low voltage & line voltage

[r46ski]

Hi Guys,

I'm new so bear with me here.
I have been told by an inspector that I cannot bring a low voltage wire (t-stat) into a box with my line voltage. The box will switch a low voltage can light. I have a small trans that I wire up and put in box with the device. He says that the low voltage wire must be rated the same as the maximum wire rating in box. Could someone please tell me some code sections to let me know if I can't do this.

 

[petersonra]

Re: low voltage & line voltage

The insulation rating on all wiring must be equal to or greater than the highest voltage in the box.

there are also some requirements on seperation of power circuits from limited energy circuits.

 

[iwire]

Re: low voltage & line voltage

Along with what petersonra has said once you mix the T-stat wiring with the line voltage wiring the entire wiring for the T-stat must be treated like line voltage all the way back to the unit it controls.

Take a look at the following code articles

725.52 Wiring Methods and Materials on Load Side of the Class 2 or Class 3 Power Source.
Class 2 and Class 3 circuits on the load side of the power source shall be permitted to be installed using wiring methods and materials in accordance with either 725.52(A) or (B).
(A) Class 1 Wiring Methods and Materials. Installation shall be in accordance with 725.25.
Exception No. 1: The derating factors that are given in 310.15(B)(2)(a) shall not apply.
Exception No. 2: Class 2 and Class 3 circuits shall be permitted to be reclassified and installed as Class 1 circuits if the Class 2 and Class 3 markings required in 725.42 are eliminated and the entire circuit is installed using the wiring methods and materials in accordance with Part II, Class 1 circuits. Class 2 and Class 3 circuits reclassified and installed as Class 1 circuits shall not be classified as Class 2 or Class 3 circuits, regardless of the continued connection to a Class 2 or Class 3 power source.

725.55 Separation from Electric Light, Power, Class 1, Non?Power-Limited Fire Alarm Circuit Conductors, and Medium Power Network-Powered Broadband Communications Cables.
(A) General. Cables and conductors of Class 2 and Class 3 circuits shall not be placed in any cable, cable tray, compartment, enclosure, manhole, outlet box, device box, raceway, or similar fitting with conductors of electric light, power, Class 1, non?power-limited fire alarm circuits, and medium power network-powered broadband communications circuits unless permitted by 725.55(B) through (J).

It would be much easer to just keep the two systems separate.

 

[charlie b]

Re: low voltage & line voltage

I do not often work with ?Low voltage? circuits. But I have seen several NEC articles that mention installing two or more different types of circuits in the same raceway or enclosure. In several locations, the NEC has a requirement that all wires must have an insulation system that is rated for the highest voltage of any circuit. I see that Bob has posted a couple of them.

The reason for this requirement is that the insulation system of any wire (specifically the higher voltage wire) might someday fail. There would then be the possibility of leakage current from that wire to ground, or to other nearby wires. If the failing insulation area comes into contact with a wire that has a lower rating for its insulation system, then the leakage current can easily pass directly through that insulation, and start flowing through the other circuit. But if all wires have the same insulation rating, then the failed wire cannot pass its leakage current into any other wire.

 

[rbalex]

Re: low voltage & line voltage

The actual ?basic rule? is:

300.3(C) Conductors of Different Systems.
(1) 600 Volts, Nominal, or Less. Conductors of circuits rated 600 volts, nominal, or less, ac circuits, and dc circuits shall be permitted to occupy the same equipment wiring enclosure, cable, or raceway. All conductors shall have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the enclosure, cable, or raceway .

(Yes, I know the FPN refers to 725.55(A))

I have great hopes that pulling Art 725 from a CMP that is dominated by the communications industry will eventually return it to some sanity with regard to POWER LIMITED circuits. There is no actual safety issue preventing conductors of a 120 volt circuit from being in the same ??cable, cable tray, compartment, enclosure, manhole, outlet box, device box, raceway, ?? with conductors of a Class 2 or 3 circuit even if the 120 V circuit is insulated with 600V wire and the Class 2 or 3 circuit is insulated with 300V wire.

The original protectionism that was developed in Art 725 for conductors was over signal quality not inherent safety. The true safety issue is isolating the terminations so that the ?unregulated? power would not ?flash-over? to the Class 2 or 3 power supplies.

