10/2 MC armorlite in attic DC from roof box to disconnect -- attic temp derating?

[x98myers7]

hi everyone,

so, working on continued planning and trying to nail down wiring from my roof junction box to PV disconnect near the inverter. i'm in michigan.

NEC approved flexible MC armorlite a while ago for attic use. yay -- https://www.solaredge.com/sites/default/files//dc_pv_output_circuits_inside_building.pdf

i have a 2kw array with 8 of these 250w newpowa -- https://www.newpowa.com/products/250w-monocrystalline-solar-panel

I'm running two strings in parallel with 4 panels in series. 8 panels total. ambient temperature correction factor using 1.25 results in a theoretical....
max amps combined parallel= 27.725a
max voc= 136.4v

I'll be able to mount a wet rated junction box on a roof hip, where I'll mount a DIN rail terminal block to connect PV cable from array through conduit to 10/2 MC -- https://smile.amazon.com/Dinkle-Assembly-Jumpers-Terminal-Blocks/dp/B07TN3RLYZ/
i'll run 10/2 MC armorlite from terminal block through attic and walls down to PV DC disconnect in the basement power room.

awesome, so, 27.725 is below standard 30amp MC4 connector rating as well as 10/2 MC armorlite at all the big box stores.

but wait... what happens with attic temperature derating for the 10/2 during the summer? how do i calculate and account for that? in michigan summer we can get pretty hot, so I'm sure attic temps get up to 140degF.

 

[ncsolarelectric]

I'm glad you asked!

 

[x98myers7]

YES! thank you.

 

[x98myers7]

okay... so the reality is.... it's complicated, haha....

for reference...

FSEC-PF-336-98

Information on the renewable energy and energy efficiency research, education, training, and certification activities of the Florida Solar Energy Center (FSEC)

www.fsec.ucf.edu

realistically, then... I have both soffit and ridge vents so I should be seeing a max ambient temp of 112+ 22= 134degF. hottest ever for MI was 112degF.

soooo I only get 0.71..... which means I can only run 21amps through it. sigh. need 8ga (40amp at 90C), and even that I can only run 28amps through.

 

[ncsolarelectric]

YES! thank you.

NEC 2017

 

[x98myers7]

why is 10/2 the "standard" if I can only run 21amps through it? are people just ignoring temp correction?

 

[x98myers7]

wait.... interesting... 8/2 MC here is rated 55amps at 90C -- https://www.wireandcableyourway.com...rmored-metal-clad-cable-250ft-or-1000ft-spool

 

[Bluedog225]

Crazy temps down here. Good to keep this in mind. Probably need to bump up my planning temp to 114-122 eff.

If I need to jump to the next category, I’m leaving.

 

[ncsolarelectric]

why is 10/2 the "standard" if I can only run 21amps through it? are people just ignoring temp correction?

#10 AWG PV Wire is rated 40A. 0.71 x 40 = 28.4. MC wire may be different from PV wire. NEC requires you to use PV wire on the DC side, and metal conduit inside a building. You can use EMT and PV wire.

 

[x98myers7]

I’m talking 10/2 mc not PV wire. I’m not going to run PV through the house. Mc wire is way more convenient and can go through walls


I can’t run DC through 10/2 Mc to the inverter? I don’t think that’s right… I cited its allowance in the op. I’m not running micro inverters. I’m taking DC all the way to a growatt 24v 3k in the basement.

 

[ncsolarelectric]

I’m talking 10/2 mc not PV wire. I’m not going to run PV through the house. Mc wire is way more convenient and can go through walls


I can’t run DC through 10/2 Mc to the inverter? I don’t think that’s right… I cited its allowance in the op. I’m not running micro inverters. I’m taking DC all the way to a growatt 24v 3k in the basement.

NEC 690.31(A) requires the wire within the array to the first junction box or combiner box to be type USE-2 or PV Wire. Outside of the array, you can change to metal jacketed MC cable, but the issue was your attic temperature. The PV wire in EMT conduit rated at 40A can meet the temperature derating requirements, the MC cable rated 30A cannot. It seems clear to me that you can transition to MC cable after the wire leaves the hot attic space, not before.

 

[A.Justice]

Would 200c silicone or ptfe wire be beneficial in a hot attic?

 

[x98myers7]

NEC 690.31(A) requires the wire within the array to the first junction box or combiner box to be type USE-2 or PV Wire. Outside of the array, you can change to metal jacketed MC cable, but the issue was your attic temperature. The PV wire in EMT conduit rated at 40A can meet the temperature derating requirements, the MC cable rated 30A cannot. It seems clear to me that you can transition to MC cable after the wire leaves the hot attic space, not before.

Got it, yes. I’ll run 8/2 mc instead.

 

[ChrisG]

@x98myers7 Did you ever get this run. I'm adding a west facing array and plan to run Array to Disconnect as PVWire then transition to thhn in PVC conduit outside underground to house then plan on using MC amorlite 10/2 in crawlspace (doesn't get above 100F in crawlspace) to the inverter.

 

[x98myers7]

yep, sure did @ChrisG -- worked exactly as expected. i posted it all in my show-and-tell thread.