Class 2 circuit wiring

[petersonra]

I tried to avoid answering the question by claiming that it was a problem for the installing contractor but i have been forced to suggest some cabling for use in some low voltage field devices.

Some of them are Class 2 circuits, other are just 24VDC or 4-20 mA signals (some are class 2, some are not).

Normally I would not make anything a class 2 circuit, but I had to in this case because it was convenient from a UL listing issue.


Most of the cables would appear to be part of the equipment as opposed to part of the building wiring. But there are some longer rungs that are probably going to be viewed as not part of the equipment as they will be routed in cable trays from one enclosure to another.

I just do not do much specifying of this kind of thing so what is a good (read cheap) choice for this kind of thing?

 

[hbiss]

The first thing that comes to mind is tray cable- type TC. That has a 600v rating and should solve all your problems.

-Hal

 

[petersonra]

The first thing that comes to mind is tray cable- type TC. That has a 600v rating and should solve all your problems.

-Hal

But it does not solve the cheap side of the equation.

Is PLTC cheaper?

is there a TC made in an 18 gauge, 2C? These are all likely to be 2 wire cables run from every which way.

 

[petersonra]

The silnece is deafening. I think I scared everyone off by asking about Class 2 circuits. It has to be one of the code sections that is about as confusing as it gets.

I spent a good hour poking around it yesterday.

I never noticed the provision before for adding OCPD to the circuit and being able to reclassify it and run it with regular wiring. Somehow that seems useful given our customer just asked us to add fuses to every I/O circuit, and agreed to pay for it.

Is there a good tutorial on this somewhere on the web?

 

[kwired]

The silnece is deafening. I think I scared everyone off by asking about Class 2 circuits. It has to be one of the code sections that is about as confusing as it gets.

I spent a good hour poking around it yesterday.

I never noticed the provision before for adding OCPD to the circuit and being able to reclassify it and run it with regular wiring. Somehow that seems useful given our customer just asked us to add fuses to every I/O circuit, and agreed to pay for it.

Is there a good tutorial on this somewhere on the web?

Reclassifying the circuit can be a convenience, but remember you are reclassifying the entire circuit not just the cable running to some other device. Items like pushbuttons, thermostats, potentiometers, that are only designed for stand alone as class 2 circuits would need to be replaced with ones rated for class 1 circuits or general lighting and power uses.

Simple example would be typical 24 volt thermostat for heating and cooling systems. If you wanted to reclassify the control circuit for some reason you might be able to do so - but the typical thermostat will not be rated to be used in its typical manner as anything but part of a class 2 circuit.

 

[petersonra]

Reclassifying the circuit can be a convenience, but remember you are reclassifying the entire circuit not just the cable running to some other device. Items like pushbuttons, thermostats, potentiometers, that are only designed for stand alone as class 2 circuits would need to be replaced with ones rated for class 1 circuits or general lighting and power uses.

Simple example would be typical 24 volt thermostat for heating and cooling systems. If you wanted to reclassify the control circuit for some reason you might be able to do so - but the typical thermostat will not be rated to be used in its typical manner as anything but part of a class 2 circuit.

all of this stuff is feeding common industrial control devices. limit switches, 4-20mA instruments, a few 24VDC solenoid valves. the class 2 thing only became an issue becasue we had some non-Ul stuff in the panels that had to be put in a Class 2 circuit.

 

[tom baker]

I use 18 GA STP PLTC all the time. Belden makes it, I keep 1,000 ft on the shelf. But I keep my 4-20 MA out of 600 volt circuit areas.
Mike Holt has a good book on limited energy and DVDs to go with.

 

[Marc Stringer]

Class 2 circuit wiring. (CL2)

Class 2 circuit wiring. (CL2)

What Gage Size / Number of Conductors / and shielding?
And what routing? (Horizontal? Between Floors (riser)? / or above a drop cieling (likely plenum?)
I'll make a recommendation. There's an upcomg book I'd refer you to, that lists all the options but it's not yet in publication.

Thermostat cable is usually Class 2 rated (CL2)
Class 3 (CL3) is a higher rating. (Also NEC Article 725)
Communications Cable is a higher rating than either (CM) - NEC Article 800.

Tappan Wire & Cable, West Penn, General Cable, and Belden all typically talk in terms of CM rated (or CMR in risers, CMP in plenums) because it covers both Class 3 (CL3) and Class 2 (CL2) -- just the same. All would be made about the same way (anyway) and there's little or no difference in the manufacturing or testing -- so we all build to / test to / refer to the Trump suit -- Communications Cable (CM or CMR or CMP as applicable.) It doesn't have to be installed in a raceway -- if you are running it in tray though -- it goes in the low-voltage tray, not with the power cables.

 

[petersonra]

I suggested some Belden TC #18 to them. I really do not want to get involved with what they are doing any more than I absolutely have to.

 

[don_resqcapt19]

It does not look like the code requires TC cable for this. 392.10(A) permits any of the cables recognized by Article 725 to be installed in the cable tray. You would have to look at the installation requirements in 735.133 and some type of barrier may be required if there are power or Class 1 circuits in the tray.

 

[petersonra]

It does not look like the code requires TC cable for this. 392.10(A) permits any of the cables recognized by Article 725 to be installed in the cable tray. You would have to look at the installation requirements in 735.133 and some type of barrier may be required if there are power or Class 1 circuits in the tray.

I don't think it is required either, but that was what I suggested to them. I really did not want to get into designing the installation for them for free. I figured TC will work quite well in cable tray and that is what they claimed they were going to do. Trying to explain all the ins and outs of this kind of thing in a few paragraphs to people who do not speak English was not something i wanted to try, especially given I am not as comfortable with it as I would like.

In any case, it appears the type of TC I suggested is the least expensive solution given their plan to run cable tray.