120 percent rule for subpanel

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Spallelectric

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If I have a 200 amp main 225 amp bussing, with a 100 amp breaker feeding a 125 amp subpanel main lug only, with 60 amp backfeed, there is no main breaker inside of subpanel. is my subpanel restricted to only 50 amps of backfeed 120 percent rule. I’m thinking not because I have no main breaker in said subpanel. Solar is landed in subpanel
 
Spallelectric said:
If I have a 200 amp main 225 amp bussing, with a 100 amp breaker feeding a 125 amp subpanel main lug only, with 60 amp backfeed, there is no main breaker inside of subpanel. is my subpanel restricted to only 50 amps of backfeed 120 percent rule. I’m thinking not because I have no main breaker in said subpanel. Solar is landed in subpanel
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There is no difference between a main breaker in a subpanel and a load breaker in a main panel feeding a MLO subpanel. It's the breaker protecting the busbar of the subpanel in either case.

The busbar qualification requirements of 705.12 have to be satisfied in both the subpanel and the main panel. If what you are wanting to use is the 120% rule (it moves around in 705.12), then your solar feed needs to be at the opposite end of the busbar in the subpanel from the feeder, and your load breaker feeding the subpanel needs to be at the opposite end of the main panel busbar from the utility feed.

125% of your maximum inverter current plus the ratings of the breakers protecting the busbars must be less than or equal to 120% of the ratings of the busbars in both panels.
 
ggunn said:
There is no difference between a main breaker in a subpanel and a load breaker in a main panel feeding a MLO subpanel. It's the breaker protecting the busbar of the subpanel in either case.

The busbar qualification requirements of 705.12 have to be satisfied in both the subpanel and the main panel. If what you are wanting to use is the 120% rule (it moves around in 705.12), then your solar feed needs to be at the opposite end of the busbar in the subpanel from the feeder, and your load breaker feeding the subpanel needs to be at the opposite end of the main panel busbar from the utility feed.

125% of your maximum inverter current plus the ratings of the breakers protecting the busbars must be less than or equal to 120% of the ratings of the busbars in both panels.
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I’m talking about the 60 amp solar backfeed in subpanel 120% only allows 50 amp back feed.
 
Yes, a 125A subpanel fed by a 100A breaker cannot be backfed by more than 40A continuous inverter current under the 120% rule, meaning if the system needs a breaker larger than 50A it's too big. An easy solution might be to downsize the 100A breaker to 90A, (technically might require a load calc).
 
jaggedben said:
Yes, a 125A subpanel fed by a 100A breaker cannot be backfed by more than 40A continuous inverter current under the 120% rule, meaning if the system needs a breaker larger than 50A it's too big. An easy solution might be to downsize the 100A breaker to 90A, (technically might require a load calc).
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Or other options may be
replace the 125A panel with a 200A MLO
Or
convert the MLO to 100A main & tap the feeder
 
tortuga said:
Or other options may be
replace the 125A panel with a 200A MLO
Or
convert the MLO to 100A main & tap the feeder
Click to expand...
Whichever he does, the requirements of 705.12 also apply to the main panel, of course. IIRC he has a 200A panel with a 225A bus, so he has 70A of headroom on the main panel bus.
 
I had a few like this recently where a sub panel was closer to the solar but just piped EMT all the way to the service to avoid this hassle. As PV systems get larger and larger seems like there would be a market for a 300A buss loadcenter with a 200A main.
 
tortuga said:
I had a few like this recently where a sub panel was closer to the solar but just piped EMT all the way to the service to avoid this hassle. As PV systems get larger and larger seems like there would be a market for a 300A buss loadcenter with a 200A main.
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A 250A buss loadcenter with a 200A main, and bus terminal/tabs located on the end opposite from the main that would accommodate another 100A (main) breaker would also work.
 
tortuga said:
I had a few like this recently where a sub panel was closer to the solar but just piped EMT all the way to the service to avoid this hassle. As PV systems get larger and larger seems like there would be a market for a 300A buss loadcenter with a 200A main.
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Honestly if they sold the 225A busbars with 175A main breakers that would be enough. That would allow 76A backfeed on a 100A breaker. In my experience very few people really need 200A from their utility. I realize the psychology of selling 175A instead of 200A won't work. It's too bad.
 
jaggedben said:
Honestly if they sold the 225A busbars with 175A main breakers that would be enough. That would allow 76A backfeed on a 100A breaker. In my experience very few people really need 200A from their utility. I realize the psychology of selling 175A instead of 200A won't work. It's too bad.
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For most inverters I think you are right, and agree that most people don't need 200A main. But Sol-Ark makes the 15K now, and it has Icc as 62.5A x 1.25 = 78.13A, for an 80A breaker. What would be nice is if loadcenter manufacturers would offer field-replaceable busbars with slightly higher capacity and at a fair price. May be another revenue stream for them.
 
solarken said:
For most inverters I think you are right, and agree that most people don't need 200A main. But Sol-Ark makes the 15K now, and it has Icc as 62.5A x 1.25 = 78.13A, for an 80A breaker. What would be nice is if loadcenter manufacturers would offer field-replaceable busbars with slightly higher capacity and at a fair price. May be another revenue stream for them.
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Yeah my suggestion would accommodate that Sol Ark. The 76A is before the 125% factor.
 
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