Transformer Feeder Sizing

[cmiller]

Depending on Transformer Rated current, Table 450.3(B) allows overcurrent protection devices to be sized in excess of 125% normally used for conductors sizing. More specifically, if a Primary and Secondary protection design is adopted, Primary protection can be up to 250% of transformer rated current.

Does this mean the feeders conductors must be sized based on Primary protection or based on 125% of transformer rated current?

For example:

A transformer operated at less than 600V has a primary current rating of 100 A. The transformer will have both Primary and secondary overcurrent protection. The primary protection device selected is 200A.

Assuming no derating for ambient temperature or number of conductors in conduit, should the feeder circuit conductors be sized based on?

A) Continuous current rating of 125 A?
B) Overcurrent protection rating of 200A?

I look forward to your reply.

Thanks

Carl

 

[charlie b]

Re: Transformer Feeder Sizing

B.

Transformer protection is different from (and is covered by a different NEC section than) conductor protection.

 

[jim dungar]

Re: Transformer Feeder Sizing

The conductors are sized based on the primary current. They need to be protected at 125% unless one of the "tap rules" is applicable.

The transformer primary protection is not always the OCPD for the primary cables, so it is allowed to be sized following different rules.

 

[engy]

Re: Transformer Feeder Sizing

B
Article 450 does not tell how to size primary conductors.
Need to revert to article 240.