Combining motor circuits

[mstrlucky74]

If you had some motors that are 20&30a circuits would you combine the circuits in one conduit if in the same area or do you run a separate conduit for each circuit???? I know the spec sometimes dictates. But I hear different opinions on this regarding motors.

 

[augie47]

If they are simply AC Poly-phase or single phase motors with across the line starting there is definitely no problem in sharing a conduit. Keep derating in mind.

 

[iwire]

I agree with Gus unless there are VFDs involved, in that case I would keep them separate.

 

[mstrlucky74]

I agree with Gus unless there are VFDs involved, in that case I would keep them separate.

Why would you keep them separate if VFD's are involved?

 

[mstrlucky74]

If they are simply AC Poly-phase or single phase motors with across the line starting there is definitely no problem in sharing a conduit. Keep derating in mind.

And what if they are 3 phase? Thanks

 

[petersonra]

Code wise it makes little difference mostly, although it could affect derating issues, but probably will not.

VFD wise, it probably makes no difference but it might.

 

[augie47]

And what if they are 3 phase? Thanks

poly phase includes 3 phase. again, as stated, as long as no drives are involved.

 

[mstrlucky74]

poly phase includes 3 phase. again, as stated, as long as no drives are involved.

And why is my question. Thanks

 

[ActionDave]

Why would you keep them separate if VFD's are involved?

Putting the feed from two drives in the same pipe would be like putting two alley cats in heat in the same room with a tom cat. Nobody is going to get along. It's just not a good idea.

 

[GoldDigger]

The high dV/dt (square edges of voltage waveform) cause high capacitive coupling between wires of the two circuits. Not good.

Tapatalk...

 

[ActionDave]

The high dV/dt (square edges of voltage waveform) cause high capacitive coupling between wires of the two circuits.

You probably stopped kids from playing with sharp objects because someone might get an eye poked out.

 

[GoldDigger]

Nope, but I did stop them from trying to poke each others eyes out.
One set of VFD output wires which are not designed for VFD use can experience standing wave problems. Putting two different VFD outputs in the same raceway, running at different frequencies at different times just increases the potential problems.

Tapatalk...

 

[mstrlucky74]

Ok, got it but we are talking line voltage wires only, not control right?

 

[GoldDigger]

Right. Motor drive voltage, which may be more or less than line voltage.

Tapatalk...

 

[Fulthrotl]

Ok, got it but we are talking line voltage wires only, not control right?

while you can put control wires in with the motor control wiring in the same conduit, if the insulation for both
is sufficient for the highest voltage present, and you can combine motors in the same conduit, provided you
use appropriate derating, but you would be a flaming fool to do that with VFD's, which modify voltage and frequency,
and parallel conductors will have induction, the longer the run, the more the induction.

if you don't understand why induction and harmonics are a problem here, then that is a problem in itself.

 

[petersonra]

while you can put control wires in with the motor control wiring in the same conduit, if the insulation for both
is sufficient for the highest voltage present, and you can combine motors in the same conduit, provided you
use appropriate derating, but you would be a flaming fool to do that with VFD's, which modify voltage and frequency,
and parallel conductors will have induction, the longer the run, the more the induction.

if you don't understand why induction and harmonics are a problem here, then that is a problem in itself.

I think you are well advised not to run multiple sets of motor feeders in the same conduit if they are VFD controlled. But, people do it on an almost routine basis, especially with smaller motors. While I believe doing so is well below sub-optimal, I have yet to see a case where there were any actual negative effects from such an installation. that does not mean i think it is a good idea or that there has never been a case were doing so resulted in some negative effect.

 

[gk351]

I think you are well advised not to run multiple sets of motor feeders in the same conduit if they are VFD controlled. But, people do it on an almost routine basis, especially with smaller motors. While I believe doing so is well below sub-optimal, I have yet to see a case where there were any actual negative effects from such an installation. that does not mean i think it is a good idea or that there has never been a case were doing so resulted in some negative effect.

There shouldn't be any problem running the line side wiring to the VFD'S in 1 conduit is there? Then obviously have 2 separate conduits going to the motors?

 

[petersonra]

There shouldn't be any problem running the line side wiring to the VFD'S in 1 conduit is there? Then obviously have 2 separate conduits going to the motors?

I probably should not have used the term motor feeders when I really meant the wires going out to the motors.

I don't see any problem running the line side conductors to the VFDs in the same conduit.