Motor Power and Control Wires in Common Conduit 7

[nightfox1925]

I am presently designing a power and control wire routing via conduits from the associated LV MCC starter unit to the pump motor/local PB in the field. The power and control wires are routed into an adequately sized conduit where they split up once it reaches the pump/PB station through a GUAT conduit body.

I got a directive from my superior that the power and control wires should be routed in separate conduits. I asked him what is the basis and informed that it is an engineering practice.

I don’t see any reason why it should not be in common conduit since the power and control wires are 600VAC insulated and are coming from a common MCC starter.

Is this a code violation or not a good engineering practice?

 

[ausgen]

In Aus, the main issue is seperation of different voltage levels. If your control voltage 24 V ac or dc, the LV in the same "wiring system" can cause an EMC issue.

 

[JBUDA54]

There is a code issue such that you may not mix different voltage insulation ratings in the same tray/raceway. ie 15kV with 5kV or 5kV with 600V or 600V and 300, etc.

Ausgen is right on the mark with the induced voltage on low level signals such as 24Vdc. Normally the low level signal cables are rated at 300V where your power cable is 600V making you segregate them. Some contractors run all 600V cable thinking they will not violate code and run into interference problems with the 4-20mA signal cables. So eventhough you may pass the code it is not good practice to mix power and signal cables.

I reccommend reading through NEC sections 310.15(B)(2)(a) and 392.11. Your supervisor may be trying to steer clear of the possibility of having to derate the power cables by Table 310.15(B)(2)(a) as well.

 

[nightfox1925]

I forgot to mention that the power conductors are 480VAC and the control wires are 120VAC originating from the starter's 480-120VAC CPT wired to the Start-Stop pushbuttons to control the AC contactor coil inside the starter unit.

 

[nightfox1925]

"I forgot to mention that the power conductors are 480VAC and the control wires are 120VAC originating from the starter's 480-120VAC CPT wired to the Start-Stop pushbuttons to control the AC contactor coil inside the starter unit."

Any comments on this one?

 

[dpc]

It is allowed by the NEC as long as the 120 V circuit is insulated for 480 V.

I don't have a big issue with it in this case and this has been commonly done for decades. I would not say it is a bad engineering practice, at least at this point in time.

However, some organizations have strict rules about separating 120 V and 480 V circuits. So when in Rome....

Also, with increased concerns over electrical safety and arc-flash, some industries are moving away from 120 V ac control and going with 24 V dc for everything, including motor control circuits.

 

[Electic]

We have done this for small motors fed and controlled with 600V insulated wires, however; with larger motors there is a hazard that large motor feeder cables will damage the small gauge control wires. Most plants don't want #12 controls mixed with something like #2 or bigger power cables.

Gary B

 

[jraef]

I have seen several installations where this was done to save conduit, only to discover after the fact that you can indeed induce enough voltage on a 120V circuit to prevent a Stop button from de-energizing the control circuit, even on 120V systems. These are usually where for some reason the control circuit has a small interposing relay in the Stop circuit, which is also not a good practice, or the motor controller is something like a soft starter which uses PCB mounted 120V relays in it's control circuit. But the issue is, if you always separate the circuits, you NEVER have that problem, even if someone changes the equipment at a later date.

 

[dpc]

I agree that this is not normally done on very large 480 V circuits - smaller motors only.

 

[nightfox1925]

Thank you very much gentlemen. Mine is just a 10HP motor with (now)3-#10AWG(power) and 3-#14AWG(control) plus 1-#12AWG ground in a 1 inch conduit.

 

[rovineye]

My cables are not in raceways or conduit, but I do keep separation distances for EMI purposes. In your case, since the control circuit is 120 volts and IF it is less than 20 amperes, I would need to separate them by at least 200 mm, or put them in separate conduits. Not by rule, but by painful experience.

 

[controlsdude]

you can get away with 480vac and 120vac in the same conduit. As long as the motors are NOT being supplied by VFD drives. Then you need to seperate the 480vac from the 120vac and also seperate the 480vac for each vfd in seperate conduit.

But if no VFDs then you can mix like other said as long as the insulation is rated for the highest voltage.

If you have DC wires then these need to be seperated into seperate conduit, I do not believe you could get the proper result of a reliable control system if this is not the case.

I was involved in a project where we had bad electricians who ran the 480vac that had vfds with the 120vac.

Visiually we had estop indicating lights that would turn on when vfd motor would ramp and down. Also problems with radom firing of devices.

later we had to go in and put MOV on the io since we had noise bleed over from the 480vac vfd wiring.

some experiences can painful.

 

[jraef]

Again, another reason to just separate them as a rule. What if someday someone wants to make that 10HP motor vary it's speed? If you want to change to a VFD and the conduits are already separate, no problem. If not, your decision now has to include the cost of running a new conduit. I say just listen to the boss.

 

[davva]

Page 32 of the attachment gives some guidance

 

[resqcapt19]

night,

Thank you very much gentlemen. Mine is just a 10HP motor with (now)3-#10AWG(power) and 3-#14AWG(control) plus 1-#12AWG ground in a 1 inch conduit

That is probably a NEC violation as to the size of the EGC. In general with power conductors sized #10 and smaller the rules in 250.122 will require that the EGC be the same size as the power conductors.

 

[nightfox1925]

The EGC size is based from Table 16 of CEC 2006 based from a 30AT MCCB rating. This application is in Canada.

 

[resqcapt19]

night,
I missed the reference to the CEC. The NEC would require a #10 if the breaker is an thermal magnetic breaker, but permit a #14 EGC if the motor is fed from a listed combination starter that uses a magnetic only device for the ground fault and short circuit protection of the motor conductors.

 

[rockman7892]

After reading these posts I am wondering why specifically you do not want to run 120V controls with 480V power feeders controlled by a vfd? Can someone explain how a vfd would change the situation of running these together?

 

[itsmoked]

rockman7892; To control motors VFDs create complex waveforms. To do this efficiently they create them from full voltage ON/OFF signals. This means high frequencies are generated due to the sharp ONs and OFFs. These higher frequencies more easily couple across the mutual capacitances and inductances normally present when regular wires are pulled thru conduits together. These coupled signals can often raise havoc with other control devices.

There's your answer.

Keith Cress
kcress -

http://www.flaminsystems.com