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?
I do not see, from the OP's description, why he is assuming a maximum continuous load. He can probably skip the 125% calculation, and size his wire to 362 amps. After all, 215 also tells us to base the wire size on the nameplate, as a minimum.
I agree with the 75 degree column.
I would not size the wire to the 400-amp breaker; in this instance, you need to oversize the breaker to allow for current inrush at start-up. Similar to the principles used for motor OCPD sizing.
So, I'm looking at smaller wire .... say, 500mcm for copper and 700mcm for aluminum. Did I err?
Reno, why would installation not hinge on 215.2? Load would be considered continuous load, correct? So my primary current of 361 amps should be multiplied by 125%, requiring primary conductors to have ampacity of at least 451 amps. Why would 125% calculation be skipped? Perhaps I'm possibly missing something.
Greg, engineer's calculated demand on the transformer is 409 amps. Secondary conductors terminate into an 800amp ocpd.
Perhaps as the electical inspector, I am just making something more of an issue than it really is. As Hotline1 stated, it's normal for engineer's to size transformers and conductors this way.
The thing you have to remember is 215 applies to the load, not the size of the transformer. It would not be unreasonable to spec a transformer that is good for 125% of the maximum computed load but that doesn't mean you would have to upsize the feeders to 156%