panel receiving voltage from solar array.

Status
Not open for further replies.
D

djd

Senior Member
If you have a 80 amp two pole breaker fed from the inverter , does the main breaker for that panel have to be sized in accordance of nec ,, ie 150 main on a 400 amp panel ?
 
djd said:
If you have a 80 amp two pole breaker fed from the inverter , does the main breaker for that panel have to be sized in accordance of nec ,, ie 150 main on a 400 amp panel ?
Click to expand...

Where are you getting 150A and 400A from? Show us your thought process. What code rules and calculations do you have in mind, that you are asking about?
 
The electrician put a 150 main in a 400 amp residential panel , the breaker for the solar array is 80 amps . his labeling for the solar states that 150 is the max size for the main . This may be code compliant but why the 400 amp , panel and that does not leave much left for the 3500 sq. foot addition . It`s a second service for this property , they tapped of a closer sdge transformer " so cal " because of distance . I just wanna find this in the code , it does not make sense to me the rest is a long story .
 
It appears based on 705.12(B)(2)(3)(b) that the main can be 400 amps, assuming that the 80 amps is equal to or greater than 125% of inverter output circuit.
 
djd said:
The electrician put a 150 main in a 400 amp residential panel , the breaker for the solar array is 80 amps . his labeling for the solar states that 150 is the max size for the main . This may be code compliant but why the 400 amp , panel and that does not leave much left for the 3500 sq. foot addition . It`s a second service for this property , they tapped of a closer sdge transformer " so cal " because of distance . I just wanna find this in the code , it does not make sense to me the rest is a long story .
Click to expand...
Ok, the rule in question is the famous 120% rule for solar. Its exact code reference has changed over the cycles of the NEC, but the substance has been constant since NEC2014. In general, the rules for interconnecting solar with other sources are in section 705.12. In the 2020 NEC, the code reference is NEC705.12(B)(2)(3)(b). This situation sounds like a misunderstanding of these rules. Perhaps the design specified a 225A busbar, and the electrician ended up installing a 400A busbar, not realizing that increasing the busbar size will increase the capacity for either the main, the PV, or mixture of both.

If you feed a panelboard at the opposite end of the main supply, which is the most logical solution, considering Kirchhoff's current law, you are permitted to use the 120% rule. If instead, you feed it from somewhere other than the opposite end, 120% becomes 100%, and NEC705.12(B)(2)(3)(a) applies instead.

To understand the rule as an equation, assign the following variable names:
M = main breaker rating
B = busbar ampacity
P = full load amps of the PV system (i.e. sum total of inverter currents), prior to applying the 125% factor that sizes a breaker.

When the 120% rule applies, this is your equation:
M + 1.25*P <= 1.20*B

Given P=64A (for an 80A breaker), and B = 400A, this means M can be as large as 400A.

When you don't have the convenience of using the opposite end of the busbar, given the same numbers, this would mean M would be limited to 320A. Since a 320A breaker doesn't exist, this would mean you'd limit the main breaker rating to 300A. The governing equation, would then be M+1.25*P<=1.00*B.

Neither of these conditions limit your main breaker to 150A on a 400A busbar, given an 80A breaker for the PV interconnection.
 
Can you describe this 400 amp residential panel. Is there a single main breaker for everything or does this panel have 2 service disconnects?
 
djd said:
If you have a 80 amp two pole breaker fed from the inverter , does the main breaker for that panel have to be sized in accordance of nec ,, ie 150 main on a 400 amp panel ?
Click to expand...
Yes, the panel MCB and PV breaker (or actually 125% of the inverter max current unless the AHJ is on an old code cycle) have to be sized according to the NEC 705.12, but missing from what you have posted is the rating of the busbar. Reducing a 400A main breaker to 150A to accommodate an 80A backfed PV breaker is almost certainly in error.
 
I hit post earlier before finishing typing.

If this is the type of 400 amp panel that has (2) 200 amp disconnects, with one feeding the internal load center and a second for a remote feeder the down sized breaker makes sense and I have done the same. The interior panel is only rated for 200 amps. If there is a 80 amp PV breaker the supply breaker would need to be 160 amps or less.
 
curt swartz said:
I hit post earlier before finishing typing.

If this is the type of 400 amp panel that has (2) 200 amp disconnects, with one feeding the internal load center and a second for a remote feeder the down sized breaker makes sense and I have done the same. The interior panel is only rated for 200 amps. If there is a 80 amp PV breaker the supply breaker would need to be 160 amps or less.
Click to expand...
I still don't know exactly what you have, exactly, but load breakers do not figure into most of 705.12.
 
curt swartz said:
Can you describe this 400 amp residential panel. Is there a single main breaker for everything or does this panel have 2 service disconnects?
Click to expand...
Singl;e main breaker . love to tell you more but really want to focus on the solar part for now. This friend put a independent service on a 3500 sq. ft addition . put 30 or solar panels on the roof.
 
ggunn said:
I still don't know exactly what you have, exactly, but load breakers do not figure into most of 705.12.
Click to expand...
What is here I wish I took pictures , im not the best writer. A new service independent of his original service on his house. IT is a 400 amp service , with a main breaker 150 amps. His solar breaker is 80 amp two pole . 30 solar panels on roof . His info for the solar labeling on the panel state that the 150 amp main breaker cannot be increased . seems odd to me because wouldn`t the 80 amp solar breaker cover any overload ? Monday I`ll click a picture . If you really have to do this why the 400 amp panel , why not just 200 hundred . I can`t believe the solar company would do this unnecessarily.
 
