Transformer Undercurrent Protection

[reboldhack]

Contractor has an new (unused) 225 kva step down, dry transformer that he wants to use in place of the 150 kva specified. If the proper primary and secondary overcurrent protection, conductors, clearance , etc are installed and sized for the 150 kva, does this violate any NEC rules? Is there something in article 230 or 450 that this violates?

 

[steve66]

Yes, see 215.2(B)(1).

Steve

 

[coulter]

Steve -
That would be for over 600V. For that small of an xfmr, I figured it likely was 480V primary.

Rebo-
Assuming the primary is 480V, I don't know of any NEC issue, but an OCP sized for 150kVA may not stand the inrush of a 225kVA.

 

[LarryFine]

. . . an OCP sized for 150kVA may not stand the inrush of a 225kVA.

The only difference should be the difference in the magnetizing current, which should be minimal. The load reflected in the primary would be the same.

 

[winnie]

The magnetizing current is a steady state value, after the system is powered up. However when energizing the transformer there will be transient current flow (inrush) that could trip undersized OCPD. The details of the inrush current will depend upon the specifics of the transformer; this may or may not be a problem. My guess is _probably_ not a problem.

-Jon

 

[coulter]

.. My guess is _probably_ not a problem.

Jon tends to guess pretty good.

My guess is, "It depends on if the primary OCP was sized at 125% or 250%. I've had very few small xfmrs trip with the OCP set to 125% (actually none I think), so I started recommending that instead of 250% (cause of the extra copper) But, 125% for a 150kVA is 75% for a 225kVA.

Check some of the threads a few months ago on xfmr inrush current. Could be you are looking at 10X FLA for inrush.

 

[steve66]

Steve -
That would be for over 600V. For that small of an xfmr, I figured it likely was 480V primary.

Rebo-
Assuming the primary is 480V, I don't know of any NEC issue, but an OCP sized for 150kVA may not stand the inrush of a 225kVA.

Thats right. I thought I remembered this as being for higher voltages only, but when I looked to see what voltage (B)(1) was for,k I missed the "Over 600 Volts" in (B) and only saw the "Under 600 Volts" in (A).

Steve