Adjustable Speed drives - sizing fuses for large drive/small motor

[jeffl.]

We build electrical panels with ASDs. Most of the time, the customer uses the same Hp motor as the drive. Occasionally, the user will buy many of the same drives for many motors. For instance a 100HP drive for his 100HP, 75HP and 60HP motors. This cuts down on his inventory.

My question is does he really need to change the fuses each time the motor is changed to a smaller rating?

Referring to NEC 2008...I am not able to produce a statement that say he must change the fuses, since the drives we use do meet 430.124 and 430.126 for overload and overtemperature protection.

So, do they really need to use a smaller fuse to meet NEC? (I worry, because it may required changing fuse blocks to get smaller fuses)

Thanks,
Jeff

 

[jim dungar]

My question is does he really need to change the fuses each time the motor is changed to a smaller rating?

No.

The fuses protect the input circuit, the drive protects the load.

The feeder to the VFD must be sized, and therefore may be protected up to, the FLA input of the drive. A 100HP must be treated as if it is feeding a 100HP motor regardless of the actual connected load.

 

[jeffl.]

Jim,

Thanks for the prompt reply.

One of our users is stating that Table 430.52 says everything must be rated off motor full load currents, but I'm in total agreement with your statement

"The fuses protect the input circuit, the drive protects the load."

The fairly new section, X. Adjustable Speed Drives, clears some of this up, IMO.

Thanks for the clarification you provided.

THanks,
Jeff