All that went through my mind, but I don't see why would you do that?
Primary Current = 37500VA/480V = 78.125A
78 x 125% = 97.5 amps.
So as I see it, to feed the transformer you would run #3 conductors with a 100 amp CB.
What am I missing?
Thanks
Mike
you are missing inrush in to the equation, in rush can be as high as 6 to 10 times the kva rating of a transformer, while true it is best to protect as close to FLA as possible it is not always cost effective, so many just wire to the max allowed in the NEC.
If we look at the inrush problem we will see that energizing the above transformer can produce inrush currents in the range of 470 to 781 amps, while this is only for a few cycles, for a 100 amp breaker it will be getting very close to the instantaneous trip curve of that breaker/fuse, so this is why we are allow to install much larger OCPDs
we protect conductors for two things, overload, and fault current, a breaker sized to 250 % will many times still trip under a bolted fault protecting the transformer, although there are cases of long primary runs that can be a problem, we protect against overloading the transformer by sizing to 125% on the secondaries and OCPD, the 400 amp OCPD in this case is way over this limit, and will not provide any over load protection for this transformer, there is one other way to protect conductors and equipment but it is mostly used by manufactures by restricting the load that can be placed upon the equipment/conductors. we see this in control panels that have a fixed set of loads that can't be changed, kind of like a fixture socket if you have only one socket, and the largest lamp that will fit it is only 150 watt, then this would be a load limiter and manufactures can reduce their size of wiring to this fixture ( why we see smaller conductors in manufactured appliances or control panels) but we have to follow the rules in the NEC, and there is only a very few places where we can use this, and fixture wire is just one place, a 18awg wire will be protected by the controlled load from over load, and with it being fed from a 20 amp breaker it will still cause the breaker to trip under fault conditions as long as the circuit is not to long.
