Is #20 AWG or #22 AWG twisted shielded pairs for 4-20mA signaling cables legal per the NEC? 1

I'll attempt to answer my own question. I'm trying not to make this a rant...but it is a little frustrating that there does not seem to be a clear cut YES or NO to this question. This wiring is to a PLC - one of the most common wiring situations on Planet Earth. There are fuses and circuit breakers protecting every circuit. I really don't understand why a lighting circuit is allowed per the NEC in some cases to operate with a #18, a small motor to operate with a #18 and yet a maximum of .020 AMP circuit, that's 20/1000 of ONE amp, has to use a #14 gauge wire except under the complicated rules of Article 725. If I'm missing something please help!

Here's the research I've done so far:

2011 NEC: 310.106 Conductors."(A) Minimum Size of Conductors. The minimum size of conductors shall be as shown in Table 310.106(A), except as permitted elsewhere in this Code."
And that size is #14 for Copper and #12 for AL conductors for conductors <2000V

I'm looking "elsewhere" if I can use anything smaller than #14:

240.4(D)(1) #18Cu is to be protected by a 7 Amp fuse...
240.4(E)(1) #16CU is to be protected by a 10 Amp fuse....
330.104 Conductors. MI cables allow to be made in #18AWG copper.
430.22(F) A less than 1hp motor in a certain situation can be fed by #18AWG
517.75 X-Ray equipment - Allows Size 18 AWG or 16 AWG fixture wires as specified in 725.49...where protected by not larger than 20-ampere overcurrent devices.
522.21(C) Permanent Amusement Parks - allows for #26AWG to be used multiconductor cables
600.31(B) Neon signs can use 18 AWG.
604.4 Manufactured lighting systems allows #18
610.12 Cranes and hoists: some wires can be #18 under conditions for circuits not operating at more than 7amperes. And electronic control circuits can be #20AWG.
620.12 Elevators and Dumbwaiters Article 620.12 allows travelling conductors to be 24AWG and even smaller if listed.
725.49 Allows for #18 Class 1 circuits
725.127 The supply side of a Class 3 transformer can be a #18
727 Instrumentation cable....specs for small wires and ITC cable, but WHY does it have to be ITC rated and meet all the conditions?
760.43 NPLFA Circuit Overcurrent Protection. allows for #18 is protected by a 7Amp fuse
760.127 The supply side of a listed PLFA Class 3 Fire Alarm transformer can be a #18

The NEC does not specify conductor size on the secondary side of a Class 2 or Class 3 power supply. Your can use what you want as long as your circuit is not involved in critical safety applications like fire alarms, and the specifically-excluded applications like elevator hoistway cables.

If it is downstream of a Class 1 power supply or a non-power-limite source, you must obey the other rules you cite in your post / 725.49 for Class 1 and Section 300 for all other.

Get yourself one of the listed & labeled Class 2 or Class 3 power supplies and knock yourself out with your tiny wires!

Best to you,

Goober Dave

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Forgot to mention - the UL standards for industrial control panels (508 & 508A) specify that anything within the secondary circuit of a Class 2 power supply is "not subject to investigation."

I've never had an AHJ mention control circuit size. He or she is usually just interested in seeing a neat and workmanlike installation.

Best to you,

Goober Dave

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#18 is ok for use on a 7amp wire...therefore the ampacity of #18 is 7 amps per the NEC.

1 milliamps is 1/1000 of 1 amp, 20 milliamps is 20/1000 or 2/100 or 1/50 th of one amp. One amp wire is 50X larger than what is required for a 20mA circuit. Must be why the hook up to at the device is always tiny wires or a circuit board.

So a #18 at 7 amp is therefore 50X 7 = 350X greater than what is required for a 20mA circuit.

Just think if we had to use this logic to wire houses....#12 is 20A wire but we need to be 350X greater than 20A x 350 = 7000 Amps!

