Transformer Disconnect and Overcurrent Protection

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beanland

beanland

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Vancouver, WA
225kVA 480V delta to 208Y/120V transformer. 480V current is 270A so 300A circuit breaker being used. No other loads served on 300A breaker. Need to provide secondary overcurrent protection so plan to use 800A Class L fuses. 300A breaker protects for overloads and line-line faults. 800A fuses protect for line-ground secondary faults. Transformer has 800A conductors feeding 800A panel.

AHJ says that I must install a disconnect, in addition to fuses, on the transformer secondary. NEC 240.21(C)(2) requires single-set of overcurrent protection within 10 feet of transformer. NEC 240.40 requires that fuses have a disconnect on the supply side. I argue that the 300A circuit breaker is a disconnect on the supply side of the fuses.

Is there a specific NEC section that requires a disconnect on the secondary side of a transformer?
 
beanland said:
225kVA 480V delta to 208Y/120V transformer. 480V current is 270A so 300A circuit breaker being used. No other loads served on 300A breaker. Need to provide secondary overcurrent protection so plan to use 800A Class L fuses. 300A breaker protects for overloads and line-line faults. 800A fuses protect for line-ground secondary faults. Transformer has 800A conductors feeding 800A panel.

AHJ says that I must install a disconnect, in addition to fuses, on the transformer secondary. NEC 240.21(C)(2) requires single-set of overcurrent protection within 10 feet of transformer. NEC 240.40 requires that fuses have a disconnect on the supply side. I argue that the 300A circuit breaker is a disconnect on the supply side of the fuses.

Is there a specific NEC section that requires a disconnect on the secondary side of a transformer?
Click to expand...

If you are not using a fusible disconnect switch, how do you plan to mount your fuses?

But to answer your specific question, there is no code article requiring a disconnect on the secondary of a transfromer.
 
Fuse Mounting

Fuse Mounting

I planned to use a fuse mounting block for Class L fuses. Bussman claims to make one. And another company, Filnor, makes a single-fuse block.
 
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