Hello, I have 3 30 hp motors 480 volt with a 60 to 100 foot distance to motor. (it is all in pvc underground in dedicated conduits run with a ground wire except going through the electrical room in emt, the motor disconnect and a gutter. In the electrical room they are now fed from buckets. I was planning to run conduit from mcc to the vfd location (about 20 feet of emt) on the wall then back to the bucket to splice in the vfd output to each motor. How many conduits are required? I am thinking best practice is to feed all 3 with one conduit but 3 separate pipes on output of vfds. (would save some pipe and labor if i can combine them for the 20 feet in emt in the electrical room)
3 vfd output wires in same conduit?
- Thread starter Garfields
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
I do not see any problem with feeding multiple VFDs in the same conduit.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
It's generally not a good idea to run three separate outputs in the same raceway due to the possibility of different frequencies being applied to each motor. Most manufacturers say as much in their instructions.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Inputs, no issues.
Outputs, separate conduits. Always.
Outputs, separate conduits. Always.
Running multiple VFD output wires in the same conduit is not recommended by most manufacturers because the noise generated from the pulse width modulated waveform can cause cross-talk resulting in possible nuisance tripping.
That said, I have seen and done multiple cables in the same conduit many times by using something like Chainflex cable. It has a TPE jacket and is CF35-UL.
That said, I have seen and done multiple cables in the same conduit many times by using something like Chainflex cable. It has a TPE jacket and is CF35-UL.
How many feet of being together is likely to cause a problem? For example there's a gutter they would pass through and cross each other for several feet.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
chainflex is a specially made VFD cable though. it is like you ran a conduit inside the conduit.
I don't know if there is a specific answer to that question.
If you can keep them separated in the gutter it probably does not matter much.
BTW, this is what the AB PF525 manual says.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
There are a lot of variables involved.
I had one years ago where the output cables mixed along side the input cables in a tray going over the roadway, so about 25ft at the most, the rest of they way they were in separate conduits. Killed the 250HP inverter duty motor. We scoped the leads at the motor, they were getting almost 1800V spikes. Separated the wires and it went away.
Difficult to argue with that. Thank you for your sharing your experience.
TwoBlocked
Senior Member
- Location
- Bradford County, PA
- Occupation
- Industrial Electrician
I'd see what the VFD's manufacturer's installation manual says. There may be other things not being considered. Just recently found out that power factor correction capacitors and VFDs don't mix.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
We had a job where they ran several VFD outputs in the same raceway on a roof and they had problems with the drives. Even if the problem is not related to the combined conductors in the raceway the electrician will get blamed for it anyway.
Inputs in same conduit pose no problem. Would never run more then 1 drive In a
conduit. Low voltage controls should be okay in one conduit.
TwoBlocked
Senior Member
- Location
- Bradford County, PA
- Occupation
- Industrial Electrician
Had a project with four 20hp (I think...) combustion blower fans for wood fiber dryers. Ran motor leads for two of them in one conduit and leads for the other two in another conduit. Nothing in the literature against it. Beforehand, talked with the sales rep and his only concern was if one was running and someone was working on the leads of the other there could be a shock hazard. Since this was an industrial setting with the guys VERY familiar with such a possibility, it was not a concern. I think they were Toshiba drives, but not positive.
ptonsparky
Tom
- Occupation
- EC - retired
I followed up a project where multiple fan motors were run with one drive, all in the same raceway. Motor failures were frequent. High ambient didn't help. Whole project removed because of multiple OSHA issues that could not be resolved economically.
Should never have power factor capacitors on the output side of any VFD'S. We had a contractor install two 40 HP drives on 40 year old critical air handlers and and they would not operate. Luckily they had bypass feature so they ran them on bypass until I came in middle of the night to troubleshoot problem. Each motor had a PF capacitor within feet of hard to access motors. Disconnected both PF capacitors and drives worked .
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