A customer is replacing an HVAC unit with new HVAC unit and they want us to reuse the existing circuit. The existing circuit they want to use is fed from a 250A circuit breaker and is 4/0 CU THHN. The new unit has a MCA of 133A and MOCP of 150A (this is all 3 phase 480V). The existing disconnect switch is old and they want us to replace. The existing switch is 200A with 150A fuses. The question I have is can I install a 200A switch with 150A fuses that is being fed from a 250A CB and 4/0 CU wire? I think the wire is ok because it is rate at 230A and they don't make a 230A CB so I can round up to the next size per 240.4(B). With 150A fuses the switch will never be overloaded, but I just can't find a portion in the code that tells me this is acceptable. It doesn't make any sense installing a 400A disconnect switch with fuse reducers and 150A fuses. I appreciate the feedback.
Disconnect Switch Sizing
- Thread starter Richard S
- Start date
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qcroanoke
Sometimes I don't know if I'm the boxer or the bag
- Location
- Roanoke, VA.
- Occupation
- Sorta retired........
Yes. The 150 amp fuses limit the current draw to the max. allowed on the nameplate. I think you're good to go.
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