Plug In Cord/ 20A Circuit Breaker

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Volta said:
That a 24 AWG wire can probably carry an amp safely, and would be fine to carry the current of a 60 watt lamp at 120 volts, but it is too fragile to be trusted in our world.
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Ok, but I thought we were talking about 16 AWG vs 14 AWG?:)

But I know (now) what you mean.;)
 
Is there a rule that states that 16AWG insulation has to be rated at 90 C? NEC Table 310-16 only has one temperature rating for 16AWG.

Is there any reason to believe that 10ft or less runs of 90 C #16AWG MTW would not be adequately protected by the 20A breaker, since it is rated to carry 18A?

Especially since the actual amps carried by the wire will be much less than the ampacity of the wire. I definitely understand why the main power chord should be at least 12/3, but I do think that there is not a safety risk, inside a grounded metal enclosure, to use 16AWG for individual circuits. If there is a short through the individual 16AWG, I think the circuit breaker would clear before the wire melted. If it were overloaded to 19 amps, it may eventually melt, but I think the risks for that are low.
 
fifty60 said:
Is there a rule that states that 16AWG insulation has to be rated at 90 C? NEC Table 310-16 only has one temperature rating for 16AWG.

Is there any reason to believe that 10ft or less runs of 90 C #16AWG MTW would not be adequately protected by the 20A breaker, since it is rated to carry 18A?

Especially since the actual amps carried by the wire will be much less than the ampacity of the wire. I definitely understand why the main power chord should be at least 12/3, but I do think that there is not a safety risk, inside a grounded metal enclosure, to use 16AWG for individual circuits. If there is a short through the individual 16AWG, I think the circuit breaker would clear before the wire melted. If it were overloaded to 19 amps, it may eventually melt, but I think the risks for that are low.
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Wrong.
A 20A circuit breaker can hold significantly more than 20A for several minutes at least and possibly it could hold up to 1.5 times its rating indefinitely. And a 16AWG wire connected to the terminals of a 15A plug or socket can get a lot hotter than the wire itself with a solid splice would. The end result is that it is not safe.
PS: you are usually more concerned about the insulation failing than the wire itself melting!
 
Failing insulation means exposed live copper. I am a believer in striving to meet UL, so I do not think I will use 16AWG on anything other than pure control circuits.

That being said, looking at NFPA79 12.6.1.1 and it describes the use of #16AWG in non-motor POWER circuits of 8 amperes or less...provided the following conditions....

NFPA 79 defines a POWER circuit as "A circuit used for supplying power from the supply network to units of equipment used for productive operation and to transformers supplying control circuits"

If I have a control transformer then is everything on the secondary (115V) considered the control circuit and therefore 12.6.1.1 does not apply?

Is NFPA79 12.6.1.1 only pertaining to the actual power chord that directly connects to the supply?

I am trying to get a feel for A) what the rule is, and B) what the general accepted practice is. Is it common to find, for example, 5A resistive load heaters inside equipment (that plugs into 20A receptacles with a 12/3 chord) using all 16AWG wiring and no fuses at all inside the equipment.
 
Would you consider the conductors inside the machine that is supplied from a 120V 20A receptacle to be part of the power circuit or part of the control circuit? I am looking at NFPA 79 and am wondering if my machine would fall under 7.2.4.1:

"A control circuit tapped from the load side of the branch circuit short circuit and ground fault protective device and functioning to control the loads connected to that branch circuit shall be protected against overcurrent in accordance with 7.2.4.2"

Going to 7.2.4.2.2 the standard reads "Control circuit conductors sizes of 18, 16, and 14AWG shall be considered as protected by an overcurrent device of not more than a 20 ampere rating."

Would my industrial machinery that plugs into the 120V 20A receptacle be a "Control circuit tapped from the load side of the branch circuit short circuit and ground fault protective device and functioning to control the loads connected to that branch circuit."...
 
fifty60 said:
Would you consider the conductors inside the machine that is supplied from a 120V 20A receptacle to be part of the power circuit or part of the control circuit? I am looking at NFPA 79 and am wondering if my machine would fall under 7.2.4.1:

"A control circuit tapped from the load side of the branch circuit short circuit and ground fault protective device and functioning to control the loads connected to that branch circuit shall be protected against overcurrent in accordance with 7.2.4.2"

Going to 7.2.4.2.2 the standard reads "Control circuit conductors sizes of 18, 16, and 14AWG shall be considered as protected by an overcurrent device of not more than a 20 ampere rating."