djd said:
What is here I wish I took pictures , im not the best writer. A new service independent of his original service on his house. IT is a 400 amp service , with a main breaker 150 amps. His solar breaker is 80 amp two pole . 30 solar panels on roof . His info for the solar labeling on the panel state that the 150 amp main breaker cannot be increased . seems odd to me because wouldn`t the 80 amp solar breaker cover any overload ? Monday I`ll click a picture . If you really have to do this why the 400 amp panel , why not just 200 hundred . I can`t believe the solar company would do this unnecessarily.
Click to expand...
wwhitney said:
We need a proper single line diagram to further comment on the install.

Cheers, Wayne
Click to expand...
send pic monday .
 
curt swartz said:
I hit post earlier before finishing typing.

If this is the type of 400 amp panel that has (2) 200 amp disconnects, with one feeding the internal load center and a second for a remote feeder the down sized breaker makes sense and I have done the same. The interior panel is only rated for 200 amps. If there is a 80 amp PV breaker the supply breaker would need to be 160 amps or less.
Click to expand...
typical 400 amp 30 circuit , where in the code , and why ?
 
djd said:
Singl;e main breaker . love to tell you more but really want to focus on the solar part for now. This friend put a independent service on a 3500 sq. ft addition . put 30 or solar panels on the roof.
Click to expand...
There is no information there that can enable us to help you. The square footage of the addition is irrelevant and the number of solar modules is nearly so.
djd said:
What is here I wish I took pictures , im not the best writer. A new service independent of his original service on his house. IT is a 400 amp service , with a main breaker 150 amps. His solar breaker is 80 amp two pole . 30 solar panels on roof . His info for the solar labeling on the panel state that the 150 amp main breaker cannot be increased . seems odd to me because wouldn`t the 80 amp solar breaker cover any overload ? Monday I`ll click a picture . If you really have to do this why the 400 amp panel , why not just 200 hundred . I can`t believe the solar company would do this unnecessarily.
Click to expand...
If the MDP has 400A busbars, and the PV backfed breaker is correctly sized and positioned at the opposite end of the busbar from the main breaker, and there are no other electrical sources in the panel, the main breaker does not need to be downsized. If load breakers are being considered in the calculation, that is in error.
 
Carultch said:
Ok, the rule in question is the famous 120% rule for solar. Its exact code reference has changed over the cycles of the NEC, but the substance has been constant since NEC2014. In general, the rules for interconnecting solar with other sources are in section 705.12. In the 2020 NEC, the code reference is NEC705.12(B)(2)(3)(b). This situation sounds like a misunderstanding of these rules. Perhaps the design specified a 225A busbar, and the electrician ended up installing a 400A busbar, not realizing that increasing the busbar size will increase the capacity for either the main, the PV, or mixture of both.

If you feed a panelboard at the opposite end of the main supply, which is the most logical solution, considering Kirchhoff's current law, you are permitted to use the 120% rule. If instead, you feed it from somewhere other than the opposite end, 120% becomes 100%, and NEC705.12(B)(2)(3)(a) applies instead.

To understand the rule as an equation, assign the following variable names:
M = main breaker rating
B = busbar ampacity
P = full load amps of the PV system (i.e. sum total of inverter currents), prior to applying the 125% factor that sizes a breaker.

When the 120% rule applies, this is your equation:
M + 1.25*P <= 1.20*B

Given P=64A (for an 80A breaker), and B = 400A, this means M can be as large as 400A.

When you don't have the convenience of using the opposite end of the busbar, given the same numbers, this would mean M would be limited to 320A. Since a 320A breaker doesn't exist, this would mean you'd limit the main breaker rating to 300A. The governing equation, would then be M+1.25*P<=1.00*B.

Neither of these conditions limit your main breaker to 150A on a 400A busbar, given an 80A breaker for the PV interconnection.
Click to expand...
Thank you for your feedback , I`ll send a pic on monday so you can see the whole situation .
 
don_resqcapt19 said:
It appears based on 705.12(B)(2)(3)(b) that the main can be 400 amps, assuming that the 80 amps is equal to or greater than 125% of inverter output circuit.
Click to expand...
My main question was sizing of the main , 150 when could gone to 200 amps couldn't find in code book ,I have 2020 .
don_resqcapt19 said:
It appears based on 705.12(B)(2)(3)(b) that the main can be 400 amps, assuming that the 80 amps is equal to or greater than 125% of inverter output circuit.
Click to expand...
The largest main for this panel is 200 , I just didn't understand why you have limit the main because of solar .
 
djd said:
My main question was sizing of the main , 150 when could gone to 200 amps couldn't find in code book ,I have 2020 .

The largest main for this panel is 200 , I just didn't understand why you have limit the main because of solar .
Click to expand...
Generally speaking, the reason is that you have to limit the total amount of current to which a busbar can be exposed from all sources. Again generally speaking, you must satisfy NEC 705.12(B) or (D), depending on your code cycle, in all panels between the point of interconnection and the service, but specifically, I don't know what you have without seeing an electrical diagram of the service and the PV system.
 
Status
Not open for further replies.
Top