Transformer Overcurrent Protection

[Nardie]

I have a vendor design I'm reviewing and it has a 10 kVA single-phase transformer being installed. The primary OCPD is 30A, the secondary is connected to a 100A main breaker panel. The panel has 10 branch circuit breakers. The way I read NEC Table 450.3(B) is that I can only have 125% secondary protection, which would be a 60A breaker. In addition, I would think I would need to install #2 AWG conductors from the 10 kVA transformer due to a 100A main panel breaker. So my question is am I reading the code wrong i.e. the secondary OCPD is wrong. This installation will per the NEC not the NESC. In addition, why would the vendor install a 100A breaker when the transformer max amps would be approximately 40A.

 

[david luchini]

You don't indicate what the primary and secondary voltages are.

 

[Nardie]

The primary voltage is 480V. The secondary voltage is 240/120V.

 

[jim dungar]

The way I read NEC Table 450.3(B) is that I can only have 125% secondary protection...

That table is often mis-read.
The table says: if your primary protection is sized at not more than 125%, your secondary protection can be any size you want it to be, even 1000%.
However, if your primary protection is greater than 125% but less than 250%, then your secondary protection cannot exceed 125%.

Yes, you are correct, the secondary conductors need to be based on the secondary protective device.