Backfeeding solar in a Main Panel with feeders at bottom

[RickwithSisuSolar]

Hi, if I had a 200A main panel that has feeders located at the bottom that go to a 150A service panel, is it still possible to use the 120% rule if I was to backfeed a 40A solar breaker at the bottom slot of the 200A main panel?

 

[ggunn]

Hi, if I had a 200A main panel that has feeders located at the bottom that go to a 150A service panel, is it still possible to use the 120% rule if I was to backfeed a 40A solar breaker at the bottom slot of the 200A main panel?

In most jurisdictions I work in, yes, as long as the subpanel fed by the feedthough lugs has a main breaker. Since the MCB is 200A and the sub is only 150A, I assume that it does.

 

[jaggedben]

In my opinion, no, you can't use the 120% rule with a 40A breaker because the breaker is not at the opposite end, the lugs are.

Options:
1) Tap the feeder to a fused disco for the solar.
2) Use another one of the busbar rules.
3) Install the breaker in the sub.
For options 1 and 2 you most likely need the main breaker in the sub as ggunn described.


The 2020 NEC has a section that addresses feed through lugs and is more restrictive than the ambiguities might allow in earlier cycles.

 

[ggunn]

The 2020 NEC has a section that addresses feed through lugs and is more restrictive than the ambiguities might allow in earlier cycles.

Can you point me to the pertinent article?

 

[wwhitney]

Can you point me to the pertinent article?

2020 705.12(B)(3)(6):

"Connections shall be permitted on busbars of panelboards that supply lugs connected to feed-through conductors. The feed-through conductors shall I be sized in accordance with 705.12(B)(1). Where an overcurrent device is installed at the supply end of the feed-through conductors, the busbar in the supplying panel board shall be permitted to be sized in accordance with 705.12(B)(3)(1) through 705.12(B)(3)(3)."

Cheers, Wayne

 

[ggunn]

2020 705.12(B)(3)(6):

"Connections shall be permitted on busbars of panelboards that supply lugs connected to feed-through conductors. The feed-through conductors shall I be sized in accordance with 705.12(B)(1). Where an overcurrent device is installed at the supply end of the feed-through conductors, the busbar in the supplying panel board shall be permitted to be sized in accordance with 705.12(B)(3)(1) through 705.12(B)(3)(3)."

Cheers, Wayne

So there needs to be OCPD on the feedthrough conductors where they leave the MDP?

 

[wwhitney]

So there needs to be OCPD on the feedthrough conductors where they leave the MDP?

Yeah, I don't know, as I find the text very ambiguous. Is "supply-end" with respect to the grid, or to the interconnected power source, or either? Also, as the last sentence is written in the permissive tone, there's nothing that actually seems to prohibit using (B)(3)(1)through (B)(3)(3) when the predicate of that sentence is not satisfied.

For reference, below is the 2023 First Draft version of the corresponding section, 705.12(B)(5). I find it to be clearer (although it still has the same problem with the permissive tone in the last sentence). What I can't tell is whether what it says is what the CMP thought they were saying in the 2020 NEC, but they messed up the wording the first time, or if this represents an intended substantive change.

Cheers, Wayne

"Connections shall be permitted on busbars of panelboards that supply lugs connected to feed-through conductors or are supplied by feed-through conductors. The feed-through conductors shall be sized in accordance with 705.12(A). Where an overcurrent device is installed at either end of the feed-through conductors, panelboard busbars on either side of the feed-through conductors shall be permitted to be sized in accordance with (1) through (3)."

 

[LarryFine]

So there needs to be OCPD on the feedthrough conductors where they leave the MDP?

I would think so anyway, if they're only rated for 150a, unless they're short enough for tap allowances

 

[jaggedben]

I would think so anyway, if they're only rated for 150a, unless they're short enough for tap allowances

See this recent thread:

Feeder conductors "becoming" tap conductors?

Soo, I designed this PV system with a load side tap after a 100A breaker at the meter main that's feeding a subpanel inside. Of course, a 100A breaker will be installed downstream at the subpanel. Basically we're tapping between two breakers, which should be totally fine as long as my PV...

forums.mikeholt.com

 

[ggunn]

Yeah, I don't know, as I find the text very ambiguous.

Whatever do you mean? It's written with all the clarity and comprehensibility that we have come to expect from the NEC.