rivecar44
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- Chicago IL, USA (MDW)
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- Engineer
A vfd manufacturer suggests sizing line side conductors based on the rated inrush current of the device instead of the rated continuous current. Any comments.
VFD input wiring
- Thread starter rivecar44
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petersonra
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- Northern illinois
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- engineer
ptonsparky
Tom
- Occupation
- EC - retired
That is a mixed up suggestion. IMO, Inrush for a VFD would be the moment the capacitors are charged for the first time. How long does that take? I can't see any reasonably sized conductor being in danger of overheating in that time.
We just sized VFD feeders at @125% of ampere on drive labels. Can remember several of our 100 HP 480 volt drives were rated for 125 amps and the motor name p!ate stated motor draw was 123 to 124 amps. If the feed was a good distance you might have to increase the feeders to a count for voltage drop. I restarted hundreds of VFD'S after performing PM'S & troubleshooting and if the ramp up speed is at least a few seconds the I rush current is greatly reduced. Have installed Amprobe on two of the three feeders and never observed a VFD inrush current higher the motor namep!ate ampere especially on longer ramp up times.
rivecar44
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- Chicago IL, USA (MDW)
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- Engineer
For a 200 Hp vfd,
for a 125 Hp VFD, input rating 135.8 Amps, max. inrush current 236 amps. Calculating conductor size using 125% per NEC of input rating current equals 169.75 Amps, then 2/O is enough. But, manufacturer suggest minimum 3/O. When I called them, tech indicated the wire range is based on the inrush current.
ptonsparky
Tom
- Occupation
- EC - retired
Looks like the min size the input lugs accept is 3/0. That makes it easy from here.
Do you expect substantial VD because of distance?
Do you expect substantial VD because of distance?
rivecar44
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- Chicago IL, USA (MDW)
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- Engineer
no. only 10 ft
Worst part all drive companies install undersized conductors in their drives. Have performed IR scans in 60 degree rooms where their under sized wire is 20 degrees hotter. When I questioned a drive company they told me that they do not have to follow the NEC. Can still remember the first 100HP VFD that I I stalled in the 1980's. Drive was rated for 135 amps and they only used #2 copper inside of drive and yep they ran hot wasting energy 24/7. I have worked on a lot of motors that years ago were feed from a starter inside of a MCC bucket that where a VFD was installed on feeder wires that were questionable ( Thought they should have been the next guage up wire ) but going by the touch screen panel the output voltage @full speed was usually over 470 volts. We had a lot of drives that had bypass function that would draw a large in rush the infrequent few times they were on bypass. Can remember a drive tech telling me that larger drives ( maybe over 50 HP ) have something built in to limit in rush current on capacitor banks.
petersonra
Senior Member
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- Northern illinois
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- engineer
My guess is the tech does not know and just made up an answer or parroted something some else told him that does not know either. It seems like for some reason they have chosen a MCCB that comes with standard size lugs that just happen to take a minimum size of 3/0.
Keep in mind in many cases the rating of the OCPD on a drive like this can be 250%.
I don't know enough about this particular VFD to be able to comment much more on it.
- Location
- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Whomever told you that is a nincompoop. Inrush current on a VFD is not a thing worthy of consideration. You might want to find a better supplier, this person is making stuff up…
The VFD feeder (input) conductor size is dictated in 430.122, it must be 125% of the rated INPUT current of the VFD, which is usually slightly LESS than the output current rating. If you size it the same as the output conductors, you are safe, albeit possibly wasteful.
The VFD feeder (input) conductor size is dictated in 430.122, it must be 125% of the rated INPUT current of the VFD, which is usually slightly LESS than the output current rating. If you size it the same as the output conductors, you are safe, albeit possibly wasteful.
petersonra
Senior Member
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- Northern illinois
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- engineer
A lot of people selling such things are pump people and know nothing about VFDs, other than that they will solve all known problems with application of pumps and that there are good commissions to be made selling them.
I have occasionally wondered why the UL listing often allows the rating of the OCPD to be 250% when there is just no danger of inrush opening the OCPD.
Agreed.
hillbilly1
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- North Georgia mountains
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- Owner/electrical contractor
Probably the manufacturer cya for excessive voltage drop if it is a long run.
petersonra
Senior Member
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- Northern illinois
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- engineer
There is no upsizing of the conductors involved.
hillbilly1
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- North Georgia mountains
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- Owner/electrical contractor
Yes, I know, but it was a boiler plate instruction from the manufacturer.
Strathead
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- Electrician/Estimator/Project Manager/Superintendent
perhaps I am missing the nuance of the question, since some much smarter than me have already answered here, but most VFD's I have worked on have fuses or another form of overcurrent protection and act as disconnects. So the OP term "feeder" is correct, but that means the size of the feeders is based on the size of the upstream breaker. It doesn't appear the OP is asking about sizing the overcurrent protection.