TABLE 450.3(B)

[m sleem]

We are in a rehabilitation project where the existing transformer is 500 kva & loaded by 40% only of its capacity. We are going to replace the existing breakers and fuses. Can we size the main breaker (frame) after transformer by 125% of the transformer capacity & 125% of 40% XFMR (Trip)?

 

[don_resqcapt19]

The code only sets the maximum size of the transformer OCPD, it does not set a minimum. You will have no problem on the secondary side by using smaller OCPD, but if you try to reduce the primary side, by that much, the OCPD will probably open when you energize the transformer.

 

[m sleem]

The code only sets the maximum size of the transformer OCPD, it does not set a minimum. You will have no problem on the secondary side by using smaller OCPD, but if you try to reduce the primary side, by that much, the OCPD will probably open when you energize the transformer.

Fine, & would cable's ampacity match the ampere trip?

 

[don_resqcapt19]

The secondary cables would have to be protected per one of the rules in 240.21(C).

 

[m sleem]

Very helpful don.