Control Circuit Conductor Protection Question

[Shackled Designer]

Salutations.

My question likely touches on some areas of the NEC, but I'm asking it in the General Electrical forum since it seems to relate more particularly to NFPA 79, specifically, section 7.2.4, Control Circuit Protection.

What does the code (7.2.4.2.2) mean when it states, "Control circuit conductors sizes of 18, 16, and 14 AWG shall be considered as protected by an overcurrent devices(s) of not more than a 20-ampere rating." Does this mean that I should use over-current protection, or does the phrase, "shall be considered as protected" mean that my control circuit may actually go without protection so long as it does not draw more than 20 amps?

I suspect that the former is the right interpretation, i.e. I should use fuses (or MCBs) that do not exceed a 20-amp rating if I happen to be using 14 AWG wire.

However, that raises the question in my mind as to why someone wrote 7.2.4.2.2 in the first place. In light of section 12.5 of NFPA 79, what is the justification for using protection rated up to 20 amps on, say, an 18 AWG wire, which is otherwise not rated for more than 7 amps? One may wonder whether the assumption is that in a control circuit, the likelihood of reaching 7 amps is considered to be almost nil. For this reason I ask whether I am properly interpreting this section.

By way of background, what leads to this question is my inclusion of a phase- and voltage-monitoring relay device in an industrial panel fed by a 480 Wye Mains supplying (probably) over 100 amps. The panel distributes power to numerous branch circuits located in the panel. The relay device would be installed on the load-side of a suitably-sized Mains circuit breaker before the separation into the several branches. The phase-monitoring relay documentation says that the device itself requires no fusing, but that "fusing for the line protection depends on the cross-section used." The device is made to support wires of 20 to 14 AWG, so my monitoring relay connect to the panel mains through a set of terminal blocks.

Since the relay is used to monitor rather than deliver the 3-phase power, I presume that it and its feeding conductors a control circuit, not part of the power circuit. Please correct me if this presupposition is in error. If I am correct, however, then section 7.2.4.2 applies.

Best regards,
The Shackled Designer

 

[GoldDigger]

It means that you have to provide an OCPD not larger than 20A and if you do the NEC will not worry about the ampacity and wire type of individual small control wires.
It does NOT mean that you can act as if there were 20A OCPD even when there is not.

 

[Shackled Designer]

Thanks, GoldDigger.

I figured that I would have to put some sort of protection in there.

Based on what you said, would it be correct to think that the people who settled on 7.2.4.2.2 figured that overloading an 18 AWG conductor, for example, with 15 amps did not anticipate very serious damage? Just curious.

Best regards,
The Shackled Designer

 

[Jraef]

Thanks, GoldDigger.

I figured that I would have to put some sort of protection in there.

Based on what you said, would it be correct to think that the people who settled on 7.2.4.2.2 figured that overloading an 18 AWG conductor, for example, with 15 amps did not anticipate very serious damage? Just curious.

Best regards,
The Shackled Designer

Understand that this deals only with conductors INSIDE of the control panel itself, you cannot use this rule for any circuit that leaves the confines of the box. Once you do that the NEC applies and you must either use 14ga minimum protected by a 15A OCPD or the special allowances for smaller wire if fused correctly. So INSIDE of a control panel, what can cause an overload? A device running at too much load? What could that be, where you would WANT to allow the protection device to be too large anyway? The only other possibility is a component failure which, in all likelihood, will involve a short circuit, and the difference in the instantaneous trips on a 20A OCPD or a 15A OCPD ate relatively insignificant.

This rule in NFPA 79 allows this because they know that the conditions inside of the panel, including the likelihood of a fire starting inside and spreading outside, are controlled by the other requirements in this code. But remember, NFPA is mostly about fire safety, not best practice. Minimalist design does not mean maximum reliability. I always look at the contact ratings of the devices in the control circuit. For example if there is a relay or output device rated only 5A, I will fuse for 5A for at LEAST that portion of the circuit and at that point, even 18ga wire is overkill.