D-rating load center?

Tobyhey74

New User
Location
Utah
Occupation
machinist
hello, new to the forum. I recently installed solar on a new construction. I installed solar on my accessory building which is where my meter base is located. I installed and 11 kW system on a 200 amp service so they D rated service breaker to 175 and put a 60 amp for solar. My home has a 200 amp shut off on the exterior and a 200 amp load center in the home. Do I have to D-rate both of those to 175 or can I leave them.
Thank you
Toby
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
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Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
I opened this thread. The poster says his electrician won't do any work unless he changes both breakers. Imo, just the breaker at the service panel needs to be replaced to 175 amps
 

jaggedben

Senior Member
The description is a little light on detail, but I agree that if you have breakers in series at both ends of a feeder only one needs to be 175A.
 

Carultch

Senior Member
Location
Massachusetts
hello, new to the forum. I recently installed solar on a new construction. I installed solar on my accessory building which is where my meter base is located. I installed and 11 kW system on a 200 amp service so they D rated service breaker to 175 and put a 60 amp for solar. My home has a 200 amp shut off on the exterior and a 200 amp load center in the home. Do I have to D-rate both of those to 175 or can I leave them.
Thank you
Toby
The term is derating. What it means is to reduce the rating of a product. Sometimes environment and conditions of use cause a product to no longer be able to be used at its full capacity. Another meaning of the term is to reduce the rating by changing a product setting or component. In this context, it is the latter, and it is done by replacing the breaker completely.

The reason they need to derate your main breaker on your panelboard, is to meet the "120% rule" in NEC705.12(B)(2)(3)(b), for protecting the busbar of the panelboard. Once you are on the supply side of the 175A breaker, you are no longer looking at a busbar that is carrying current. Instead, you are looking at feeder conductors. Busbars require such a rule, because panelboards are routinely populated with branch breakers that add up to a lot more than the busbar rating. The 120% rule is an industry compromise to keep the total load and heating among all the branch breakers within reasonable limits, yet still have some headroom to interconnect sources.

The 200A exterior shutoff upstream of your 175A breaker, is there to be your service disconnect and to protect the feeder conductors from it to the 175A breaker. The feeder conductors are existing and presumably already compliant to be protected by a 200A service disconnect, so there is no need to change the trip rating of the 200A exterior shutoff.
 
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kwired

Electron manager
Location
NE Nebraska
To simplify what Carultch said into something pretty basic - you can draw up to 200 amps from the utility without tripping anything. You can also draw an additional 60 amps from the PV equipment without tripping anything. Somehow they came up with a 120% rule as he mentioned as it is not likely you would actually have 260 amps on that bus.

There can be more than that to this but that is a good start at understanding what can happen.
 
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