Motor Ground Fault Short Circuit protection question

Leadfoot, we have been talking about the branch-circuit short-circuit ground-fault protection. This is not the same as the overload protection.

But, I did find in the NEC 430.6(C) For motors used with VFDs, the ampacity of conductors, switches, branch-circuit short-circuit and ground-fault protection shall be based on the maximum operating current maked on the motor or control nameplate, or both.

I sure don't like being wrong. But it's worse giving the wrong information and not correcting yourself.

Tark said:

Leadfoot, we have been talking about the branch-circuit short-circuit ground-fault protection. This is not the same as the overload protection.

But, I did find in the NEC 430.6(C) For motors used with VFDs, the ampacity of conductors, switches, branch-circuit short-circuit and ground-fault protection shall be based on the maximum operating current maked on the motor or control nameplate, or both.

I sure don't like being wrong. But it's worse giving the wrong information and not correcting yourself.

Click to expand...

Please excuse me, but I'm going to be putting together a system with several drives in it in the near future. I need to know as much about this code as possible before we build it. I want to get it right.

What does this mean?

430.6 (C) Alternating –Current Adjustable Voltage Motors.

For motors used in alternating-current, adjustable voltage, variable TORQUE drive systems, the ampacity of conductors, or ampere rating of switches, branch circuit short-circuit and ground-fault protection, and so forth, shall be based on the maximum operating current marked on the motor or control nameplate, or both. If the maximum operating current does not appear on the nameplate, the ampacity determination shall be based on 150 percent of the values given in Table 430.249 and Table 430.250.




Does an adjustable speed drive with a power converter as described in section 430.2 also fall into the category of an adjustable voltage, variable TORQUE drive system? Why didn't the NEC specify Adjustable Speed Drive System as they defined it in that section? I thought this referred to a hard firing SCR type system when I read it, which I believe isn't considered a power converter since it is using the exhisting line power and chopping it up. In that case, sizing the branch circuit conductors and protection to the motor would be the way to go.

Another question... Section 430.122(A) of "Section X" dealing with Adjustable Speed Drives specifies that Branch/Feeder circuit conductors be not less than 125% of rated input to the power conversion equipment. Does that mean that if I use a 20HP 460v drive on a 5HP motor, I need to size my wire at 6AWG, but use a 12 amp circuit breaker?