# Thread: VFD cabinet feeder sizing

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## VFD cabinet feeder sizing

Having a dispute on the proper sizing of a feeder from a fused bus disconnect to a VFD panel. The input current to the VFD is 207A. The molded case circuit breaker ahead of the VFD is sized at 400A per the mfg recommendation. The only load in the VFD panel is the VFD; there are no other continuous or non-continuous loads. So the thought is that the feeder is sized (per section 215.2) for 125% of 207A = 258.75A (300MCM). The other contention is it should be sized at 125% of 400A = 500A. (1000MCM) but that just doesn't make sense. The breaker isn't the load, it's sized for the VFD. Thoughts?

2. Any idea where "The molded case circuit breaker ahead of the VFD is sized at 400A per the mfg " comes from if the load is 207A?

Is there a bypass for across the line starting too?

3. Originally Posted by ckline520
Having a dispute on the proper sizing of a feeder from a fused bus disconnect to a VFD panel. The input current to the VFD is 207A. The molded case circuit breaker ahead of the VFD is sized at 400A per the mfg recommendation. The only load in the VFD panel is the VFD; there are no other continuous or non-continuous loads. So the thought is that the feeder is sized (per section 215.2) for 125% of 207A = 258.75A (300MCM). The other contention is it should be sized at 125% of 400A = 500A. (1000MCM) but that just doesn't make sense. The breaker isn't the load, it's sized for the VFD. Thoughts?
The rule in the NEC (430.122) is that the conductors are sized at a minimum of 125% of the VFD input amps, not 125% of the OCPD.

430.122 Conductors - Minimum Size and Ampacity.
(A) Branch Feeder Circuit Conductors. Circuit conductors
supplying power conversion equipment included as
part of an adjustable-speed drive system shall have an ampacity
not less than 125 percent of the rated input current to
the power conversion equipment
125% of the OCPD is a ridiculous concept, VFD or no VFD.

2nd issue, as raised by ron.
If you read the VFD installation manual that says "400A" for that drive, you will likely see that is a MAXIMUM size recommendation, because the VFD mfr does not know whether you will be using a bypass or not, but they DO know that the VFD has not been tested or listed with a breaker LARGER than 400A. There was no real reason to actually go that large if there was no bypass, 300A would have been just fine.
Last edited by Jraef; 05-23-19 at 12:28 AM.

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Thanks for the feedback!
There is no bypass, just the VFD.
The VFD came "factory assembled" and mfgs always default to worst case. Since the drive can output up to 200% over current depending on the setup parameters, they size the OCPD for that. VFDs always seem to be a bit of a sticking point since they can be setup and configured for their specific application. Input currents can differ simply based on whether you're running in normal duty or heavy duty mode. But code doesn't take that into consideration. That same drive i mentioned in my initial post can be set to run in a mode that limits input current to 170A, but the way the code reads, we'd still have to size the incoming feeders at 125% of 207A since both 207A and 170A are on the drive nameplate.

5. I think code should allow conductor sized at less than drive rating if conductor and overcurrent protection are at least 125% of actual load expected. Conductors are protected, drive is certainly protected if it can handle more than this anyway. I have done this before and where it comes into play is if you already have a drive that is larger then the application calls for. In my application one time it made no sense to use a drive that originally was supplied by a 400 amp breaker (we no longer used the equipment it was driving, but had a need to try varying speed of a smaller motor to see if it gave us favorable results) to supply a new load that held just fine on a 100 amp breaker, and had conductors sized accordingly.

6. Originally Posted by ckline520
Thanks for the feedback!
There is no bypass, just the VFD.
The VFD came "factory assembled" and mfgs always default to worst case. Since the drive can output up to 200% over current depending on the setup parameters, they size the OCPD for that. VFDs always seem to be a bit of a sticking point since they can be setup and configured for their specific application. Input currents can differ simply based on whether you're running in normal duty or heavy duty mode. But code doesn't take that into consideration. That same drive i mentioned in my initial post can be set to run in a mode that limits input current to 170A, but the way the code reads, we'd still have to size the incoming feeders at 125% of 207A since both 207A and 170A are on the drive nameplate.
The 200% does not factor into it, drives will only deliver that 200% for 2-3 seconds before they shut down to protect themselves. If you think about it, if I size conductors for FVNR starting of a motor, I size them at 125% of the NEC table of FLCs for that HP size. But that motor may take 600% current for 2-3 seconds on startup, yet I am not required to oversize the conductors. That is baked into the sizing rules, so 200% for 3 seconds is irrelevant to those rules.

When you USE a drive at its lower Constant Torque drive current rating, you use the lower input current rating of that drive in sizing the feeder conductors. Yes, both input current values may be shown, but they will apply to different HP ratings for the motor. If you use a CT rated drive on a VT load of the same HP as the CT rating of the drive, yes, you will have to size everything based on the HP used, which was your decision in using more VFD than was necessary. It’s the same if you use a VFD that is 2x the size of the motor FLA because you are feeding it with single phase, your input conductors will have to be sized for the input current rating of the VFD (something that lots of people miss).

The OCPD put into a combo VFD unit made by the mfr is more of an economic decision than anything else. For THEM, there is going to be no difference in cost between a 300A breaker and a 400A breaker, so they might as well use 400A just in CASE the user wires their own bypass in the field. From a protection standpoint though, it’s all somewhat irrelevant in that the VFD has better protection in it for the motor circuit than the OCPD is going to provide, and VFDs had to be listed AS the motor OCPD starting back in 2005. So the breaker is really only providing protection for the VFD itself and since by definition of anything goes wrong inside of the VFD it is ALREADY toast, all you are really doing is preventing a fire at that point.

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