LV MCC Incomer sizing - 100% rated breaker

[cherryg222]

We have a situation where the transformer (dry type) is rated at 1500/1770kVA (85/115degC). The secondary side current @ 480V for 1770kVA is 2129A. As per NEC Article 450.3(A), the secondary side OCPD needs to be sized at 125% of FLC i.e 2661A. If we choose to use 100% rated breaker (UL 1066 for Power CB supports 100% rated breakers), can we adopt a 2500A CB with 2500A bus, for the 480V MCC?
What is troubling me is that there is no exception to 450.3(A) with specific reference to 100% rated CBs.

 

[RRaghunath]

I think you can definitely use 2500A rated CB.
The overload is always supposed to be for a limited time, hence need not be governing the continuous rating of ACB / busbars.

 

[waross]

The 125% rule is the maximum. You can go smaller, depending on the calculated load on the MCC.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cuky2000]

Be aware that the breaker is tested by UL at 75degC terminals. If the feeder is designed with ampacity using conductor insulation such as THHN rated for 90^oC, the breaker still needs to be selected for 125% or the feeder not to exceed 75^oC.

 

[cherryg222]

Thanks all for your feedback. We are following the 75degC ampacities. Along with a 100% rated breaker and enclosure, we will be able to meet the requirement. I still had the question lurking for the bus of 2500A. I am guessing that the bus requires no derating and is good for continuous 2500A load based on temperature rise test.

 

[jraef]

Correct, bus in the MCC requires no de-rating.

But you seem solely focused on the conductors. Your TRANSFORMER is not rated to deliver 2500A continuously.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[cherryg222]

Jraef...My concern is upto 2129A only. Any current carrying part with any deration should be able to meet that amps. If the breaker is 100% rated in it's enclosure, and the bus bars are rated at 2500A and certified for it, everything will fall into place. We know that the design is 2x100% which would mean less than half the loading of the transformer during normal operation. My concern is the rating of 1770kVA should not have any unseen, hidden bottlenecks downstream.