Amperage for Enclosures

[mda3vk902]

Hi all,

Where in the NEC can I find rules for choosing an amperage on my electrical enclosures? This is to size an AC combiner panel with fuses, though I imagine similar rules would apply to other enclosures like a Utility Accessible Disconnect.

2,416A of inverter current with 3,020A OCPD total. Can I use a 3000A enclosure, or do I have to upsize to 3200A?

Thanks!

 

[JakeX]

See 2023 NEC 705.30(B). Each branch breaker in the combiner panel should be 125% of the full AC output current of the inverter feeding that branch breaker. The rating of the main circuit breaker of the combiner panel can be calculated in the same manner and equates to the sum of the branch breaker ratings (e.g. two 50A branch breakers, then the main is 100A). The bus rating should be equal to or higher than the main circuit breaker rating.

 

[mda3vk902]

Thanks for the quick reply. These are the rules for sizing the OCPDs (here fuses). This does not discuss the enclosure rating though. Is the enclosure sized according to the overcurrent protection devices?

For example, my AC combiner scenario, the sum of the branch circuit fuses will be 3,020A. However, on the output of the AC combiner, we'll likely use 3,000A fuses. Would this mean we can use a 3,000A enclosure? Where in the code does it define the amperage of enclosures we need?

 

[jim dungar]

Thanks for the quick reply. These are the rules for sizing the OCPDs (here fuses). This does not discuss the enclosure rating though. Is the enclosure sized according to the overcurrent protection devices?

For example, my AC combiner scenario, the sum of the branch circuit fuses will be 3,020A. However, on the output of the AC combiner, we'll likely use 3,000A fuses. Would this mean we can use a 3,000A enclosure? Where in the code does it define the amperage of enclosures we need?

The conductors need to be sized to the OCPD. Does your enclosure have conductors in it?

 

[mda3vk902]

Yes, the enclosure has conductors

 

[ActionDave]

I don't understand the question. Conductors carry amps so they are what gets sized by the amperage they can handle. Enclosures, conduit, junction boxes, are sized by the volume they need to contain the conductors.

 

[mda3vk902]

That would explain it. I was under the impression the enclosures were also rated for a specific Amperage. Thanks

 

[JakeX]

Thanks for the quick reply. These are the rules for sizing the OCPDs (here fuses). This does not discuss the enclosure rating though. Is the enclosure sized according to the overcurrent protection devices?

For example, my AC combiner scenario, the sum of the branch circuit fuses will be 3,020A. However, on the output of the AC combiner, we'll likely use 3,000A fuses. Would this mean we can use a 3,000A enclosure? Where in the code does it define the amperage of enclosures we need?

The bus rating should be equal to or higher than the main circuit breaker or fuse rating, so I would specify a switchboard with 3200A bus. If you are asking for the physical dimensions of the switchboard, you need to contact your preferred manufacturer (e.g. Eaton, Schneider, Siemens, ABB-GE, Etc.) and ask them to give you a switchboard layout.

 

[Strathead]

That would explain it. I was under the impression the enclosures were also rated for a specific Amperage. Thanks

Just FYI, enclosures are actually, junction boxes, cabinets and cutouts per the Electrical code. Then you have wireways and gutters which are technically raceways. You can read the requirements for indoor, outdoor, metal and non-metallic in the various code sections.