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NFPA 79 conductor sizing confusion

NFPA 79 conductor sizing confusion

NFPA 79 conductor sizing confusion


I have been studying NFPA 79 with my goal being to understand the conductor sizing requirements for an Industrial Control Panel. I understand that in the USA, this might also be governed by UL508A, but at the moment, I am just trying to understand NFPA 79.

I am having a hard time understanding what (with my limited knowledge) seems to be a contradiction between NFPA 79 sections 12.6.3 and 12.6.4.

12.6.3 says "Conductors shall not be smaller than 18 AWG for control circuits within control enclosures or operator stations.".

12.6.4 says "Conductors for electronic control input / output and control devices shall not be smaller than permitted in through" These subsections ( through make it sound like conductors smaller than 18 AWG are permitted in some cases., for example, says "Conductors within control enclosures shall not be smaller than 26 AWG".

Does the difference between 12.6.3 and 12.6.4 lie in whether the conductor is being used for a "control circuit" or for an "electronic control input / output and control device?"

"Control Circuit" in NFPA 79 is defined as "The circuit of a control apparatus or system that carries the electric signals directing the performance of the controller but does not carry the main power current." In the project I am working on, the "Control Circuit" uses 120 VAC. For all of my pushbuttons, selector switches, and PLC I/O, I am planning to use 18 AWG wire.

Where I would like to use smaller gage wire (if acceptable) is in cases where electronic devices within my control enclosure require some signaling, typically conveyed using a contact closure from a relay (dry contact) PLC output module or from a standalone relay. For these signals, I like to use two conductor cable with either 20 or 22 AWG conductors. Am I safe to assume that conductors for this type of signal would fall under 12.6.4, making 20 or 22 AWG conductors acceptable?

Any advice will be greatly appreciated.

Thanks in advance,

RE: NFPA 79 conductor sizing confusion

It is impossible to source CAT 5 cable with 18 ga wire. 24 gauge is typical even for rough service CAT5 cable.

RE: NFPA 79 conductor sizing confusion

Thank you FacEngrPE,

That is a good example of a case where conductors smaller than 18 gage are clearly considered acceptable.

The way that NFPA sections 12.6.3 and 12.6.4 are written still makes it difficult (at least in my mind) to differentiate between circumstances where 12.6.3 applies (no conductors smaller than 18 gage) and circumstances where 12.6.4 applies (smaller conductors are acceptable).

I still suspect the difference between 12.6.3 and 12.6.4 might lie in whether the conductor is being used for a "control circuit" or for an "electronic control input / output and control device?" An ethernet cable is clearly used for low voltage / low current electronic signaling.

For many "typical" control devices (speed controllers, temperature controllers, etc), one is often required to create a contact closure between two terminals to initiate a particular function. In many cases, the device has an internal 5 VDC power supply. To initiate a function, you create a contact closure to pull a particular device input down from 5 VDC to 0 VDC (common). In cases like this, I usually use two conductor cable with 20 or 22 gage conductors to convey the contact closure from a PLC relay output module to the device. In many cases, the size of the device terminals would make accommodating 18 gage wires difficult, especially if/when you have to land two conductors at one terminal. Like your ethernet cable example, I can't imagine that this would be challenged by any authority. It is just not clear to me how I can explicitly justify it based on how 12.6.3 and 12.6.4 are worded.

I am not concerned that my use of 20 or 22 gage conductors will be challenged in cases like this, that it won't perform the desired function, or that it will be unsafe - I'm really just trying to build up my understanding of the code (in this case, NFPA 79).

I appreciate your help.

Best regards,

RE: NFPA 79 conductor sizing confusion

Thank you IRStuff - that is a good suggestion. I will reach out to alphawire and see if I can get an application engineer to shed some light. I appreciate your help.

Best regards,

RE: NFPA 79 conductor sizing confusion

Generally speaking, electrical codes do not apply to anything that is low voltage limited energy.

LOW-VOLTAGE LIMITED ENERGY CIRCUIT – A control circuit involving a peak open-circuit potential of not more than 30 volts ac rms, 42.4 volts peak, or 60 volts dc supplied by a primary battery or by an isolated secondary circuit, and where the current capacity is limited by an overcurrent device, such as a fuse, or by the inherent capacity of the secondary transformer or power supply, or a combination of a secondary winding and an impedance. A circuit derived from a line-voltage circuit by connecting a resistance in series with the supply circuit to limit the voltage and current is not identified as a low-voltage limited energy circuit.

If you're building panels then I suggest you buy and follow UL508A, or better yet actually take the course and become a listed panel shop. UL508A is how to build a panel. NFPA 79 is how to install that panel.

RE: NFPA 79 conductor sizing confusion

Thank you Lionel,

I did read UL508A per your suggestion in another thread I started when I was trying to determine whether I should include main fuses for the main disconnect in an Industrial Control Panel I have been designing. This led me to the realization that the disconnect I was planning to use was not suitable for use as a main disconnect in an Industrial Control Panel. For this reason, I decided to use a UL98 rated fused disconnect.

This is an "in-house" project for which I will not be subjected to any regulatory scrutiny. I am confident that my new control panel will be safer and better designed than the 30 year old panel I will be replacing. After reading UL508A, I feel pretty good about my design in terms of compliance. I work for an equipment manufacturer rather than a panel shop. I have designed our control panels for about 15 years, but I have them assembled and wired by a UL listed shop. I've never gone as far as actually getting one of our panels UL listed, but this shop tells me if there are any red flags about my designs. (Although I now know there have been cases when I've used main disconnects that would not be considered suitable for the main disconnect of an Industrial Control Panel.) We have never run into any issues with regulatory authorities when our equipment has been installed, but I do everything I can to make each new panel design better and less likely to be challenged than the last one.

As I design these control panels, install them, and run the interconnect wiring between the control panel and the equipment myself, I've found it to be somewhat challenging to figure out when I should be looking at UL508A, NFPA 79, or NFPA 70 National Electrical Code. Your advice (and from others on this forum) on the distinction has been extremely helpful in my learning curve, and is very much appreciated.

Thanks again,

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