Would my industrial machinery that plugs into the 120V 20A receptacle be a "Control circuit tapped from the load side of the branch circuit short circuit and ground fault protective device and functioning to control the loads connected to that branch circuit."...
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Think about it.
The cord is plugged into a power receptacle not a control connector.
If the conductors supply power to a device they are power conductors not control conductors.
If the conductors are used to control a device such as in the circuits that terminate at a relay/contactor coil then they are control conductors.
NO.
I am curious, will you give design credit to the members of the Mike Holt Forum for this product you(we) are designing by proxy?
 
Not sure why you would say that ehunter. I have already determined (and stated) that I will not use #16AWG on anything other than control circuits. I have seen 16AWG on some designs done by experienced engineers and I am trying to understand WHY they did it. I have a little over 1 year experience as an engineer and NEC/NFPA is not taught in University. I am still wrestling with some terms, and am determined to understand the 4 corners of every NEC/NFPA topic, and am then determined to pay that knowledge forward to the junior Engineers who come after me. I appreciate the advice of my peers and elders. No need to be rude Sir.
 
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I don't think he was intending to be rude. Write that off to the internet if that's what you perceived. I've been following this thread and it does seem odd that an EE gets so hung up on wire size. The original inquiry as to whether the NEC applies to your design was valid and that question was asked and answered. It seems like you have more direction now and I'm sure everyone's glad to help. Your design needs to account for all conditions present which can include startup load, operating load, ambient temps, ambient atmosphere (explosion hazard, for example), and on and on. There's conductor size, material, solid or stranded, insulation, etc. It is the duty of the engineer to foresee these conditions and design accordingly. We all wish that we hit the nail on the head with a first design but that does not always happen. You'll need to build your prototype and test it to the limits you want it to withstand and either accept or amend your design. Having a 3" piece of conductor inside a machine is much different than having a 30' conductor in a wall.
 
I'm not hung up on a wire size. I have found that almost everytime I have questioned my predecessor on a design issue he has proved to be right in the end. The forum is very helpful in this regard, opposed to just carrying on because it is how things have always been done. Not everyone in the industry is as proactive in their careers as the people that take the time to be part of this forum, that is why I like to come here to ask questions. I don't just drop a question and wait for answers, I use the forum as a tool to help as I actively search for answers myself.
 
Thanks everyone. I definitely have more traction on this issue, and will design in accordance with NFPA 79.
 
A slightly different situation. Now I am looking at a 15A receptacle, but at 230V. I have no control transformer, everything is 230V...heaters, motors, control devices. I am being told to use 14AWG on everything. Here is my problem, the control relays I have supplied to me spring cage connection rated at 10A, and will only accept down to #16AWG. I can order the same relays with screw terminals and use down to 14AWG conductors. I have a large supply of spring cage connection Relays however, and would like to use them.

I will use screw terminals for the motors and heater circuit 14AWG conductors so they will be protected by the 15A supply breaker.

The rest of the components are control relays, SSR's, and timers. If I can consider these components "a control circuit tapped from the load side of a branch circuit short circuit and ground fault protective device" I can use NFPA79 7.2.4.2.2 I can safely use the 16AWG with the 15A OCPD, and use the spring cage relays.

I would like some input on whether or not I am declaring this control circuit correctly. Is it due diligence enough to say "this circuit is controlling a timer, this circuit is controlling a relay coil, there fore they are control circuits."

I do not consider heaters and motors inside my machine to be part of the control circuit. The control relays, timers, and ssr are all protected by the same 15A receptacle. Can I declare some items within this machine "control circuitry" and use 7.2.4.2.2, or does it all have to be considered a non control circuit.

Is everything from the OCPD protecting the conductors supplying the receptacle considered a branch circuit? Would any individual circuits whose load is a control relay or timer then be considered a tapped control circuit?
 
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Also, if anyone could offer any insight into chapter 9 of NFPA79. It is confusing to me because chapter 7 talks about control circuit conductor protection, chapter 8 talks about grounding, and then BAM chapter 9 talks about control circuits again. It seems like chapter 9 adds to the definition set forth for control circuits in chapter 7. Chapter 9 says that a control circuit supplied from an AC source must use a control transformer unless the voltage is 120VAC or less.

Chapter 9 gives an exception to the above rule. The exception says: "other voltages shall be permitted, where necessary, for the operation of electronic, precision, static, or similar devices used in the control circuit"

My 230V circuit does not have a transformer. I am not sure my heaters are considered static because they are controlled to a %output by SSRs. My motor ouptuts vary depending on load. But then again, heaters and motors would not be considered part of the control circuit. The control relays, ssr's and timers would be considered static/electronic.

Does Chapter 9 apply to Chapter 7 or am I reading this wrong? Does this in any way prevent me from treating any of my 220V circuit (heaters and motors excluded) as a control circuit? Thanks in advance for the help.
 
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