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24VDC and 120VAC Control Circuits on the Same Conduit
Using a limit switch with 2 contacts 24VDC and 120VAC control circuits on the same conduit

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!

1 out of 2 members thought this post was helpful...

soyxan,

The general practice is not to run AC and low-level DC (usually defined as less than 50 VDC) in the same conduit, but like everything in life there are very few hard and fast rules. The real problem arises when there is high current flowing in either circuit, and particularly if that current is being switched off and/or on, and how long the conduit is that the two circuits are run through.

If you used twisted, shielded pair wiring for one or both circuits and properly grounded the shield drain wire there probably wouldn't be any issues. If the coil for the emergency motor stop circuit isn't large (say, 0.5 A or less) and isn't being continually switched on and off, then it's probably not going to an issue.

The length of the conduit the two signals are run together in can also be an issue. If it's a few meters, that's probably not an issue. If it's 30- or 50- or 100's of meters, that could also be a problem--but one that might be mitigated with the use of properly grounded twisted, shielded pair wiring.

As for a normative, well, that's subject to a LOT of things. Local or regional or national technical regulations or specifications can vary widely from place to place. Also, some companies have their own guidelines (often times NOT published and NOT included in any equipment specification, and quite frequently put in place after a bad experience from a previous piece of equipment caused lots of problems--which is HARD to try to counter). Your routing may be just fine, but because they had bad experiences with similar routing in the past they just may not accept it. No matter what any normative may say.

So, the amount of current being drawn by either circuit (DC or AC) as well as the frequency of switching on and off of either circuit as well as the length of the conduit through which the two circuits are run and the type of wiring used can affect the probability of problems caused by running the two circuits in the same conduit.

Hope this helps!

1 out of 1 members thought this post was helpful...

CSA's response covers it, but wise to separate the voltage classes when you have the option.

By Bob Peterson on 1 February, 2017 - 11:13 pm
2 out of 2 members thought this post was helpful...

Well if you're in the United States there's a good chance that both of these are class 1 circuits and can certainly be run together by code. It's possible the D C circuit might be a Class 2 circuit which would be a different case. Whether it's a good idea or not is something else but with plc DC inputs it probably won't matter. Note use of the word probably.

By W.L. Mostia on 2 February, 2017 - 11:08 am
1 out of 1 members thought this post was helpful...

One other thing to consider since the wiring typically ends up in the same junction box. It is not good engineering practice (a maintainability & safety issue) to mix signal levels when one of them represents a shock hazard and one does not. All it takes is a careless technician working on the DC circuit to get on the 120 VAC by accident and then you will have a potential shock victim. Guidance from IEEE 518 would recommend separation.

Noise can be coupled in two ways in signal cables, inductively (current based) and capacitively (voltage based). Twisted pair helps minimize the inductive coupled noise and an aluminum shield helps minimize the capactively coupled noise, if properly grounded. So if you must put them in the same conduit, use twisted pair, shielded cable for both and clearly identify the 120 VAC circuits at your terminations and provide appropriate guarding (>50V per NEC).

William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)
SIS-TECH Solutions, LP

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