Derate for 2 sets of motor leads in single conduit - motors do not run at same time

[Jim Mayer]

This is one of those "I know the NEC mentions this situation somewhere but I can't find it" questions:

I will be feeding 2) 3-phase 480 VAC motors from a single control panel. The machine is such that these motors will never run at the same time. I am planning to run all 6 motor leads in a single conduit from the control panel to the machine and then T off to the individual motors at the machine. I am under the impression that I do not need to derate for more than 3 current carrying conductors since the motors will not run simultaneously.

 

[Carultch]

This is one of those "I know the NEC mentions this situation somewhere but I can't find it" questions:

I will be feeding 2) 3-phase 480 VAC motors from a single control panel. The machine is such that these motors will never run at the same time. I am planning to run all 6 motor leads in a single conduit from the control panel to the machine and then T off to the individual motors at the machine. I am under the impression that I do not need to derate for more than 3 current carrying conductors since the motors will not run simultaneously.

NEC2014 identifies in Table 310.15(B)(3)(a)'s footnote, that you do not have to count conductors that CANNOT be simultaneously energized. The example they had in mind when proposing this change, is the travelers on 3-way switch circuits, that by the nature of the switching, it is impossible to carry current in all three current-carrying conductors at the same time.

Is there some part of your system that prevents both motors from operating at once?

 

[Jim Mayer]

NEC2014 identifies in Table 310.15(B)(3)(a)'s footnote, that you do not have to count conductors that CANNOT be simultaneously energized. The example they had in mind when proposing this change, is the travelers on 3-way switch circuits, that by the nature of the switching, it is impossible to carry current in all three current-carrying conductors at the same time.

Is there some part of your system that prevents both motors from operating at once?

Thank you for the prompt response and for finding the footnote!

Yes, the motors are interlocked by the control system. Both in PLC code and hardwired safety circuits. I am comfortable with not applying the derate in this circumstance since they cannot and will never run simultaneously.

 

[petersonra]

in most cases at least for smaller motors it won't matter since you usually use 90 deg C wire and get to start the derating process with a number that magically works out that you don't have to make the wires bigger even with 6 CCC.