PV or THHN

[badwithusernames]

I am installing three strings on my barn roof, which will be feeding 2 inverters in my house which is about 150' away. The run from barn to house is buried PVC40 conduit. Two of these strings will be combined to feed one inverter, though in the future I may want to separate them (different inverter with more MPPT). Originally the plan was for a combiner in the barn (for the two strings) and THHN from the combiner to the inverter. If I eliminate the combiner box in the barn, I can run each string straight to the DC disconnect at the inverter. The disconnect can accommodate 2 strings. I assume PV wire should be used all the way?
Thanks. SD

[solarix]

PV wire is at least twice the cost of THHN and PV wire is not smoke rated so not allowed indoors. Use a j-box under the array and run a 10ga THHN pair for each string.

[badwithusernames]

PV wire is not smoke rated so not allowed indoors.

There is the bit of info I was not able to find. I will make the transition on the roof and run THHN between the buildings. Thank you.

[solarix]

BTW, use EMT conduit above ground on DC solar circuits.

[badwithusernames]

BTW, use EMT conduit above ground on DC solar circuits.

I have been told this, and planned to. Is this due to UV deterioration, grounding, thermal expansion, or something else?

[solarix]

I think its because the code is afraid of the evil DC solar power and wants to guard against cutting into it by mistake.

[bob-n]

Common THHN is rated for 600V. Make sure that your strings are lower voltage than the rating of the THHN you are planning to use.

[foo1bar]

PV wire is at least twice the cost of THHN and PV wire is not smoke rated so not allowed indoors. Use a j-box under the array and run a 10ga THHN pair for each string.

At least some PV wire is rated under multiple listings and I think can be used within a conduit indoors using one of the other ratings.

But the cost thing is bigger argument to go with a cheaper wire - especially at 150' length.

Also, I'd consider putting the inverter in the barn and running (cheaper) aluminum wire back to the main panel. Also you would have to do a little more calculation to see how much power is lost in those runs to see what the right tradeoff is for size of wires vs. cost. And on the plus side, it's probably a good time to look at doing a subpanel for AC use in that barn for other purposes along with the solar backfeed.

Aluminum wire does have some bad history - especially when it was used at smaller gauges (ie. 10 AWG and 12AWG). But properly terminated AL wiring is as safe as copper. It doesn't take that much effort to put some noalox on the wires, make sure the connector is AL rated, and properly torque the connector.

[inetdog]

Make sure that your THHN is actually multiply rated as THWN or THWN2 with the W for water resistance if it is going to be used outdoors, even in conduit.

[emartin00]

I have been told this, and planned to. Is this due to UV deterioration, grounding, thermal expansion, or something else?

On the roof it is heat related. PVC conduit will start to sag due to the high temperatures seen on the roof.

[badwithusernames]

At least some PV wire is rated under multiple listings and I think can be used within a conduit indoors using one of the other ratings.

But the cost thing is bigger argument to go with a cheaper wire - especially at 150' length.

Also, I'd consider putting the inverter in the barn and running (cheaper) aluminum wire back to the main panel. Also you would have to do a little more calculation to see how much power is lost in those runs to see what the right tradeoff is for size of wires vs. cost. And on the plus side, it's probably a good time to look at doing a subpanel for AC use in that barn for other purposes along with the solar backfeed.

Aluminum wire does have some bad history - especially when it was used at smaller gauges (ie. 10 AWG and 12AWG). But properly terminated AL wiring is as safe as copper. It doesn't take that much effort to put some noalox on the wires, make sure the connector is AL rated, and properly torque the connector.

No doubt there are better ways to do this, but given a few hurdles from the local utility and township permit department, I am choosing the path of least resistance. Ha.
The barn WOULD be the perfect location for the inverters, but the sub panel is only 100A so not large enough to feed with this system. Even if I could, the AC disconnect would still have to be on the house next to the meter. My current design is for 2 smaller inverters with a single MPPT each, because it's what I can afford. Depending on how this works out, we may upgrade later to an inverter with multiple MPPT, and add some panels to the house, so having the inverter in the basement isn't really a bad thing. Digging the trench and pulling wire is pretty simple work for me, so I'm going with it.
Aluminum wire creeps me out, I'll happily pay up for the correct copper conductors. Just checked w my spuule house, the cost for 10awg THWN and THHN are the same, PV wire is not even 30% more.
SD

[foo1bar]

Aluminum wire creeps me out, I'll happily pay up for the correct copper conductors.

Really aluminum wiring is safe and very cost effective for larger ampacity wires.

There was some use of aluminum wire in the 70's where it was getting installed in situations that it wasn't appropriate for.
It was used on every light switch, outlet, etc - with no use of anti-oxidizing paste. And that lead to bad results.
Also the current aluminum wires you buy are a better alloy for the purpose than the alloys used in the 70s.

Aluminum gets used for a *lot* of 50A-200A feeder wires now. With proper connectors, proper torquing, and anti-ox paste I think it's as good as copper and a *lot* cheaper.

ex: 4 AWG aluminum for $.26/ft vs. 6 AWG copper for $.57/ft. (same ampacity - but less than half the price.)
Those are actual current prices - and just selected from a single website. But I think they're typical.


It sounds like you have 100A subpanel in the barn.
So if you'd want to increase that to a larger panel anyhow, now may be the time to do it. Doing it as part of your solar installation means that it would qualify as an expense related to your solar installation and therefore part of what you could use for the tax break.