different field, not high-power solar.

For power cabling both wires need to be exactly the same resistance or the current will take the path of least R and overheat the wires.

Parallel wiring is ok but it has to be exactly the same type and length along with being properly terminated.

[Ampster]

For power cabling both wires need to be exactly the same resistance or the current will take the path of least R and overheat the wires.

Parallel wiring is ok but it has to be exactly the same type and length along with being properly terminated.

Help me translate that into Ohm's law or physics. I think what you are saying is that if one wire has more resistance than the other wire with less resistance could overheat if the current exceeds the rating of that wire. I suppose one could calculate how much resistance it would take to force that current using I=V/R. My guess is if one used the same size wire of the same length and terminated them properly the current difference would be trivial and the risk of overheating one conductor would be reduced.
I admit my question to @Takis was a little snarky or seemed like baiting him to test his logic. I have more knowledge of your experience to be able to shed some light on this for the greater good. Apparently according to one source it is allowed under NEC for commercial. I am not asking for an endorsement.

[SunEagle]

Help me translate that into Ohm's law or physics. I think what you are saying is that if one wire has more resistance than the other wire with less resistance could overheat if the current exceeds the rating of that wire. I suppose one could calculate how much resistance it would take to force that current using I=V/R. My guess is if one used the same size wire of the same length and terminated them properly the current difference would be trivial and the risk of overheating one conductor would be reduced.
I admit my question to @Takis was a little snarky or seemed like baiting him to test his logic. I have more knowledge of your experience to be able to shed some light on this for the greater good. Apparently according to one source it is allowed under NEC for commercial. I am not asking for an endorsement.

2 identical wires in parallel should each carry 1/2 the current. But if one of those wires is weaker then the other it may carry more current and exceed it's rating.