Motor calculation
- Thread starter brianthejabroni2
- Start date
ptonsparky
Tom
- Occupation
- EC - retired
You need to look at the VFD instructions.
David Castor
Senior Member
- Location
- Washington, USA
- Occupation
- Electrical Engineer
The feeder to the VFD has to be sized based on the maximum rated input current to the VFD, not the motor. (125% of the rated VFD INPUT current)
ptonsparky
Tom
- Occupation
- EC - retired
The VFD will provide the motor over current protection.
Going to use this VFD as an example:
https://www.grainger.com/product/33M548?gucid=N:N:FPL:Free:GGL:CSM-1946:tew63h3:20501231
It's max output is 1170A, so you size wires based on 125% of that which is 1462.5A, you would use 4 sets of 500s
If VFD is soft start you can just use 1500A fuses, the VFD already usually has overload protection so you won't have to worry about overload.
https://www.grainger.com/product/33M548?gucid=N:N:FPL:Free:GGL:CSM-1946:tew63h3:20501231
It's max output is 1170A, so you size wires based on 125% of that which is 1462.5A, you would use 4 sets of 500s
If VFD is soft start you can just use 1500A fuses, the VFD already usually has overload protection so you won't have to worry about overload.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Yep.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Incorrect on both items.
The rules for sizing the conductors from the VFD to the motor are no different: 125% of the MOTOR FLC from the appropriate NEC table for that HP.
The rule for sizing the conductors FEEDING the VFD are fixed at 125% of the VFD rated INPUT current, which is often different from the OUTPUT current rating that the VFD is sold by. That information is typically in the drive manual or technical data.
The rules for selecting the input protective device require STARTING with that installation manual / technical documents of the VFD. That’s because what you can use and what size it can be is dictated by the listing of the VFD. For example many are NOT listed for use with circuit breakers alone, they require special high speed fuses in their listings, ESPECIALLY at that size range. So just because the NEC says you can use a CB doesn’t mean it’s the correct device.
Then the listing will also dictate a maximum size of protective device the drive is tested with, which may or may not match up with the NEC limits in 430.52. So bottom line, there is no simplistic answer, other than what others have already said: RTFM (Read The F###### Manual).
ptonsparky
Tom
- Occupation
- EC - retired
I've experimented with VFDs, etc, but it was almost always something I could afford to replace if I screwed up. I'd be pretty hesitant about launching into this project without considerable help.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
There's not a whole lot of difference between a small vfd and a large vfd other than the amount of smoke the large vfd lets out if something goes really wrong.
ptonsparky
Tom
- Occupation
- EC - retired
And $$$$$$.
Mixed up between input and output, and was too lazy to read the manual so I made up an allowable fuse size lol.
ramsy
Roger Ruhle dba NoFixNoPay
- Location
- LA basin, CA
- Occupation
- Service Electrician 2020 NEC
Qualified peer reviewers may react, if we ass-u-me with authority.
Many of us present statements as fact, but rarely get corrected by someone who can, which perpetuates speaking with authority.
Maybe phrased like "just for fun", "my guess", "without more info", or "it seems to me" would allow more room for error.
No one showed us a VFD in this ball park, stuck their neck out, or baited others to articulate the procedure. Thanks for taking the bait.
TwoBlocked
Senior Member
- Location
- Bradford County, PA
- Occupation
- Industrial Electrician
Contact you VFD distributor. This is going to be a big ticket item and they will give you specific help. There are line and load filters to consider, too. Be good to include the thermal protection switch as a permissive into the VFD, also.
eric stromberg
Senior Member
- Location
- Texas
It's possible to have a 200 amp VFD, maximum rated input, but to have a 125 amp motor. As long as the VFD manufacturer has a rating that will drive the motor, say 140 amps, you would use 1.25 x 140 as the input rating. Some manufacturers have tables that cover this. I know of one VFD manufacturer whose tables have ratings for 100%, 95%, 90%, 85%, 80%, 75%, 70%, 65%, 60%, 55%, and 50%. You pick the rating that is the next one over the motor requirement. This is the input rating of the drive. This is what you base the conductor size for the conductor feeding the drive.