smallest allowed conductor for control circuits

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I am an Electrical Technician for a company that builds process systems. I build industrial control panels and they get mounted to skidded systems and we run PVC coated RMC. We have to follow UL 508A for the panel building, but UL doesn't cover anything as far as conduit and the related. Inside the panels, for a power circuit, 14 AWG or larger must be use, and for control circuits, 18 AWG or larger unless it's a multi-conductor cable. I was taught that nothing smaller than 14 AWG THHN was allowed in conduit. But I'm not sure I believe that, and am trying to find where in the NEC it tells me. If it is 14 AWG, then fine. I just want the proof.

Any help would be great!!

THANKS! I've learn A LOT from this forum in the past year, just reading.

Justin Reading
Electrical Technician
 

RUWired

Senior Member
Location
Pa.
It's the allowed for what that I'm missing, How about a cat5 in a conduit gauge 20-22,

nothing preventing that ? If that makes no sense, then, I have missed the question!! :)


Cat 5 would be a bad example to use. It is only rated for 300 volts and class 1 circuits are required to be 600 volt rated. Besides that i hear what your saying. I think the logic to the minimum required i posted is that ocp is required for class 1 circuits and conductors smaller than 18 awg do not have any ocp ratings listed. To me that would limit it to 18 awg.

Rick
 
WOW

I didn't expect these many responses this quickly. That's cool.

Justin you could look at article 725.49 for circuit conductors and 725.43 for OCP and also table 430.72(B).
Welcom to the forum
Rick

I will read the articles when I get to work tomorrow morning....

justin, You can run any size wire in conduit that you want, follow the code rules for fill etc.

I'm not that familiar with the NEC. More familiar with UL 508A, but there are minimum conductor sizes allowed depending one what kind of circuit it is. As I mentioned before: 14 AWG for power circuits and 18 for control circuits. This is inside an Industrial Control Panel though. Unless it is a multi-conductor cable, such as CAT5. Benaround: I'm asking about single conductor THHN.

Are these systems listed and labelled ? If so, the NEC might not apply (?)

The panels alone are being "UL 508A Listed" YES. But I want the skids wired up safely and to whatever code I should be following for conductors in whatever kind of race way I'm using, which I'm assuming is the NEC.

Cat 5 would be a bad example to use. It is only rated for 300 volts and class 1 circuits are required to be 600 volt rated. Besides that i hear what your saying. I think the logic to the minimum required i posted is that ocp is required for class 1 circuits and conductors smaller than 18 awg do not have any ocp ratings listed. To me that would limit it to 18 awg.

Rick


If the OCP is only at 1 AMP, that brings be down to ??AWG (don't have chart in front of me to look). But I'm assuming inside the panel and inside conduit will (?) be different.


Thanks again for all the quick responses.

Justin Reading
Electrical Technician
 

RUWired

Senior Member
Location
Pa.
If the OCP is only at 1 AMP, that brings be down to ??AWG (don't have chart in front of me to look). But I'm assuming inside the panel and inside conduit will (?) be different.


Justin Reading
Electrical Technician
IMO, your minimum size conductor is will be 18 awg. As mentioned before, see table 430.72(B).

Rick
 
Thank you all for your replies. After looking at the referances given, I have determined that 18 AWG is the smallest allowed for control circuitry.

Thanks again for all your help!!

Justin Reading
Electrical Technician
 

pfalcon

Senior Member
Location
Indiana
I am an Electrical Technician for a company that builds process systems. I build industrial control panels and they get mounted to skidded systems and we run PVC coated RMC. We have to follow UL 508A for the panel building, but UL doesn't cover anything as far as conduit and the related. Inside the panels, for a power circuit, 14 AWG or larger must be use, and for control circuits, 18 AWG or larger unless it's a multi-conductor cable. I was taught that nothing smaller than 14 AWG THHN was allowed in conduit. But I'm not sure I believe that, and am trying to find where in the NEC it tells me. If it is 14 AWG, then fine. I just want the proof.

Any help would be great!!

THANKS! I've learn A LOT from this forum in the past year, just reading.

Justin Reading
Electrical Technician

OP,
You reference that you are building industrial panels. Many of the minimum wire sizes in industry were developed to prevent mechanical damage in industrial installations. These requirements are above and beyond the scope of the NEC.

See NFPA79:2007 12.6 Conductor Sizing
http://www.nfpa.org/aboutthecodes/list_of_codes_and_standards.asp?cookie_test=1

You will find a direct match to the sizes you stated. NFPA79 is not a legal requirement unless the receiving company has officially adopted the standard. Even if not adopted the NFPA79 standard is considered to be sound industrial practice.
 
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