Thermostat wiring in same conduit with line voltage

[steve66]

Can a 24 volt DC thermostat cable to a condensing unit or a rooftop unit run in the same conduit with the power wiring to the CU or RTU?

I believe the answer is no, but I wanted to verify.

At least one CU seems to only have one knockout for a conduit penetration, so I'm not sure the electrician can supply it with two separate conduits.

 

[GoldDigger]

Can a 24 volt DC thermostat cable to a condensing unit or a rooftop unit run in the same conduit with the power wiring to the CU or RTU?

I believe the answer is no, but I wanted to verify.

At least one CU seems to only have one knockout for a conduit penetration, so I'm not sure the electrician can supply it with two separate conduits.

If the thermostat and the relay in the condensing unit are both OK to be driven from a Class 1 instead of Class 2 power source, then you can reclassify the thermostat circuit as Class 1, run it using ONLY Chapter 3 wiring methods (suitable for the power voltage) from end to end, and put that Chapter 3 wiring in the same conduit as the power wires.
This is only allowed because the thermostat wiring is operationally related to the power wire, where the function of both is making the condensing unit run when needed.

 

[Dennis Alwon]

Generally there is 2 separate places for the T-stat wiring. I am not sure how you would enter or exit the conduits if the T-stat wiring is in a separate compartment

 

[infinity]

If the control circuit is Class 2 ONLY you must keep it separate even if you use a conductor with 600 volt insulation.

 

[augie47]

A lot of folks look at 300.3(C) and think with a properly rated insulation they are home free but they aer unaware of the Art 725 restrictions.

 

[JFletcher]

Can a 24 volt DC thermostat cable to a condensing unit or a rooftop unit run in the same conduit with the power wiring to the CU or RTU?

I believe the answer is no, but I wanted to verify.

At least one CU seems to only have one knockout for a conduit penetration, so I'm not sure the electrician can supply it with two separate conduits.

Not unless one of the exceptions for 725.136(A) is present.

 

[steve66]

sounds pretty cut and dry.

Thanks for the responses and references.

 

[shortcircuit2]

725.136(I)(2) allows the class 2 and class 3 circuit conductors to be separated by flexible tubing. The industry does this with the new MC cable. Why would it be prohibited in raceway?

 

[Fulthrotl]

725.136(I)(2) allows the class 2 and class 3 circuit conductors to be separated by flexible tubing. The industry does this with the new MC cable. Why would it be prohibited in raceway?

it's a manufactured listed assembly. not field fabricated.

i'm trying to think if i've ever seen t stat in with power in the field.
i can't bring to mind ever seeing it.

 

[steve66]

725.136(I)(2) allows the class 2 and class 3 circuit conductors to be separated by flexible tubing. The industry does this with the new MC cable. Why would it be prohibited in raceway?

Yes, I had thought having the contractor separate the thermostat wire with some type of tubing, like smurf tube or innerduct, but its an existing installation, and the existing conduit is only 3/4". Hard to squeeze anything else in there with the power wiring.

In the end, the contractor wound up making another roof penetration for the t-stat wiring.