How should I calculate wire/breaker sizing for 50HP motor on a 75HP VFD?

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lonewolf454

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My superior had the bright idea of replacing a hydraulic motor system with a VFD/motor combo. It is going to be a 3PH/460/50HP motor and PF700 75HP VFD. Problem is in the A-B book it calls for 100A breaker min for 75HP VFD. So I assume I will need not #4 AWG wire to VFD but instead #3. But that is recommended minimum. Per NEC, for 50HP (65FLA) motor, I calculate 81.25 for wire size, (#4) Fuse 113.75 (125A semiconductor) Breaker 162.5 (175A). Now if this breaker is to be 175A per NEC, then will I need to size wire from Breaker to VFD for 175A?

I plan on #4 AWG from VFD to motor, but with larger VFD than needed, questions arise... Any suggestions?
 
NEC 2011. 430.122(A) indicates the conductors feeding the drive shall have an ampacity not less than 125% of rated input current. The protection ahead of the drive could be sized considerably higher than the current rating of the wires, just as with any other motor installation.
 
In this case the 50hp will not inter into the calculations. As per ptonsparky the NEC will require you to size the conductors and breaker for a 75 hp motor.
 
bob said:
In this case the 50hp will not inter into the calculations. As per ptonsparky the NEC will require you to size the conductors and breaker for a 75 hp motor.
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Slight correction, the NEC requires that you size the conductors and breaker for a 75HP drive, and more specifically, the Max Amp Rating of the specific drive you are using. But in essense you are right, the motor connected to the drive does not figure into it.

Most VFDs are UL listed to provide the motor thermal OL for a single motor connected to it, and most of the MAJOR suppliers also list the VFD to provide the Short Circuit Protection for a single motor connected to it.
 
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I'll have to set it up using 50HP data so it will fault/trip when motor has problems.

So, I should use 3AWG wire from 175A breaker to 100A class J fuses, to VFD and on to motor with 3AWG in case some brainstormer decides to up the motor to 60/;75 hp later on. Correct?
 
I agree with the others.

I've done a drive for a well that was a bit oversized. It required 300mcm to the drive, but only 4/0's to the motor. Seems weird, but I just go with it....

This is an easy one to get popped on by the inspector if you're not paying attention.
 
lonewolf454 said:
I'll have to set it up using 50HP data so it will fault/trip when motor has problems.

So, I should use 3AWG wire from 175A breaker to 100A class J fuses, to VFD and on to motor with 3AWG in case some brainstormer decides to up the motor to 60/;75 hp later on. Correct?
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As far as you are concerned, it has nothing to do with "some brainstormer", it has only to do with this:
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 to the
power conversion equipment.
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So as has been said now several times, forget the motor. Find out the input current of the drive and start there. Everything else is meaningless until this section is satisfied. So using the input current as a reference point, multiply it by 1.25 and select the next wire size up that is closest. Fusing for the VFD is going to be whatever the VFD manual says is the right fuse for the VFD, in fact it may be very specific because the VFD was UL listed with those specific fuses.

What you do from the drive to the motor is now important only to you. When you program the drive for the motor nameplate FLA, the drive will become the protection for those conductors. If you want to size them for a 50HP motor, so be it. If you want to accommodate "some brainstormer" by putting in conductors appropriate for a 75HP motor, he will thank you for it some day in the future. And remember, that brainstormer could end up to be you!
 
I calculated 90.37A based on drive current rating 60HP HD, so we should be good with 3AWG. This is 60HP HD rating (forget 75HP Standard duty). 60HP is the largest the brainstormer will be able to go. :cool:

The boss wanted to use the 75/60HP VFD because we already stock this, less inventory to keep up with. Forget that it costs more in initial cost, to wire, and energy...
 
lonewolf454 said:
I calculated 90.37A based on drive current rating 60HP HD, so we should be good with 3AWG. This is 60HP HD rating (forget 75HP Standard duty). 60HP is the largest the brainstormer will be able to go. :cool:

The boss wanted to use the 75/60HP VFD because we already stock this, less inventory to keep up with. Forget that it costs more in initial cost, to wire, and energy...
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You don't calculate the drive input current...you take it from the drive nameplate.
 
lonewolf454 said:
... The boss wanted to use the 75/60HP VFD because we already stock this, less inventory to keep up with. Forget that it costs more in initial cost, to wire, and energy...
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Perfectly valid reason in my book. It costs money and/or ties up budget dollars to have multiple similar spares of things that don't fail very often.

But the energy use in the larger drive is probably not going to be more in this case. The motor energy use is the same, losses in the VFD are based on that. In fact because the wire size is higher, you may actually end up with ever so slightly less I2R losses in the wires. Not enough to get excited about though. The only time the energy use might go up noticeably from using an over sized drive is if you have a big jump in the VFD frame size where it has bigger cooling fans etc. I don't think that would be the case from 50-60HP.
 
ptonsparky said:
90.1 amps @ 75 HP
72.3 @ 60 HP heavy duty.

90.1*1.25=112.625

IMO #2 cu @ 75 degree.
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One thing I've been wondering with regards to 430.122 and the way VFDs are name plated now, will an inspector insist on the conductors being sized for the "75HP Standard Duty" current rating, or the "60HP Heavy Duty" current rating, if that's the way you are using it? It used to be that VFD mfrs had two different versions of the VFDs, VT and CT with different nameplates even if they were the same drive. Now they all are tending to but these labels on them that say both ratings. So which one is the one the AHJ will insist on being followed? I would imagine the highest one since that's the intent of the section, but can any of our inspector members fill in this blank?
 
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