Solid or Stranded, THWN or Romex (for AC arrays with Microinverters)

[dapug]

As you know, Enphase uses a 2 conductor #12. My design spec says to use #12 THWN to run the lines to the Enphase Combiner. BUT... there is some confusion on the topic...

Why is THWN required when running AC in the attic? Is it not allowed to be exposed in the Soladeck JB where it ties into the Enphase Q cable?

Do you need conduit in the attic?
I assume yes if individual strands of THWN
I assume no if using Romex is ok, just use wire staples.

My understanding is Romex cannot be used in outside conduit (PVC and LFNC) (areas where not running through the attic), so that part makes sense.

Stranded or Solid?
I've heard solid for AC, but stranded for DC. Most THWN by solar suppliers is stranded, but they assume DC. Mine is ALL AC all the way, so... which to use?
For AC I heard solid or stranded is personal preference. Why would one prefer stranded where rigidity isn't a concern?

#10 or #12 from Soladeck to Combiner box?
I've seen others in these forums say to use #10. Why? Enphase itself is #12. I'm only going about 40ft from the array to the combiner box, is #12 not sufficient?
my design spec doesn't call for anything heavier than #12 to a 20A breaker on any given array

[Mike90250]

it's not about solid or stranded for AC or DC, it's what are the connectors on components rated for ?

Solid is stiffer and harder to pull thru conduit. Stranded is more flexible

Larger wire has less loss ( less voltage drop) for longer runs.

[bob-n]

Mike is right, in all regards. He often is.

THWN is not required in the attic. But it is a very good choice. You need to put the wire in conduit and label the conduit. It is hard to pull romex through conduit. THWN is made for pulling through conduit.

There is no code requirement for solid or stranded. Stranded is slightly more expensive and much easier to pull through conduit. That's why most people use stranded.

Note that Home Depot and Lowe's sell THWN, and may be cheaper than solar suppliers.

For a 12A breaker, code allows AWG 12. You could use fatter wire and get more power from your solar array. It's up to you whether it is worth the extra money for more copper.

For a 40 foot run, two conductors, AWG 12, at 15A, you will lose 28.6 watts in the wire.
For a 40 foot run, two conductors, AWG 10, at 15A, you will lose 18.0 watts in the wire.

I hope this helps.

[dapug]

Love it, super helpful, thank you both!