Circuit Sizing for Oversized Transformer

[rwilsond]

Assume a balanced connected load of 25 kVA on the secondary of a 45 kVA, 3-phase, 480-208/120V transformer. Does the NEC prohibit less than that of the transformer's primary rating for sizing wiring and CB? For example, can a 45A CB, and 50A ampacity wiring be used to supply the transformer's primary (which has a FLA rating of 54A)?

If there is a code violation, what paragraph/section of the NEC?

 

[augie47]

Re: Circuit Sizing for Oversized Transformer

as far as I know, no violation.
You may find the inrush current on the transformer causes you a problem. hopefully your breaker has adjustable trip curve.

[ November 07, 2005, 07:00 AM: Message edited by: augie47 ]

 

[templdl]

Re: Circuit Sizing for Oversized Transformer

One of the most common problems that I have ran into with field applications is the undersizing of the pri. OCPD for DTDTs.
This is the way that I look at it.
The inrush of a 150degC 45kva DTDT will be in the neighborhood of about 10 times it's FLA. So 54 x 10= 540a. So a 45a OCPD will have a instantaneous trip of 10x its rating +-20% which puts it in the range of 360-540a which most likely would result in nuisance tripping when the transformer is energized. The problem gets even worse with 115 and 80degC and K-factor transformers where the inrush can range from 600a-approaching 1000a.
It appears as though the cable size that is being selected to feed the DTDT is the limiting factor.
If so can it be resized? If one elects to use the 125% rule, 54a x 1.25=68a where you can use a #6 cable and allowed to apply a 70a OCPD which should give you the head room that you need.
But, then there is the 250% rule if you have secondary protection which you do have but then the 25' tap rule must be a consideration and the size of the feeder would be determined by the secondary protection.
Also, my guess is that your panel most likely is limited to 100a rated breakers.