UL508 Motor Disconnect SCCR After VFD

[herding_cats]

Adding MOCPD on the LINE side is fine. Fuses, disconnects, breaker, whatever. Putting a fuse or disconnect on the LOAD side of the drive creates the potential to destroy the bus on the drive. There is no NEC or UL regulation or code requirement that I know of. Been a YASKAWA dealer for 10+ years. Never heard of it.

 

[EC Dan]

With all of this said, I'm still a bit hazy on how I can put forth an argument to my UL inspector where I can state that I do not need to install the recommended ampere rated Class J fuses ahead of the motor disconnect if it is used after a VFD.

SB4.2.1 Exception No. 5: Components installed on the load side of a variable-speed drive where the variable speed drive has built-in electronic short circuit protection, are not required to have a short circuit current rating.

 

[Jraef]

SB4.2.1 Exception No. 5: Components installed on the load side of a variable-speed drive where the variable speed drive has built-in electronic short circuit protection, are not required to have a short circuit current rating.

Boom… there you go!

 

[petersonra]

Look closely at the supplement in UL508a that tells you how to calculate the SCCR of the control panel. There is a note or exception in there that covers equipment downstream of a vfd.

 

[JovialBulge]

Thank you all for the advice and answers here!