83% rule for dwelling unit (310.15(B)(7))

[zemingduan]

I am always confused how this rule applys to/ affects the service/feeder conductor, service rating, service disconnect mean rating, service overcurrent protection rating.

For example, a big single family building which has a calculated load of 135A @ 120/240V 1ph in according to NEC 220.82.

- According to 230.79 the service disconnect mean rating need to be 150A.
- According to the 310.15(B)(7)(3), in no case the service/feeder conductors are not required to have an ampacity greater than 150A x 83% = 124.5A.

Let's say if we use an enclosed circuit breaker as the service disconnect mean. We have to use a 150A circuit breaker instead of a 125A breaker since the disconnect mean rating need to be at least 150A. But we can use #1 Awg Cu (130A ampacity) as the service and feeder. Do you agree?

Let's say if we use a fused safety switch as the service disconnect mean. Does that mean we can use a 150A safety switch with 125A fuses? The disconnect mean rating is still 150A.

Thank you!

 

[augie47]

Agree except with a calculated load of 135 you need 150 amp fuses. The #1 Cu would still be correct per 310.15(B)(7)
(and it would actually be a 200 amp switch with 150 amp fuses as there is no 150 amp switch)

I am always confused how this rule applys to/ affects the service/feeder conductor, service rating, service disconnect mean rating, service overcurrent protection rating.

For example, a big single family building which has a calculated load of 135A @ 120/240V 1ph in according to NEC 220.82.

- According to 230.79 the service disconnect mean rating need to be 150A.
- According to the 310.15(B)(7)(3), in no case the service/feeder conductors are not required to have an ampacity greater than 150A x 83% = 124.5A.

Let's say if we use an enclosed circuit breaker as the service disconnect mean. We have to use a 150A circuit breaker instead of a 125A breaker since the disconnect mean rating need to be at least 150A. But we can use #1 Awg Cu (130A ampacity) as the service and feeder. Do you agree?

Let's say if we use a fused safety switch as the service disconnect mean. Does that mean we can use a 150A safety switch with 125A fuses? The disconnect mean rating is still 150A.

Thank you!