How can I rate a main distribution panel serving 3 sub-main panels in an office building?

Question

Assume each sub-main panel is rated 125 Amps and have load of 39.4 KVA.

120/220 3ph 4 wire 60 Hrz

Thanks

Answer 1

You can only load the panel to 80% of its rating.

3 panels x 39.4 kVA / panel = 118.2 kVA

118.2 kVA ÷ .220 kV ÷ √3 = 310 amps.

310 amps ÷ .80 = 387 amps.
The next availabe panel size is 400 amp.

Technically this could get a bit more complicated since you have 120/240 V 3-phase, 4-wire service. You can not completely balance the single-phase load across all three phases. If there is too much single-phase load, you might have to move up to a larger panel.