solar interconnect via feedthrough lugs

[wwhitney]

electrical engineer didn't approve the method, so I could not use the lug kit method, he said the breaker in sub panel can be only 70A max. is this true? what if I use 400A load center with 200A main breaker.

The 2017 NEC (CEC) is silent on how to handle subfeed lugs. If you treat the MB of the panel supplied by the subfeed lugs as if it were a breaker in the panel (a reasonable approach, although not supported by any text in the NEC), then you are limited to a 70A breaker.

That's because the only way to qualify the 200A bus shown in the service panel is with the 100% rule, and 125A + 70A < 200A. I think your PV may only need a 110A breaker? (100000*2*1.25/240 = 104A) Then you could instead use a 110A breaker for the PV and a 90A breaker in the subpanel. [Or, since the subfeed lugs aren't gaining you anything in this interpretation, a 90A breaker on the service panel bus.]

But it seems to me it would be much simpler to interconnect the PV via intercepting the 200A feeder to the subpanel not shown, leaving the bus and breakers in the service panel unchanged. The feeder interconnection rules are more flexible.

Cheers, Wayne

 

[Designer101]

The 2017 NEC (CEC) is silent on how to handle subfeed lugs. If you treat the MB of the panel supplied by the subfeed lugs as if it were a breaker in the panel (a reasonable approach, although not supported by any text in the NEC), then you are limited to a 70A breaker.

That's because the only way to qualify the 200A bus shown in the service panel is with the 100% rule, and 125A + 70A < 200A. I think your PV may only need a 110A breaker? (100000*2*1.25/240 = 104A) Then you could instead use a 110A breaker for the PV and a 90A breaker in the subpanel. [Or, since the subfeed lugs aren't gaining you anything in this interpretation, a 90A breaker on the service panel bus.]

But it seems to me it would be much simpler to interconnect the PV via intercepting the 200A feeder to the subpanel not shown, leaving the bus and breakers in the service panel unchanged. The feeder interconnection rules are more flexible.

Cheers, Wayne

we cannot install 90A breaker there as it will not be enough for the whole house as we relocated all the loads there.

yes intercepting 200A breaker was my first idea, Plan reviewer didn't understand that approach, trust me he was nuts. we later used two 7600 inverter and connected each one of them one in sub panel and one in main bus bar of the lower bus bar. we already installed this project last year and passed.

I was asking this question as i am still confused about the word in NEC 2020 ABOUT FEED THROUGH LUGS, DOES IT MEANS THAT WE NEED TO HAVE ONE OCPD JUST ABOVE THE FEED THROUGH CONDUCTORS IN THE MAIN PANEL, AND HOW DOES THAT MAKE DIFFERENCE WITH 70 A limitation??

 

[wwhitney]

I was asking this question as i am still confused about the word in NEC 2020 ABOUT FEED THROUGH LUGS, DOES IT MEANS THAT WE NEED TO HAVE ONE OCPD JUST ABOVE THE FEED THROUGH CONDUCTORS IN THE MAIN PANEL, AND HOW DOES THAT MAKE DIFFERENCE WITH 70 A limitation??

It would make no difference on the 70A limitation, but the (2020) 705.12(B)(3)(6) requirements would have to be met in order to allow any PV supply to that service panel busbar. The 2020 wording is confusing, but it certainly doesn't refer to the location of OCPD on the bus side of the feed-through lugs. It would be OCPD on the feed through conductors.

Rather than try to further parse that language, I think it makes more sense to look at the version in the 2023 First Draft version, which is at least a bit clearer [comments like this are mine to correlate the section numbers between 2023 and 2020]:

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) [formerly 705.12(B)(1)] 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 705.12(B)(1) through 705.12(B)(3) [formerly 705.12(B)(3)(1) through (3)].

Still not very clear, as a literalist reading says that the first sentence allows connections without restriction, rendering the last sentence moot.

Cheers, Wayne

 

[Designer101]

It would make no difference on the 70A limitation, but the (2020) 705.12(B)(3)(6) requirements would have to be met in order to allow any PV supply to that service panel busbar. The 2020 wording is confusing, but it certainly doesn't refer to the location of OCPD on the bus side of the feed-through lugs. It would be OCPD on the feed through conductors.

