# Thread: Buss Sizing and Disconnects

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## Buss Sizing and Disconnects

If I am needing a 1000 amp buss to accommodate 120% rule .My
Main breaker on my 1000 amp subpanel 400 amps.
Do I need to install a 1000 amp buss rated disconnect even though I have a 400 amp main breaker?
The disconnect is ahead of the subpanel
Thank you

2. I do not understand the setup you have. How does the 1000A relate to the 400A?

Can you narrate a oneline diagram or even better provide one?

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Originally Posted by harborrick
If I am needing a 1000 amp buss to accommodate 120% rule .My
Main breaker on my 1000 amp subpanel 400 amps.
Do I need to install a 1000 amp buss rated disconnect even though I have a 400 amp main breaker?
The disconnect is ahead of the subpanel
Thank you

If you connect at the opposite end of the bus from the main supply:
(Main supply breaker) + (result of calculation that sizes PV interconnection breaker) must be less than or equal to 120% of busbar rating

1000A busbar & 1000A main breaker means that 200A is the maximum PV interconnection breaker that you could use in this unit.

If the panelboard has a main lug only, and an external device taking the place of the main breaker, the rating of that external device takes the place of the main breaker rating in this calculation.

"result of calculation that sizes PV interconnection breaker" = 1.25* Inverter max continuous current. If you have to round up to get to a standard size, you can use the result of the calculation prior to rounding, rather than the breaker itself.

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Originally Posted by harborrick
If I am needing a 1000 amp buss to accommodate 120% rule .My
Main breaker on my 1000 amp subpanel 400 amps.
Do I need to install a 1000 amp buss rated disconnect even though I have a 400 amp main breaker?
The disconnect is ahead of the subpanel
Thank you
If I understand your question, no. If you have a 1000A panel with a 400A main breaker, you have (1.2)(1000A) - 400A = 800A of headroom in that panel. If you are connecting to it through a backfed breaker, the breaker and disco only need to be sized to the Imax of your inverter(s) and 125% of your Imax must be less than 800A.

That said, however, your MDP is also subject to the 120% rule; 125% of your inverter Imax plus the rating of the main breaker in the MDP must also sum to less than 120% of the bus rating of the MDP and the breaker in the MDP feeding the sub must be at the opposite end of the bus from the MDP main breaker.

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Originally Posted by harborrick
If I am needing a 1000 amp buss to accommodate 120% rule .My
Main breaker on my 1000 amp subpanel 400 amps.
Do I need to install a 1000 amp buss rated disconnect even though I have a 400 amp main breaker?
The disconnect is ahead of the subpanel
Thank you
The math isn't adding up. If you have a 400A feed then that's also your max backfeed. So the 120% rule couldn't require more than 666A. (800A/1.2). Even if the 1000A bus is because of equipment availability or something that doesn't change the bus requirement. As far as the feeder or switch needing to me more than 400A that isn't required either unless perhaps you're still on the 2011 Code and the AHJ is pushing an unusual interpretation, especially with respect to the disconnect. Never had an AHJ require that, and it makes no sense electrically. The most operating amps that switch could see is 400.

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Originally Posted by jaggedben
The math isn't adding up. If you have a 400A feed then that's also your max backfeed. So the 120% rule couldn't require more than 666A. (800A/1.2). Even if the 1000A bus is because of equipment availability or something that doesn't change the bus requirement. As far as the feeder or switch needing to me more than 400A that isn't required either unless perhaps you're still on the 2011 Code and the AHJ is pushing an unusual interpretation, especially with respect to the disconnect. Never had an AHJ require that, and it makes no sense electrically. The most operating amps that switch could see is 400.
Realistically, answering this question without seeing the entire wiring diagram is a bit of a shot in the dark, and my answer was no exception. Context is important and we have none.

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Originally Posted by ggunn
Realistically, answering this question without seeing the entire wiring diagram is a bit of a shot in the dark, and my answer was no exception. Context is important and we have none.
I thought his description was clear enough. But it's a fair point that there could be important items could be left out, especially since the givens don't make much sense.

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Originally Posted by jaggedben
I thought his description was clear enough. But it's a fair point that there could be important items could be left out, especially since the givens don't make much sense.
He only talks about the subpanel; there could be other issues in the main panel.

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