Transformer secondary side overcurrent protection and feeder overload protection

[fandi]

Hello All,
A 150KVA 480V-208/120V Delta-Wye transformer is fed from a 225A breaker on the 480V side.
On the secondary side:
- max secondary side current is 150KVA/(1.732*208V)=416A.
- (2) sets of #3/0 CU in parallel so the max ampacity is 400A < 416A. To me, the wires are undersized. On a side note, per 240.4(F), for Delta-Wye transformers, secondary conductors are not considered to be protected by the primary OCPD.
- There's a MLO 600A panel within 10ft of the transformer secondary. This is where it seems conflicting: a) Per 240.21(C)(2), no OCPD at the secondary but b) Per 408.36(B), an OCPD is required to protect the 600A panel. Note that the exception in this subsection is not applicable because this is a Delta-Wye transformer. To me, an OCPD is required per 408.36(B).
Any thoughts?
Thank you.

 

[infinity]

Your secondary conductors are 400 amps so they must land in an OCPD of 400 amps or less.

 

[jim dungar]

To me, an OCPD is required per 408.36(B).
Any thoughts?
Thank you.

You are correct.
A lot of people to not realize that 240, 408, and 450 all must be satisfied, particularly when dealing with transformer secondaries feeding panels.

This has only been the case relatively recently, maybe less than 6 code cycles. So designers that always do things the way they were first taught (who needs continuing education) may still be putting MLO panels on transformer secondaries.

 

[augie47]

You are correct that 408.36 would require a main in the MLO panel.
Since you have met the rerquireemt for transformer protection in Art 450 the secondary conductors would need to meet the 240.21(C) rules. If you keep the parallel 3/0s that would mean a 400 amp device.
You could use a 600 amp main in the panel as long as you supplied it with condcutors rated not less than 600 amps.

 

[fandi]

Thank you all.