Rather than try to further parse that language, I think it makes more sense to look at the version in the 2023 First Draft version, which is at least a bit clearer [comments like this are mine to correlate the section numbers between 2023 and 2020]:



Still not very clear, as a literalist reading says that the first sentence allows connections without restriction, rendering the last sentence moot.

Cheers, Wayne

wayne, one question here,
suppose hypothetically if we use 400A sub panel connected via feed through lugs protected by 200A main breaker( relocate all house loads in this 400A main panel)and connect solar PV in the sub panel instead of main panel.
then in the 400 A sub panel we can connect up to 400*1.2-200=280A Solar ( though meter sockets might be rated only 200A, so we are limited to 200A Only)
is this valid , i think so because the main panel will not have any Load breakers and can be treated as same as service entrance and its has 200 bus bar to allow any source current in opposite direction from grid ??

 

[ggunn]

wayne, one question here,
suppose hypothetically if we use 400A sub panel connected via feed through lugs protected by 200A main breaker( relocate all house loads in this 400A main panel)and connect solar PV in the sub panel instead of main panel.
then in the 400 A sub panel we can connect up to 400*1.2-200=280A Solar ( though meter sockets might be rated only 200A, so we are limited to 200A Only)
is this valid , i think so because the main panel will not have any Load breakers and can be treated as same as service entrance and its has 200 bus bar to allow any source current in opposite direction from grid ??

No. 705.12 still must be observed in the main panel. You are limited to 40A maximum of inverter current X 1.25 into a 200A main panel with a 200A MCB no matter how it gets onto the other end of the bus. Whether it's a backfed breaker in the main, a tap on the feedthrough conductors, or fed into a subpanel is irrelevant.

 

[Designer101]

No. 705.12 still must be observed in the main panel. You are limited to 40A maximum of inverter current X 1.25 into a 200A main panel with a 200A MCB no matter how it gets onto the other end of the bus. Whether it's a backfed breaker in the main, a tap on the feedthrough conductors, or fed into a subpanel is irrelevant.

I understand that but the confusion lies here is suppose I replace the main panel with only the disconnect only panel(200A disconnect) then it would work. (400*1.2-200) or 120 percent rule. ( you can even call this load side tap size wires per tap rule???)
now back to original why cannot the Main panel be treated as the disconnect only Panel after removing all the loads (after all its the same basics)
isnt it? i hope you get what i am trying to say.

 

[wwhitney]

If you remove all breakers from the main panel except for a feeder breaker (or feed thru lugs), then the main panel qualifies under the "sum of all breakers" rule. So you can interconnect up to 100% of the busbar rating (or main breaker rating if it is lower).

Instead of using a single downstream 400A panel, you have the option to use two smaller panels, a 200A main breaker panel for the loads, and a generation panel. The solar interconnection then occurs somewhere along the 200A feeder from the main panel to the load panel, using the feeder rules.

Cheers, Wayne

 

[Designer101]

If you remove all breakers from the main panel except for a feeder breaker (or feed thru lugs), then the main panel qualifies under the "sum of all breakers" rule. So you can interconnect up to 100% of the busbar rating (or main breaker rating if it is lower).

Instead of using a single downstream 400A panel, you have the option to use two smaller panels, a 200A main breaker panel for the loads, and a generation panel. The solar interconnection then occurs somewhere along the 200A feeder from the main panel to the load panel, using the feeder rules.

Cheers, Wayne

First option'
ya we can do that but the load breakers will be limited on to the feed though sub panel.
, i already posted that situation earlier, so this option is very limited.

second option.
if i use feeder tap options using two small sub panel it will comply to the lower portion but how about upper portion on the main Panel how is that compliant??this is where whole my confusion lies.

 

[wwhitney]

First option

Sorry, isn't that just what you proposed last night, "relocate all house loads in this 400A main panel"? I'm agreeing that if you do that, the service panel is fine.

And as to what you called my second option, that was only in the context of already having emptied the service panel. Meaning instead of a single 400A subpanel, using (2) panels with the feeder interconnection.

Cheers, Wayne