I know this is absurd. But I still don't understand why there is ANY objection by ANYONE, or why complicated rules seem to be in place that limits the use of small wires to feed these types of circuits. I understand the valid mechanical strength argument and perhaps any noise argument...but as the NEC clearly states 90.1(C) - it is not a design guide.

And thanks for the response Goober Dave, but I think you in some way serve to make my point...there are many qualifiers as you point out - and that's my objection. oh well.

Sure thing, bdn2004. I believe in most cases, the exceptions that require larger wire even for Class 2 circuits are almost entirely for physical endurance of critical or life-safety systems.

The most critical thing to note is that the source of power for your tiny-current circuit must be limited (Class 2, Class 3). If the voltage source instead is Class 1 or unlimited, it is capable of sending larger levels of current down those tiny wires in the event of a fault. With a 1000 VA source, you could easily start a fire if your wires are too small to survive the let-through current of the upstream protective device without burning and arcing. The Class 2 and Class 3 power sources are designed to keep fault currents at low enough levels to prevent this.

Best to you,

Goober Dave

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If you try to use logic when applying the NEC, you will be often frustrated. If it makes you feel better, you can always submit a change proposal to the appropriate code-making panel.

dpc is totally correct. The Code is what it is. Don't even try to apply logic to it, that will only drive you crazy. Just accept it, and go have a drink after work. Sure, suggest your change, don't expect it to be accepted though. Happiness isn't in understand the whys and wherefores of the code; happiness is in accepting the code and using the code approved solution for every problem and moving on. After all, when the client asks you can always just say that that's what the code requires; can't do a thing about it.

I know a guy who was on a CMP years ago. He was on the panel that made rules for cable tray. I remember him saying that any change was fought that would allow cable trays to be used somewhere where conduit had exclusively been used per the NEC. In this case some hazardous areas.

Could it be that some of the Code might be in place due to the interests of those that might gain benefit or harm from the use of their product? But isn't that fascism?...but that would never happen in the land of the free...nah. couldn't be.

Could it be that some of the Code might be in place due to the interests of those that might gain benefit or harm from the use of their product?

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Of course. That is why the Code Making Panels have representatives from manufacturers, contractors, consultants, UL, etc. Each member represents the special interests of their group to some extent - kind of like Congress or your state legislature. Why are Congressmen from Ohio pushing for more M-1 tanks that even the Pentagon does not want? You'll only need one guess. But not even close to fascism.

The definition I use for fascism is this: the merger of state and corporate power. The NEC when adopted by the locality becomes the law....state power. So requiring people to do stuff - making a Code requirement, when it's justified or not, but at the same time would benefit a particular industry immensely...is certainly flirting with that definition of fascism if not outright.

I'm not against the NEC or it's aims - it's been a great thing for this entire world in making safe, consistent electrical installations. But it certainly has the potential for abuse by nefarious players who could potentially bang their drums for hazards that don't really exist or very obscure chance of ever occurring, then demanding one size fits all solution through the law - the NEC.

I remember reading once that there is a requirement (a law) at all gas pumps there be a sticker that cell phones can not be used. But in world history, there has not even been one fire or explosion at a gas station that could be blamed on a cell phone. Now how did such a law become a law? But in theory if the right elements were present in the right mixture it could be an ignition source. Electricity by it's by it's very nature - by not being visible lends itself easily to such theoretical arguments.

It's not just the NEC and the other widely-adopted codes.

I lived in a city in which the local code board consists of master electricians. The NEC was the adopted code, but local code contained clauses that override the NEC.

One of my favorites: In non-residential construction, the maximum number of general-purpose 15A or 20A receptacles on a 120V / 20A circuit shall be 3.

How much more does that make a new building cost in terms of the electrical contractor's portion, all of which by law have to be managed by a master electrician? We can't even put one on each wall of a room with one circuit.



Best to you,

Goober Dave

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DRWeig said:

We can't even put one on each wall of a room with one circuit.

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Just build triangle shaped rooms.