Fusing and wire size calculations

[Carultch]

Second the conductors between the transformer secondary fuse must have an ampacity equal to or greater than the rating of the fuse. The "round-up" rule in 240.4(B) does not apply to these conductors, so the 500kcmil conductors are not large enough for the 500 amp fuses.

Is the transformer a 3 wire to 3 wire delta/delta transformer? Is so, you could use the primary OCPD to protect both the primary and secondary transformer windings as well as the secondary conductors as long as you size the conductors per 240.21(C)(1).

Suppose the transformer is a topology (1-phase or 3Wdelta-3Wdelta) that allows the primary OCPD to protect the secondary wires by proxy, yet there still is an OCPD on the secondary for other reasons. Do you still need to size the wire to the secondary OCPD, even though the wires are otherwise considered protected when you eliminate the secondary OCPD?

 

[don_resqcapt19]

Suppose the transformer is a topology (1-phase or 3Wdelta-3Wdelta) that allows the primary OCPD to protect the secondary wires by proxy, yet there still is an OCPD on the secondary for other reasons. Do you still need to size the wire to the secondary OCPD, even though the wires are otherwise considered protected when you eliminate the secondary OCPD?

The code would only require that the secondary conductors be large enough so as to be protected by the primary OCPD per the rule in 240.21(C)(1). It does not matter that there may be a larger or smaller OCPD on the load end of the secondary conductors.

There may be other rules that also apply to the size of these conductors, for example, for a drive the conductors must have an ampacity that it at least 125% of the input current rating for the drive.