Transformer Secondary Over Current Protection

[brian john]

Have a situation where a customer has the following installation

150 KVA 480-208/120, 3 phase Delta/wye transformer with the following

PRIMARY
200 amp circuit breaker over current protection
3/0 copper conductors
SECONDARY
4-200 amp fusible safety switches with 175 amp fuses
2/0 copper conductors

In regards to the secondary over current protection what is the maximum size fuse that can be installed in each device.

the maximum single device would be 600 amps (125% of 416 A=520)

Do we take the 520 amp and divide by 4 or am I totally off base?
__________________

 

[DARUSA]

Primary OP 225amp. 4/0 copper wire
Sec OP 600 amp 2 set of 300 kcmil.
No you don't have to divide!!!

 

[augie47]

You are fine as is. The transformer (primary & secondary windings) are protected per 450.3(b) as the primary overcurrent is less than 125%.
The only thing you need to watch from there is protection of secondary conductors per 240.21(C) and the 2/o is rated 175 amps.
The total secondary OCP is not relevant to the transformer protection as the primay OCP has that covered..