24VDC and 120VAC control circuits on the same conduit

soyxan

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Jul 2010
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Madrid
Posts
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Hi,

We are using a limit switch with 2 contacts. One of them is wired to a PLC (24VDC) for alarming and the other one is wired directly to the CCM and is used to feed a coil (127VAC) that will act as an emergency stop for a motor.

As both signals are for "control" purposes. Can we run them in the same conduit? If so can you point me to a normative that says so?

Thanks in advance!
 
Run 120VAC only through the limit switch and run the alarming side to a small 120V relay in the panel that switches panel 24V to the PLC input.
Problem solved.

soyxan said:
Hi,

We are using a limit switch with 2 contacts. One of them is wired to a PLC (24VDC) for alarming and the other one is wired directly to the CCM and is used to feed a coil (127VAC) that will act as an emergency stop for a motor.

As both signals are for "control" purposes. Can we run them in the same conduit? If so can you point me to a normative that says so?

Thanks in advance!
Click to expand...
 
GTUnit said:
Run 120VAC only through the limit switch and run the alarming side to a small 120V relay in the panel that switches panel 24V to the PLC input.
Problem solved.
Click to expand...

Yes, that will work but we have like 60 limit switches with this configuration and if I prefer to not make can any change (if there is not any problem in doing the way it is now)
 
The coil of the motor contactor could also be 24V. In my opinion you are asking for problems running 120v and 24VDC in the same switch. JMO
 
Racer5039 said:
The coil of the motor contactor could also be 24V. In my opinion you are asking for problems running 120v and 24VDC in the same switch. JMO
Click to expand...

Yes but the 120VAC comes from the CCM and we do not have 24VDC available on it.

My question is regarding if it is forbidden to mix both signals in the same conduit. No problem with the switch AFAIK
 
Unless some rule has changed recently, in the USA, you can run 120 vac and 24 vdc in the same conduit ONLY if the insulating rating of the wires match.
It is frowned upon by many.

PLEASE note - this is the USA, not your location.
You must get a copy of your local code to see if it is allowed.
you must then get with plant engineering and safety to see if it is allowed.

regards,
james
 
Ok, code aside; you can realistically do anything you want.
The risk you take is the 24V signal wires picking up inductive noise and there is the possibility of that noise switching on your 24V input.

You can also get a 120VAC input card for the PLC and feed all this 120V IO into that.
 
Is it possible for you to do a limited number of switches and see if you get any noise on the 24vdc circuit?
The conduit will not be the issue, you could use shielded wire for the 24vdc...it will be inside the switch and the switch enclosure.
 
Last edited:
There is no problem mixing voltage in the same conduit, just make sure the wires is isolated for the higher voltage. In your case having 24 and 120v there shouldn't be an issue since a "common used" wire is 300/500V rated.

My issue is: Are you running two cables to every limit switch? I don't want to mix diffrent voltage sources in the same cable.
 
GTUnit said:
Ok, code aside; you can realistically do anything you want.
The risk you take is the 24V signal wires picking up inductive noise and there is the possibility of that noise switching on your 24V input.

You can also get a 120VAC input card for the PLC and feed all this 120V IO into that.
Click to expand...

I've never had issues in the past running 120v control and 24v signal wires together assuming they're discrete inputs. Any noise created wasn't enough to trigger false DC inputs for me.

With that said, I wouldn't run 120v control power next to unshielded analog DC inputs.
 
I ran into a case where someone had run 24vdc stop button wires in the same conduit with 480v motor wires. It worked fine until they added a VFD for that motor. Then if the motor was running at 60 hertz everything was fine, but if the motor was slowed down it would induce enough on the wires to hold the PLC input on.
 
T Gibbs said:
I ran into a case where someone had run 24vdc stop button wires in the same conduit with 480v motor wires. It worked fine until they added a VFD for that motor. Then if the motor was running at 60 hertz everything was fine, but if the motor was slowed down it would induce enough on the wires to hold the PLC input on.
Click to expand...

Yea learnt his one the hard way with a system with 120 motors.!!
From then on ALL DC and AC was run in separate conduit.
 
T Gibbs said:
I ran into a case where someone had run 24vdc stop button wires in the same conduit with 480v motor wires. It worked fine until they added a VFD for that motor. Then if the motor was running at 60 hertz everything was fine, but if the motor was slowed down it would induce enough on the wires to hold the PLC input on.
Click to expand...

I could see that being an issue. I won't run any kind of 3-phase or even heavy single phase loads (IE: Motors) with 24vdc, but control power hasn't caused any problems with me in the past.

I had an issue in the past with a motor driven by up/down contactors that would cause enough noise to send a start signal to a VFD and turn on a separate motor. I solved that by putting MOV's on the contactors.
 

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