Combining Wires in Conduit


Thread Starter

Mark Ray

We end up with a variety of cables in conduits in a large industrial plant more and more. So what guidelines or resource do you use to decide what cables are OK to combine in a conduit or cable tray? There are RIO,
Ethernet, A/V, Analog, RS232, Devicenet, ASI, phone lines, etc... plus 24vdc.

Well, 3 phase and 110ac surely belongs by itself but what about the other intelligent cables. A little direction would be helpful to sort out all the different
combinations that are faced in todays industry.

Thanks, Mark

I have been in the airport baggage industry for over 10 years as well as other industries that have the same conditions.

We specify that a contractor can ONLY run 480 and 110 in the same conduit / trough. All other comms, including DC may be run in a separate conduit / trough together.

Keep in mind, in a motor control cabinet, the comms must cross at a 90 degree angle from power sources.

We do this on large scale projects from 10 to 2000 motors over miles and miles of conveyor and have never had an issue.

When in doubt always consut the NEC for the latest regulations.

Good luck...
Boy, Mark is this a good question... for which I don't know solid answers either but I'll add some practical experience.

Industrial Network cables don't mix well with anything else... Profibus, DH+, Modbus, whatever.

24VDC doesn't go well with communications cables. I've seen big 24Vdc power supplies sink the heck out of numerous I/O cards, PCs and networking equipment when a conduit cut happened or something rubbed the wrong way during a cable pull.

Any high frequency cable does not mix well with Ethernet (AV, some digital PBX nets, etc.).

RS-232 will work with damn near anything (yea, yea, I know the -232 standards).

Some VFDs have a weird tendency to propagate noise back up the control lines (particularly with high speed changes in the frequency). Thus, I alway put control cables for VFDs in their own pipe.

Steppers should always have their own pipes (lot of noise there so pretty obvious but don't electricians do the strangest things?).

Someone will tell me if I'm wrong but I think the NEC says (from a fire standpoint) that anything under 30v nominal can go in the same pipe. Beware of this when dealing with the tradesmen.

Just a synopsis of my experiences. I hope you get some answers as to clear-cut standards because I'd be interested as well.

John Kelley

Matthew Hyatt


Visit, a great resource site and tons of technical and code information.

Rule of thumb is that low voltage and communications should always be seperated from high voltage control and power wiring. Even though the code allows for low voltage to be in the same conduit as a control (120vac) the low voltage wires must be at least 600vac rated - it may be 300vac, but why risk it?. It is just simpler to not mix singal, comms and low voltage control with high voltage and power wiring. Even for short runs, it is far better to keep stuff seperate. Still, talk with loca linspectors, electricians and review the NEC rules and code. Depending on the project cost adn such, it may be best to put all comms related lines in one conduit, all analog signals - inputs and ouputs in another with 24vdc control and DI / DO and all 120 vac control in another and all high current / power in a seperate conduit.

this can of course seem impossible once you get into a MCC or MCP or a control panel, but typically, you will not have a main power feed conduit or buss connections inter-mixed with other signals anyways.
There are no cut and dry rules for the question and the answers you have received are all good. The NEC book has different chapters for each type of wiring method and just like the code each wiring method should be separated from each other so one will not adversly influence the other during the normal operation. The large grouping is power, control, Istrumentation and communications. All my life I have been installing wires and cables in these large groupings. When it comes to instramentation and communications there are groups within groups. If the type of cable can influence or be influenced by other cables in a conduit or tray don't install the cable. With no clear rules these are general guide lines.

can you tell me the particular part of the code that states you can combine a low voltage and communications wiring. I have a project where 220Vac is in the same conduit with the communications cable which uses a Twisted Shielded Pair Cable.
fel flores,

The real problem is if the current flowing through the 220 VAC circuit is "high" and/or switching. What's high current? In my opinion, anything above approximately 3-4 Amperes, and it's worse if it's switching (on and off, say for starting a small induction motor).

If the conduit is metal, that can even make matters worse as it seems to "concentrate" the effect of current and/or switching.

Also, if the low voltage signal cable is properly shielded and the shield drain wire is properly grounded (earthed) then problems can be minimized, but there are no guarantees. If the "communication cable" is not shielded and the shield is not grounded (earthed), that by itself can be a problem. Some Ethernet cables can be purchased with shields capable of being grounded (earthed).

Again, it's not necessarily the level of the voltage (though if it's 380/440 VAC, three-phase power even at low current that can be a problem depending on the cable and how it's shielded)--it's the current and the nature of the current flow.

But, in many cases, high voltage cables are run in close proximity to low voltage signal cables as then enter or leave a control panel, and if that's only for a meter or two it's not usually a problem, though a good practice is still to try to separate them as much as possible.

But, regardless of whether code does or doesn't allow, it's simply not good practice to chance running high voltage cables and low voltage signal cables in the same conduit or cable tray. It's just asking for trouble. Codes can "allow" a lot of different things, as they are subject to interpretation even if they seem quite literal.

What you've described may work for decades with no problems whatsoever, and it may be problematic from day one. I've seen similar installations suddenly develop problems when someone plugs in a large box fan or a drill motor to a convenience outlet whose power feed wires passed in close proximity to communication wires. And, this wasn't even in conduit. (The latter caused a large plant to shut down, and it took some real sleuthing to find out the cause of the trip. It was only because it was noticed that there was construction works going on very near and by asking questions of the workers in the area--who were just looking for an unused outlet for their drill motor for two holes to be drilled--that we were able to pinpoint the cause of the trip to the Customer's "satisfaction." They demanded that the convenience outlet be removed from the cabinet, after their request for proposal demanded there be a convenience outlet available in the cabinet.) Things can, and do, change over time as people make modifications or misunderstand the intended applications of circuits.

Hope this helps!
I think NEC section 16 is pretty clear, 16-212 in particular. I can't find it at the moment, but somewhere in the code it specifies that the insulation must be up to or over the greatest voltage. if you were to run a communication cable with a 600 Volt cabl,e the communication cable would need to be insulated for 600 Volts also.

Aside from that you would be crazy to run communication cables with high power motor cables.
In general, separation is specified in...

Class 1, Class 2, and Class 3
Remote-Control, Signaling, and
Power-Limited Circuits

Depending on purpose of wiring, there may be some other more specific requirements and perhaps some exceptions in other Articles, but that's where you'll start for power vs. power limited and low voltage.