Micro Inverters

[Canton]

I have installation with (30) PV panels on the roof. This installation is under NEC 2014. Originaly there was going to be a String-Inverter installed but the client has opted for Micro-Inverters due to shading, avoiding the Rapid Shutdown requirement and some other things. This is the first installation I have done with Micro-Inverters and a few things are standing out at me.

There are (15) micro-inverters installed for the (30) panels (2 pv panels per micro-inverter) and they have been divided up into (3) strings. These (3) strings of micro-inverters have a maximum output current of 11 amps each and the trunk cable is #12. All (3) strings of trunk cable go to my combiner box on the roof and are then combined onto #8 THHN (33 amps max AC output current, 40 amp OCPD) that is piped down the side of the house to a AC fused disconnect on the ground (40 amp) and then into the house panel onto a 40 amp backfed breaker.

The questions I have are:

Do I need an OCPD in the combiner box (20 amp in-line fuses) on the #12 trunk cable before it is spliced onto the #8 THHN?

I thought all Fuses had to be readily accessible, can I put them in the combiner box on the roof? (I thought I read an exception at one time)

Should I bring the (3) sets of #12's down and put (3) 20 amp backfed breakers in the Panel (400 amp panel, will not violate 120% rule) or should I combine them in combiner box next to the disconnect on ground level with OCPD?

Thanks for any advise

 

[jaggedben]

...that is piped down the side of the house to a AC fused disconnect on the ground (40 amp) and then into the house panel onto a 40 amp backfed breaker.

You don't need both the fused disco and the 40A breaker. You could go with an unfused disco, or perhaps no disco at all if not required by the AHJ or utility.

[/QUOTE]The questions I have are:

Do I need an OCPD in the combiner box (20 amp in-line fuses) on the #12 trunk cable before it is spliced onto the #8 THHN?[/QUOTE]

Yes. You cannot protect the #12 wire with a 40A OCPD. Consult the trunk cable manufacturer's info for the max OCPD size and max number of inverters on the cable. Probably you are okay with 20A.

[/QUOTE]I thought all Fuses had to be readily accessible, can I put them in the combiner box on the roof?[/QUOTE]

The do not have to be readily accessible if they are adjacent to the equipment they supply. See 240.24(A)(4).

Should I bring the (3) sets of #12's down and put (3) 20 amp backfed breakers in the Panel (400 amp panel, will not violate 120% rule)

Make sure your AHJ or utility doesn't demand a single disconnect for the whole system. But otherwise I would probably bring them all to the panel, just because it simplifies things. Sourcing proper components for a rooftop combiner might be more of a pain than it's worth. Keep in mind that roof level temperatures are much higher than normal, so your AHJ may demand that you use OCPDs that are approved for use at those temperatures.

On the other hand....

I've put a load center on the roof when I could put on the north side of a tilt-up array, such that it could be upright like a normal outdoor loadcenter, and not have to meet those concerns. And we usually run #10 down from the roof, mainly for reasons of voltage drop/rise, which means that combining to #8 would possibly allow a smaller conduit size down from the roof, which you might deem to be worth it.

or should I combine them in combiner box next to the disconnect on ground level with OCPD?

That's another option. For me it would probably depend on the details, such as the length of the runs and the locations of the various equipment.

 

[jaggedben]

Sorry for bad formatting, this should be more readable...

The questions I have are:

Do I need an OCPD in the combiner box (20 amp in-line fuses) on the #12 trunk cable before it is spliced onto the #8 THHN?

Yes. You cannot protect the #12 wire with a 40A OCPD. Consult the trunk cable manufacturer's info for the max OCPD size and max number of inverters on the cable. Probably you are okay with 20A.

I thought all Fuses had to be readily accessible, can I put them in the combiner box on the roof?

The do not have to be readily accessible if they are adjacent to the equipment they supply. See 240.24(A)(4).

 

[Canton]

Sorry for bad formatting, this should be more readable...



Yes. You cannot protect the #12 wire with a 40A OCPD. Consult the trunk cable manufacturer's info for the max OCPD size and max number of inverters on the cable. Probably you are okay with 20A.



The do not have to be readily accessible if they are adjacent to the equipment they supply. See 240.24(A)(4).

I will put the 20 amp 0CPD in the combiner box. Thank you for the code reference, I could not find it.