My calculations for this transformer's primary conductors & primary overcurrent protection, do not match what was submitted by the engineer. Any advice would be appreciated.
Transformer is as follows:
300kva, 3 phase, 3 wire, 480V primary with
3 phase, 4 wire, 208/120V secondary.
Transformer is being fed off of existing 800 amp switchgear.
Submitted plans show primary overcurrent device to be
400 amp, 3 pole breaker & (3) 750kcm AL primary conductors.
I'm coming up with 361 amps of current, for this 300 kva transformer. According to 215.2(A)(1), the current gets multiplied by 125%, which now brings my 361 amps to 451 amps, which under table 310.16, requires...
1250kcm AL or 700kcm CU (using 75C column for terminal ratings.)
And table 240.6, requires a 450 amp overcurrent device.
Am I missing something?
