VFD Feeder Sizing

[mls1]

I am a bit confused by the 2014 NEC with regards to VFD feeders and I'm hoping light can be shed. The feeder conductors must be sized for 125% of the DRIVE rating or 125% of the MOTOR FLA if a bypass device is used and the MOTOR FLA is higher than the drive rating. So far this makes sense. Overload protection for the MOTOR can be provided by the DRIVE and must also be provided IN THE BYPASS CIRCUIT per Article 430. The disconnecting device to the DRIVE must be sized to 115% of the DRIVE rated input current. Still good so far. But then 430.130 short circuit protection has to be based on the MOTOR per 430.52. That creates two questions for me:

1. Does this basically imply that the feeder breaker must be undersized or at least have an adjustable instantaneous that can be set down to within Table 430.52 for the motor fed by the DRIVE?
2. Since 430.52(C)(3) is referenced that would seem to imply that a combo starter with HMCP can be used to feed a VFD but 430.130(3) says "self protected combination controller shall only be permitted where specifically identified in the manufacturer's instructions for the power conversion equipment." I don't recall VFD manufacturer's listing combo starters for use with their equipment. Is this something I've missed and am I interpreting this correctly?

Thanks!

 

[jraef]

The VFD mfrs are not concerned with what the protection for the VFD or the motor is when you are in Bypass. So your Bypass circuit is on its own so to speak. Many small VFDs are listed in series with the little self-protected starters because of cost and size, and because regular CBs will not come smaller than 15A anyway, which may be too large to adequately protect a very small VFD (without fuses in between).


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[mls1]

Thanks, jraef. I've been discussing this with a number of different suppliers and the lack of direct info has been surprising. One thing I did learn is that Allen Bradley started switching over to thermal mag breakers for their integrated VFDs about a year ago citing the difficulty of recertifying the tested MCP under UL508C. It would seem the easy answer is just use thermal mags and call it good but some industrial customers are insistent on combo starters with the reason being that if the VFD craps out on the mid-watch they can just run the T-leads straight to the starter and get the line back up. Is this a common practice others are seeing and is it being done in compliance with the NEC?

 

[jraef]

The reason Rockwell switched is simply because UL ceased to accept mag-only (MCP) breakers ahead of VFDs (and soft starters by the way), regardless of brand. That change actually took place a few years ago and any new product releases needed the full thermal-mag protective devices, but UL allowed legacy approvals to continue for a while. That clock ran out last year.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington