×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Branch Circuit Protection for multiple VFD's

Branch Circuit Protection for multiple VFD's

Branch Circuit Protection for multiple VFD's

(OP)
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?

RE: Branch Circuit Protection for multiple VFD's

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.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources