Main Breaker/disconeect size calculation for industrial machine.

[John Controls]

I am looking to find an optional (allowable) method for calculating the disconnect size for a specific type of industrial machine. The machine includes both VFD controlled motors and heating loads. Typically, I have always used 125% of the heating load + 125% of my largest motor + everything else at FLA. Reality is, that the heating only draws near 100% during initial heating and at that time the motors are not permitted to run. After the unit has come up to heat, the duty cycle of the heating reduces to about 30% at which time the motors are permitted to run. The heating is controlled via a solid state contactor utilizing either phase angle control or zero cross cycle count control and at no time does this machine draw more than 75 amps, as it is seen from a clamp on amp meter. My calculations demand I size the main at 175 amps but another machine builder is offering a competitive machine that only uses a 100 amp main, making their machine a more attractive purchase option. My Boss is unhappy. I thought I remembered seeing something in the NEC that permitted deration based on the control design limiting the current but I am not finding anything. Ideas?

 

[augie47]

There are certain areas of the NEC such as 430.26 that reference demand factors, but IMO, your situation falls more in line with design factors not covered by the NEC but rather by Codes regulating control panels such as UL508.
It would be worthwhile to note the requirements in NEC Art 409 especially 409.110
It might also be worth noting that many jurisdictions would require control panels such as the one you describe to be listed by a NRTL such as UL. NEC Art 90.7 provides that factory installed wiring of equipment need not be inspected other than for alterations or damage as the design is governed by Codes other than the NEC