Stat wire with 480v to roof top A/C

[Gaffen99]

I looked at a job to change existing roof top A/C units, the 24 volt control is penetrating the roof to the unit in the same conduit. The line voltage is 480/277, now if the control is run as thhn, is this OK? What if it is standard 18/5 stat wire? What class does this fall under, the low volt control I mean?
Thanks

 

[StephenSDH]

I don't have my code book on me, but my understanding is the insulation of all conductors in the same raceway must be rated for the maximum voltage present in the raceway or greater.

So the stat cable probably isn't rated for 480 or greater.

THHN is rated for 600v so you could pull THHN for you stat contacts.

You can mix 480AC with 24DC, but you should be aware that if there is communications involved, interference can come into play.

Just found an old forum
http://forums.mikeholt.com/showthread.php?t=77835

 

[StephenSDH]

I'm not as sure about that 24vdc in the same conduit. Can somebody correct me if needed.

 

[augie47]

I believe its a bit more complicated. You need to look at Art 725 and determine the Class of the low voltage circuit and then see if it can be run with the power circuit.

 

[nakulak]

if the thermostadt/control wiring is class 2, then I believe it is not allowed (725.55 2005) (check the manuf literature). Also, if the control wiring includes shunt trip connected to FA then you are probably screwed by 760 too.

however, if the wiring is class 1 for controls, then I believe that 600v conductors would be allowed (I haven't seen thermostadt wiring above 300v, but I think it is made)

 

[Gaffen99]

Is the class determined by power source alone?

 

[hbiss]

Yes BUT...

You can "reclassify" CL2 and CL3 to Class1 by (in this case) using THHN or TFFN and then you are allowed to run it in the same conduit or raceway, boxes etc ONLY IF that wiring is functionally associated with the power wiring. (Art 725.26(B))

So the question is: is the thermostat wiring associated with the power wiring of the A/C unit? As long as the power wiring, thermostat wiring and raceway serve only the same A/C unit the answer is yes.

-Hal

 

[LarryFine]

As long as the power wiring, thermostat wiring and raceway serve only the same A/C unit the answer is yes.

I'd agree if the T-stat wire is rated for 600v.

 

[augie47]

Yes BUT...

You can "reclassify" CL2 and CL3 to Class1 by (in this case) using THHN or TFFN and then you are allowed to run it in the same conduit or raceway, boxes etc ONLY IF that wiring is functionally associated with the power wiring. (Art 725.26(B))
-Hal

I can't disagree with that, but I also can substantiate it by NEC. Can you give me a Code reference please ?

 

[raider1]

Gus, take a look at Exception #2 to 725.130(A) (2008) or in the 2005 see Exception #2 to 725.52(A).

Chris

 

[nakulak]

the wiring can certainly be reclassified, but the power source may not be able to be reclassified as a class 1 source (especially if it is integrated with the control board of the hvac unit). regardless, this begs the question, do you really want to run the control wiring in the same conduit. If the control board stands to be fried from any fault, it may not be a good practice. if the control wiring is associated with shunt trip for fire alarm, it is not allowed (imho).

 

[hbiss]

Sorry, we are still on 2002 here. The 08' reference would be Art. 725.48(B)(1) and 725.130(A) Exception #2 and the FPN that follows.

-Hal

 

[Gaffen99]

Is it feasible to just change the transformer to a class 1 if needed. I still don't think it should be run together just doesn't seem right in the sense that these unit are controlled by a remote company. 480 wont mess with thier communication to the unit?

 

[LarryFine]

Is it feasible to just change the transformer to a class 1 if needed. I still don't think it should be run together just doesn't seem right in the sense that these unit are controlled by a remote company. 480 wont mess with thier communication to the unit?

You could always ask them. If it's been an issue in the past, I'm sure they're already aware of it.

 

[hbiss]

Is it feasible to just change the transformer to a class 1 if needed?

I don't see how that would help you. It would be the same result as reclassifying CL2 or CL3 to Class 1. Also remember that Class 1 is not the same as power and running it in the same raceway with power is still subject to having to be associated with the power wiring.

As for interference, what exactly does this LV wiring do? Most that I have seen only activates a 24 volt contactor coil to turn the A/C unit on and off. If such is the case I can't imagine a problem with interference.

-Hal

 

[tom baker]

While 300.3(C)(1) states
(C) Conductors of Different Systems.
(1) 600 Volts, Nominal, or Less. Conductors of ac and dc circuits, rated 600 volts, nominal, or less, 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.
Exception: For solar photovoltaic systems in accordance with 690.4(B).
FPN: See 725.136(A) for Class 2 and Class 3 circuit conductors.

Whats most often missed is the FPN.
Also see 90.3 for what I consider to be the most important rule in the NEC

It is allowed to reclassify a Class 2 circuit to a Class 1, but I how does this effect the UL listing by marking over it?