Transformer protection/pre-assembled?

[Jayyy]

First time in this scenario. I have a pre-wired 25kva, 3~ 208 delta to (multi-tap) 400v 3~ Y transformer with multiple breakers and primary switch/disconnect incorporated into it. The machine that it is feeding is 3~ 4wire, 400v at 5.5 (KW) rated at 13 amps.
Originally the trnsfrmr was designed to handle more than one machine. We are now using one trnsfrmr per machine for redundancy reasons and I would like to protect the trnsfrmr correctly.
Do I still have to protect the trnsfrmr for its nameplate or can I protect it for the nameplate of the machine ( or load being served) at 30 amps from the panel?

 

[augie47]

The transformer should have protection per 450.3(B). If you wish to use a lower overcurrent protection device you can do so. On your primary, you don;t wish to be so low as to cause nuisance tripping.
As far as the secondary to you machine, if 450.3(B) is satisfied you may protect the secondary conductors at the conductor ampacity.
A 30 amp breaker on your 25 kva transformer is fine if that's what you want.
In a nutshell: protect the transformer per 450.3(B) and the conductors per 240.21 and the machine per nameplate.

 

[jim dungar]

Per 450.3(b), you only need to protect your transformer using a primary device set at not more than 125%. If this is done your secondary device, if one is used, can be any size and only needs to protect the secondary conductors.

But, 450.3(b) has an allowance for using a primary device set greater than 125%. This route requires a secondary protective device be provided, sized at 125% max (this device may be the same as the one protecting the secondary conductors).