Branch Circuit Protection for multiple VFD's 1

[Scotty61]

Regarding NEC 2008, I can't seem to find information on the use of a single branch circuit protection device with 2 or more LV VFD's. Article 430.53 leads me to believe this is not permitted. Could anyone shed some light on this subject?

 

[jraef]

It depends (doesn't everything?). Most of the major VFD manufacturers have their VFDs UL listed to provide the Short Circuit Protection for the motor it is connected to, but that doesn't relate to protection of the circuit feeding the VFD. So if you have multiple VFDs on one branch circuit, and the branch circuit wiring and protection device fits within the rules for protecting the total circuit, then no problem.

As an example (I'm making this up as I go here, so give me some slack). You have 2 x 5HP 460V VFDs. Each of those VFDs protects the motor and motor leads, so now you must just protect the VFD itself and the wires leading to it. Your 5HP 460V VFDs are rated 10A max. each, so the wiring (and therefore the SCPD) must be a minimum of 125% of that max. amp rating of the VFD, i.e. 12.5A. So you COULD protect each VFD with a 15A CB and 14ga wire. But you could also use 10ga wire to feed them both from one 30A CB; the 30A rating covers the 125% minimum of any VFD and will carry the total circuit load (125% of the largest plus the total remaining load). Each VFD then re-protects the motor circuits down stream from themselves even though the upstream SCPD might be technically too large.

But not all VFDs are so listed; you have to ask and it is not something the average salesman will know definitively. Don't accept a flippant answer, get it in writing. if they can't produce a document saying the have that listing, then you have to size each circuit per it's own rules.


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