Undersizing wire/breaker for equipment

[menoknow]

Long story short I am currently hooking up power for some new manufacturing equipment in Washington State. Just finished hooking the first (4) production lines up to power with (4) more on a boat on the way here. However I have a bit of a crazy idea. I've been monitoring the current draw for over a month as I've been connecting these to power and they are drawing way under the nameplate rating. Significantly under the nameplate value. The nameplate value of the largest piece of equipment in one of the production lines is 250 amps, but the entire line consisting of (6) different pieces of equipment is only drawing 25 amps at 480V. Currently the largest piece of equipment is hooked up with 350kcmil aluminum conductors, but I was thinking on the future installations is there any reason I can't underpower some of the equipment. Say hook the largest one up with 3awg copper conductors on a 100 amp breaker?

There is a 75HP motor in that piece of equipment, but I see absolutely no inrush current on the power monitor. (3) have been hooked up and running at full tilt for almost 2 weeks now, and my monitor has not detected a peak over 80 amps. I'm assuming this has to do with the beast of a frequency drive controlling the motor, but it seems a little ridiculous to have to fully size these circuits going by the NEC.

I don't think it would be a safety concern as the wire would be fully protected whatever ampacity I choose as long as the breaker is properly sized. Anyone have any thoughts on this? Would I be violating the NEC if I were to do this? in the past I've hooked up equipment rated at 225 amps on a 200 amp circuit and pass inspection with flying colors.

 

[petersonra]

The code has very conservative load calculations. But, they are what they are, and if you want to abide by the code you have to follow them.

It's not unusual for inspectors to miss stuff like you described. It's not like it is a safety hazard because the wires are all properly protected even if not to code.

VFDs make the situation even stranger since, for instance, if the motor is drawing 75% of FLA the VFD only draws as much current as is required to supply the amount of energy being used by the motor, so you can have these crazy things going on where a motor is running at 75 Amps and the drive is only drawing 10. You don't get to take credit for any of that though in your load calculations.

You can take credit for it later on kind of in an indirect way as the code allows you to determine existing loads by monitoring as opposed to by calculation.

 

[don_resqcapt19]

....

There is a 75HP motor in that piece of equipment, but I see absolutely no inrush current on the power monitor. (3) have been hooked up and running at full tilt for almost 2 weeks now, and my monitor has not detected a peak over 80 amps. I'm assuming this has to do with the beast of a frequency drive controlling the motor, but it seems a little ridiculous to have to fully size these circuits going by the NEC.

....

What is the time resolution of your power monitor? The resolution of many monitors is too long to see the inrush current.

If the 75 hp is on a motor starter, the inrush could be within the instantaneous trip range of the 100 amp breaker. The code would permit a 250 amp breaker with 1 AWG copper wire for a 3 phase, 480 volt, 75 hp motor.