Feeder Conductor ampacity

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electro7

Senior Member
Location
Northern CA, US
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Electrician, Solar and Electrical Contractor
Understood ggunn. Yes I knew that, hence the upgrading of both sub-panels B1 and B2 to 200A rated, and downsizing the main from 400A to 350A.

My concern is 100A of solar OCPD exporting through #4 THHN. Existing feeders from B1 to B2 are #4 THHN and are protected by 100A breaker. B2 is in a guest house. Inverter max output currents are 47.5A and 32A, two inverters. What are your thoughts? 100A of solar OCPD exporting through #4 THHN okay?
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
electro7 said:
Understood ggunn. Yes I knew that, hence the upgrading of both sub-panels B1 and B2 to 200A rated, and downsizing the main from 400A to 350A.

My concern is 100A of solar OCPD exporting through #4 THHN. Existing feeders from B1 to B2 are #4 THHN and are protected by 100A breaker. B2 is in a guest house. Inverter max output currents are 47.5A and 32A, two inverters. What are your thoughts? 100A of solar OCPD exporting through #4 THHN okay?
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If the main breaker on the sub is sized appropriately for the feeder conductors it should also protect them from the backfed current from the inverters.
 
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electro7

Senior Member
Location
Northern CA, US
Occupation
Electrician, Solar and Electrical Contractor
ggunn said:
If the main breaker on the sub is sized appropriately for the feeder conductors it should also protect them from the backfed current from the inverters.
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Are you saying it is OKAY to interconnect 100A of solar OCPD on sub-panel B2 that has a 100A main breaker with #4 THHN feeder conductors? B2 sub-panel is on a guest house- dwelling unit according to code definition.
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
electro7 said:
Are you saying it is OKAY to interconnect 100A of solar OCPD on sub-panel B2 that has a 100A main breaker with #4 THHN feeder conductors? B2 sub-panel is on a guest house- dwelling unit according to code definition.
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I'm saying that I believe, in general, if the conductors feeding a panel are correctly sized for the OCPD protecting that panel, then if you land a backfed breaker in that panel and the bus is qualified under one of the sections 705.12(D)(2)(3), the feeder conductor size should be OK.
 
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