I have a three branch system. Branch one requires a certain fuse rating by the manufacturer and is the highest fuse rating for all three branches. The branch two is exactly like the branch one. The branch three is an AC motor with a variable speed drive controller.

If I am reading the code correctly, my feeder fusing to all three branches will be the sum of:
1. the fusing requirement of branch one.
2. the full load rating of branch two.
3. the full load rating of branch three.

Here is the question. Since branch three uses a variable speed drive controller, do I use the KVA rating of the controller or the FLA of the motor connected to the controller?

 

Sizings must be based on the VFD input rating, not the motor FLA, per NEC 430.2

You should closely read all of article 430, especially 430.53 and 430.24 and other parts regarding multiple motors/motors and other (combination)loads.

Very good information is available on the Cooper/Busman website, especially the publication "SPD.pdf"

Also try "NEMA.ORG":http://www.NEMA.ORG . Download standard ICS 2.2000. Unlike most other standards bodies, many of the NEMA standards and docs can be downloaded for free.

Regards

 

Responding to Rick Miller's query (Tue, Aug 20, 11:59 am):

Your interpretation of the sum you presented is correct. The following are presented for emphasis:

a) Branch three current rating shall be taken as the manufacture's full-load input rating for the power converter, not the full-load nameplate rating of the motor.

b) If the VFD is bypassed, then branch three rating shall be taken as the larger of the two sub-circuits to the motor.

c) Note that Article 430-62 has no provision permitting the next higher commercially available fuse.

Regards,
Phil Corso, PE
(Boca Raton, FL)