Dick Biermann, currently member emeritus of the TCC and its former Chairman was previously the Chairman of CMP16. He and I have had some long discussions over this and he appears to have been the lone voice against some of the current Art 725 excesses.

Some Art 725 circuits may also be Art 800 through 830 circuits of course. Those are the articles where it is appropriate to apply the signal protectionist rules. Other protectionist rules such as emergency signaling are adequately addressed elsewhere. Many of the Art 725 rules are grossly excessive. Originally 725 was intended to provide "looser" wiring methods because the circuits have much lower probabilities of causing a fire (see their definitions). An entire new Article (727) had to be developed, with the intent of returning at least some of the original 725 intent.

 

[r46ski]

Re: low voltage & line voltage

Thanks for the reply.

quote:
--------------------------------------------------------------------------------

300.3(C) Conductors of Different Systems.
(1) 600 Volts, Nominal, or Less. Conductors of circuits rated 600 volts, nominal, or less, ac circuits, and dc circuits shall be permitted to occupy the same equipment wiring enclosure, cable, or raceway. All conductors shall have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the enclosure, cable, or raceway .

I believe this pretty much answers my question. The t-stat wire used is more than likely not rated at 600v like the NM-B.

 

[don_resqcapt19]

Re: low voltage & line voltage

Bob,

There is no actual safety issue preventing conductors of a 120 volt circuit from being in the same ??cable, cable tray, compartment, enclosure, manhole, outlet box, device box, raceway, ?? with conductors of a Class 2 or 3 circuit even if the 120 V circuit is insulated with 600V wire and the Class 2 or 3 circuit is insulated with 300V wire.

I think that there is a safety issue here. If there is a fault in the common cable or raceway the higher voltage could be imposed on the low voltage equipment creating a shock and fire hazard. In the case of thermostats, maybe even an environmental hazard. The mercury vial in the stat could rupture causing the release of the mercury into the building.
Don

 

[don_resqcapt19]

Re: low voltage & line voltage

r46ski,
I think that 725.55(D) permits the installation that you have proposed.
Don

 

[electricmanscott]

Re: low voltage & line voltage

Is t-stat wire a chapter three wiring method?

 

[curt swartz]

Re: low voltage & line voltage

If the thermostat wire is being used to supply the light from the transformer article 411 would prohibit this installation if this wiring is going to be in the walls. Thermostat wire is usually 18 or 20 AWG which is pretty small for most low voltage lighting applications.

 

[rbalex]

Re: low voltage & line voltage

Originally posted by don_resqcapt19:
Bob,

There is no actual safety issue preventing conductors of a 120 volt circuit from being in the same ??cable, cable tray, compartment, enclosure, manhole, outlet box, device box, raceway, ?? with conductors of a Class 2 or 3 circuit even if the 120 V circuit is insulated with 600V wire and the Class 2 or 3 circuit is insulated with 300V wire.

I think that there is a safety issue here. If there is a fault in the common cable or raceway the higher voltage could be imposed on the low voltage equipment creating a shock and fire hazard. In the case of thermostats, maybe even an environmental hazard. The mercury vial in the stat could rupture causing the release of the mercury into the building.
Don

Don,

Sorry I missed your response last night.

This may just be my industrial bias, but to me your scenario seems to be a stretch to consider it ?practical safeguarding? even in residential installations. I just don?t believe there are enough data available to justify considering it as a commonly serious hazard and it sounds rather speculative to me. With enough ?what-if?s, I?m fairly sure I could develop a scenario that would make virtually any energized installation potentially dangerous and probably even a few de-energized ones.

 

[George Stolz]

Re: low voltage & line voltage

Allied makes NM two gang boxes with nifty orange plastic dividers in them, ask your friendly parts dude, he can look them up. They provide UL listed separation of deadly 24V power from 120V.

We use them for fireplaces sometimes. The normal item we use is a two gang, 1/2 HV box, 1/2 LV open attachment, all-in-one unit. I'd fret a little over having any LV lighting in a p-ring type situation, though.

 

[don_resqcapt19]

Re: low voltage & line voltage

Bob,
Yes it would be somewhat unlikely, but many faults within a raceway involve the other conductors within that raceway before the OCPD clears the fault.
Don

 

[nhrawingman]

Re: low voltage & line voltage

This is very dealt with by purchasing gang boxes from SmartHome:

http://smarthome.com/25401.html