mixing 4 to 20 milliamp wires with 120 volt wires

Status
Not open for further replies.
F

factoryrat

Member
Location
Detroit
We are replacing a large control valve in our factory. The old valve only had two wires in a single, small, cable for a 4 to 20ma signal. The new valve requires an additional pair of wires for 120v. Can both the 4 to 20 ma wires and the 120v wires be run in the same, existing, conduit? It would make the job a lot easier if we could combine the two circuits in the existing conduit and not have to run an additional conduit.
 
It would really depend on where in article 725 these wires would fall. More specifically class 2 or 3 conductors would require separation.
 
Another issue to consider is electrical noise which could be a problem if the 4-20mA and 120V runs parallel for a long distance even if the 4-20mA cable is shielded.
 
We use a cable with twisted pairs and an insulation rating of 600 vac so we may connect low voltage wires within the same box without a divider. There has never been a noise issue we know of, but we do endeavor to minimize any parallel runs unless a separation of more than 18" is maintained. We have also connected many control devices using 4-20 ma as a control whose wires terminate in control boxes with 480 volts.
 
Last edited:
gwpowell said:
Another issue to consider is electrical noise which could be a problem if the 4-20mA and 120V runs parallel for a long distance even if the 4-20mA cable is shielded.
Click to expand...

4-20 mA circuits are such a low impedance that noise is almost never a problem, at least as a practical matter.
 
I can not find any voltage rating stamped on the 4-20ma cable. Therefore I do not know what the insulation rating is. Do I have to assume it might be a lower voltage cable and pull the 120v wire in a seperate conduit just to be safe? I want to do it the right way.
 
yeah, we regularly pulled twisted 16 or 18 twisted that was 600 volt. It was on the spool as noted above.
 
If you want to do it right, run a second conduit for the 4-20ma signal wiring.
Twisted pair if possible.
You are already questioning the application, which is a good sign that you want to do it right. My gut tells me you will.
 
yanici said:
yeah, we regularly pulled twisted 16 or 18 twisted that was 600 volt. It was on the spool as noted above.[/quote

Agreed, but it sounds to me like he doesn't have the spool to check on the voltage rating. Without that information I would suggest separate conduit.]
Click to expand...
 
Typically, 4-20 mA are Art 725 Class II limited energy system, not "low voltage"
A class II circuit requires separation, even if wired with 300V or 600 volt rated insulation, see part II of Art 725 and 725.54 The power supply determines if its a class II system. You can introduce a Class II circuit into an enclosure or junction box for termination, but it has to be separated by a barrier and minimum of 1/4" maintained at terminals.
Also a Class II circuit can be reclassifed as Class 1 remote control and signaling circuit, and in that case its wired with a chapter 3 wiring method and can share the same enclosure, box or raceway with power and lighting if functionally related. I am camping at the ocean, wi-fi campground and don't have my NEC with me, so can't quote much else from memory.
Also, 4-20 mA is very immune to noise as its a twisted pair wiring, but the issue is the 4-20 ma devices can be damaged if the insulation fails or 120 volts is applied to the 4-20 "low voltage" equipment.
 
tom baker said:
but the issue is the 4-20 ma devices can be damaged if the insulation fails or 120 volts is applied to the 4-20 "low voltage" equipment.
Click to expand...


Not only can the 4-20 ma conductors be damaged, but a voltag can be imposed on the conductor, and be imposed onto the Class II equipment potentially causing a fire or shock.
 
Status
Not open for further replies.
Top