What does NEC say about parallel PV feeder cables?

4 Posts
Aug 31, 2012 12:50 pm
What does NEC say about parallel PV feeder cables?

I'm installing 14 290W Suntech PV panels 200' from the equipment room. The panels will be wired as seven series-pairs, and all seven pairs will connect to a common point inside a Midnight combiner box.

Due to the distance, I'm using individual #2 type XLP-UF aluminum wires from the PV array to the Outback load center in the house.

Sadly, due to the fact that the wire is aluminum, the power loss through the #2 wires will be 10% -- too high for me.

So I want to use TWO #2 wires in parallel for each feeder (two for positive and two for negative), to cut the resistance and the power loss in half.

Can someone tell me if the code allows this? When I asked Outback tech support, I was told that I can't use parallel wires (and that I need a combiner box, which told me he probably was confused).

All I want to do is to connect TWO wires to the combiner box's positive TBB and TWO wires to the negative TBB, then connect the other ends each pair to the same positive and negative TBBs inside the load center in the house. According to the aluminum wire table, this gives me the same resistance as 2/0 wire.

NOTE: The aluminum ampacity table shows that ONE #2 wire is large enough for the 53 amps of current my PV array can produce, so I'm not trying to parallel the wires to handle more current -- only to reduce power loss in the cables.

Thanks in advance for any definitive information.

1 Posts
Aug 31, 2012 01:47 pm
Re: What does NEC say about parallel PV feeder cables?

Hi Mike,

From what I recall in EE classes there's nothing in Ohm's law that differentiates between a single multi stranded wire and multiple single stranded wires. From a theoretical standpoint it's all about resistance and/or impedance.

However the real question is whether the wire or the Electrical Inspector would provide the most resistance. As always we suggest that you check with the "authority having jurisdiction" where you live. If you have a professional electrician doing the work and the local authorities will sign off on it knowing how you implemented the system then you're good to go.

If not there's a good chance they'll have suggestions for alternate approaches. We had the best building inspector (until he recently retired) in the town I live in. Not only did he know his stuff but he was incredibly patient and helpful even to DIYers. He never lost sight of the fact that he worked for the taxpayers of the town and that his job was to make sure that they were safe. So he was always willing to come out and inspect things during construction since he always said it was better to get it right from the beginning than to tell someone they did it wrong when they finished.

Try contacting your inspector. You may be surprised at how accommodating many of them will be....

- Jay

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