dswilliams
Member
We have a fixed speed, 3 phase, 480 volt motor for a water pump that is getting a VFD installed on it. Can we use existing conductors?
VFD retro conductor sizing
- Thread starter dswilliams
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ptonsparky
Tom
- Occupation
- EC - retired
430.6(C)
You may have to change the wire on the input side of the drive, but from drive to motor should be ok, if it was properly sized before. Don't forget the load reactor per installation instructions.
You may have to change the wire on the input side of the drive, but from drive to motor should be ok, if it was properly sized before. Don't forget the load reactor per installation instructions.
The input current will most likely be lower than before but I don't know what the code requires.
The other thing consider is the type of cable between the drive and the motor.
Normally we use three or four core armoured cable which is typical for here (UK) but I know that in USA it is quite common to use individual conductors in conduit and that isn't generally recommended for variable speed drives.
The feeder conductors to the VFD must be sized to the VFD, not the motor. 430.122
chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
Why do you say that, have I missed that reading the 'books' that come with them?
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
In most part thats because Europe seems to encourage the use of cables over conduit and shielded cable keeps the radiated RF level lower. Same reason why stuff going to EU has all those miserable ferrite beads on everything.
Not to say that there are no other advantages to VFD cable over some wires in a piece of conduit.
Possibly. I'll dig up some images from manuals when I not quite so tired.
TxEngr
Senior Member
- Location
- North Florida
DSW -
Assuming that the cable was sized for 125% of the FLA of the motor, it should be fine from an ampacity standpoint. But as was pointed out, on PWM based VFDs, it is generally recommended to use a drive rated cable consisting of 3 conductors with 3 symmetrically arranged grounds in a continuous armored cable. This is to reduce the possibility of reflected waves creating a voltage spike that will kill the motor and/or cable feeding it. If you have single insulated conductors, I would consider replacing the cable, particularly if it's over 50 feet long. You might also want to make sure your motor is inverter duty rated. Or just run it till it fails and replace it then with a rated motor.
Assuming that the cable was sized for 125% of the FLA of the motor, it should be fine from an ampacity standpoint. But as was pointed out, on PWM based VFDs, it is generally recommended to use a drive rated cable consisting of 3 conductors with 3 symmetrically arranged grounds in a continuous armored cable. This is to reduce the possibility of reflected waves creating a voltage spike that will kill the motor and/or cable feeding it. If you have single insulated conductors, I would consider replacing the cable, particularly if it's over 50 feet long. You might also want to make sure your motor is inverter duty rated. Or just run it till it fails and replace it then with a rated motor.
weressl
Esteemed Member
If the motor is not inverter rated, you could have a problem with that too. You need to look at the ASD manual and see what their wiring requirements are. The distance of the drive from the motor can add to your problem. Existing wire type, insulation and wiring method was missing from your question and without all the above information no answer can be given.
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