Panelboard feeder sizing

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nkalghatgi

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Mechanicsburg, PA, USA
I have an MCC which feeds the panelboard through a transformer. The panelboard schedule is as follows : Phase A: 70A, Phase B: 60A, Phase C: 65A. The panelboard is rated at 125A,208-120Y,3 phase.

While sizing the primary & secondary conductors to and from the transformer, what currents should I consider?
 
What size is the trany and do you want the owners to have maximum ampacity for future stuff. Panel is rated 125 amps so you could use that or at least 100 amps
 
base on transformer rating

base on transformer rating

I'm not sure exactly what the NEC states, but by the CEC, the conductors need to be based on the rating of the transformer. These conductors have to be rated for at least 125% of the current rating of the transformer (primary and secondary side).
 
The closest standard size 3 phase 208Y/120 transformer that would cover your load with a bit of growth potential would be a 30kva which would also fit well with your 125 amp panel.
As your profile indicates you are a qualified engineer, by using Art 450 and 240, sizing your conductors and over-current devices should be a snap.
 
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SalixKetuur said:
I'm not sure exactly what the NEC states, but by the CEC, the conductors need to be based on the rating of the transformer. These conductors have to be rated for at least 125% of the current rating of the transformer (primary and secondary side).
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Interesting. Are multiple taps from the secondary allowed?

If so, may they each be of a lower ampacity than 125% of the current rating, perhaps with the sum total equaling no less than 125% of the transformer's secondary rating?
 
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