OCPD for a VFD

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mull982

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When sizing an OCPD for a VFD should the continuous current rating of the VFD be used? I notice that there are several current ratings on the VFD and that for each rating it lists two currents for ND/HD. What do these ND/HD terms mean?

What section of the code specifies these OCPD ratings for a VFD. I've always assumed it was 125% of the VFD current rating.
 
ND and HD mean "Normal Duty" and "Heavy Duty", a variant of what used to be referred to as Variable or Constant torque.

On the VFD nameplates, it should tell you a maximum HP or kW related to those ratings. If you are using the VFD at the kW rating associated with one rating or the other, use the maximim amps for that use. Then use 125% of that value to select your OCPD. For example, the nameplate likely says "50HP 62A HD: 60HP 74A ND" (I am making those up). So if you use it on a 60HP motor in a variable torque application, size everything on 74A.

You can read the NEC on-line at
http://www.nfpa.org/freecodes/free_access_document.asp?id=7005SB

You have to sign up and you can't copy/paste anything, but it's free.
 
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