LV Transformer Primary Conductors and Protection

[VTer]

We have a 45kVA, 480V delta – 208Y/120V 3-ph 60Hz transformer.
NEC 450.3(B) allows maximum overcurrent protection on primary to be 250% if we limit the secondary protection to 125%.
We have a spare 100A/3P CB that we would like to use which would meet the code requirement. However, it is unclear to me if the primary conductors have to be sized for the 100A OCPD or can they still be rated for FLA x1.25 = 70A. We also have spare #4AWG THHN conductors that I would like to utilize for primary, but they are only good for 85A at 75C. I cannot find any NEC paragraph or article that allows me not to oversize the conductors to match OCDP size. When we oversize the primary OCPD, does the primary conductor have to be oversized as well and which NEC article specifies this?


"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[dpc]

The conductors must be sized per the upstream breaker size. About the only exception to this in the NEC is for motor feeders.

 

[VTer]

Thanks dpc. I was hoping I was missing some exception in the code that would allow me to use the smaller conductors.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[wroggent]

I agree with dpc. You might get some relief out of 240.21(B)(3) but that's probably about it.

 

[dpc]

For your #4 AWG (copper, of course), assuming the terminations at both ends were rated for 75 deg C, the max breaker size would be 90 amps based on the next larger size above the ampacity of 85 A. I still use 60 deg C ampacities up through 110 A breakers, just to be safe.

 

[VTer]

Thank you gentlemen.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla