Transformer Primary Conductors (for above 1000V)

[mshields]

For a typical substation transformer, I was thinking that they could be 125% of FLA even when the primary OCPD was something greater. This would by the defiunition of a tap be just that, a tap. And yet while secondary conductors are covered in the tap rules 240.21, I don't see anything about primary conductors. Bottom line: Do transformer primaries need to be fully rated for the primary OCPD?

 

[electrofelon]

For a typical substation transformer, I was thinking that they could be 125% of FLA even when the primary OCPD was something greater. This would by the defiunition of a tap be just that, a tap. And yet while secondary conductors are covered in the tap rules 240.21, I don't see anything about primary conductors. Bottom line: Do transformer primaries need to be fully rated for the primary OCPD?

Generally yes, transformer primary conductors are protected at there ampacity just like any "normal" conductor. There is However a provision to supply a transformer with a feeder tap, see 240.21(B)(3). I have never used this rule.

Also keep in mind for MV transformers, the feeder supplying the transformer must be rated for the nameplate FLC.

 

[david luchini]

For a typical substation transformer, I was thinking that they could be 125% of FLA even when the primary OCPD was something greater. This would by the defiunition of a tap be just that, a tap. And yet while secondary conductors are covered in the tap rules 240.21, I don't see anything about primary conductors. Bottom line: Do transformer primaries need to be fully rated for the primary OCPD?

Above 1000V, the transformer primary conductors are only required to be 100% of the primary rated current. The primary OCPD can be greater than primary conductor ampacity. It is not a tap.

 

[wwhitney]

Above 1000V . . . The primary OCPD can be greater than primary conductor ampacity. It is not a tap.

So why does 240.101(A) allow a fuse up to 3 times the ampacity, or a breaker with long time trip setting up to 6 times the ampacity? What's the philosophy here?

Thanks, Wayne