[Aegis]

I'm used to fusing the primary of a transformer and then that transformer feeds a panel right next to it.

I'm hooking up some 400v equipment and I have 600v to 400v transformers. I fuse the primary accordingly but my question is, should I fuse the secondary as well?

16.7KVA 600v to 400v Transformer

20A fuses on the Primary

The Equipment is 400v 3phase, 30A.

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[CMP]

What style is the transformer? Auto- Isolation- Delta wye- wye- Delta Delta? Post the transformer tag.

 

[just the cowboy]

If I could I would.
My reason would be 600-400 may not be common to get.
If you have a fault on secondary without fuses it has to go thru xformer to blow fuse and could cause damage to xformer.
If you have it fused on secondary it MAY react fast enough to take some of the fault current off of the xformer.

 

[CMP]

My guess is it’s a wye autotransformer.

 

[Aegis]

What style is the transformer? Auto- Isolation- Delta wye- wye- Delta Delta? Post the transformer tag.

One is an auto transformer (600v to 400v), the other is a delta Wye (600v to 230v).

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[Aegis]

If I could I would.
My reason would be 600-400 may not be common to get.
If you have a fault on secondary without fuses it has to go thru xformer to blow fuse and could cause damage to xformer.
If you have it fused on secondary it MAY react fast enough to take some of the fault current off of the xformer.

Makes sense. One of the transfkrmers is a 600v to 230v, 12.5 KVA. Fused at 15A there will be 39A on the secondary, the 230v equipment is only 11A however. It would make sense to choke it down a little too.

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[wiz1997]

The NEC has an table for sizing secondary overcurrent protection of a transformer.

This may not apply to the CEC, but it might help.

NEC 450.3 (B) Transformers 600 Volts, Nominal, or Less...references you to table 450.3 (B).

Table 450.3 (B) under the Secondary Protection column shows:

Currents of 9 amperes or more can have an overcurrent device rated at a MAXIMUM of 125% of the transformer rated current.

Currents of less than 9 amperes can have an overcurrent device rated at a MAXIMUM of 167% of the transformer rated current.

These are MAXIMUM device ratings, you can always go lower.

 

[Navyguy]

I know, if calculated correctly the primary fuse will blow during a secondary fault, so you do not need to fuse the secondary. Having said that, I always fuse the secondary for just reason @just the cowboy said. There is an extra cost for sure, and even maybe a space issue in some locations, but trying to source a transformer in a failure situation seems a lot more difficult then it should be, so I think of this as cheap insurance.