I cant find this in the NEC.
Is there a reference in the NEC to separate 24vdc and 120 vac into their own conduits?

Try Article 725. 2005 NEC
725.26
725.41
725.55
Depending on the class of the 24volt.

Thanks for the quick feedback.
Unfortunately I only have the 2008 NEC and 725.26 / 725.55 are no longer listed.

2008 NEC Article 725
725.48
725.136
Try thoses
Also read Article 725 Definitions for
Classes 1,2,3.

Thanks.
It looks like the answer is yes it is acceptable for our situation.

Sorting out when it's Ok and when it isn't seems an interesting subject.

Can you provide a few more project details that describe the application?

Thanks

A rigid coduit connects a control panel with a metering device.
The device uses 120VAC for control power and has a contact that will have 24vdc going through it and used back at the control panel as an input to a PLC.

Since these circuits are functionally related, even if the 24 vdc is class 2 or class 3, you could take advantage of exception #2 to 725.130(A) and also apply 725.48(B)(1) and run the 24 vdc as a class 1 circuit.

Shop_Test - As I understand it the two could in certain applications be serving the same device and be combined, but the NEC calls out certain separation devices and or distance that must be maintained between the conductors?

Tom- In the case of wiring as a Class 1 circuit (upgrade the wiring), NEC2005 725.27(A) would require 18 or 16 AWG and 725.27(B) would require 600 volt rated insulation of specific types?

Am I barking up the right tree?

Same temperature rating, your good. Personally, I always separate power from control. Regardless of the situation. Just my way.

Yes in the past I have always kept them separate also.
Problem is that we are working in a plant utilizing plant electricians who were fighting me on the issue. They wanted to use 1 conduit.

The NEC2002 and NEC2005 sections are quite clear in the matter. It more or less says "thou shall not mix wires" when talking about low voltage and line voltage under both 725 for Class 2 and 3 discussions, and 800 for comms wires. LAN wiring sections and methods may also come into play as we give lighting fixtures, security, and access control devices IP addressing in the BAS. There are several exceptions that narrowly bound the field conditions where it might be OK that also require physical barriers and separation distance, and there is also an exception under 725 if Class 1 wiring methods are used, which also is quite clear about minimum wire gauge (16 or 18) and insulation rating (600) - NEC2005 clearly also says de-rating methods shall not be allowed.

Not sure what NEC2008 might have that varies the game, haven't looked yet. But then it would fall back on what version the juristicion State is currently adopting anyway.

If the plant mechanics want you to either deviate from the Code, or exploit a loop hole, I would suggest you get it in writing and signed off by a responsible party at the plant (how often would management back the loop hole in the case of safety standards or process performance "vulnerabilites" ?).

IEEE 518 and 1100 may also be helpful, along with TIA/EIA 862.

I have never seen an exterior residential condensing unit with two electrical conduits attached, they only ever have one, it would be carrying 230V and 24V conductors.

It seems that often times the low voltage control wire is simply zip tied to the insulation of the refrigerant lines?

If the low voltage relay wires are in with the power, this is probably a code violation since it is unlikely that the furnace/ac installer ran those wires in compliance with a chapter 3 wiring method and the exceptions that were in my original reply.

Around here, the installers cable tie the low voltage cable to the sealtight conduit as permitted in 300.11(B)(2)

I see a problem brewing when the Class 2 wiring is tied to the "sealtight" due to the fact that the conductors are exposed to sunlight and most of these installations are installed this way using a two conductor cable that does not have a sunlight resistant jacket.

Ann,

Most of the manufacturer's cut sheets say more or less the same thing RE field wiring.

For example:

Q

Power Supply
1 All wiring should be made in accordance with the
National Electrical Code. Consult the local power company
to determine the availability of sufficient power to
operate the unit. Check the voltage at power supply to
make sure it corresponds to the unit’s RATED VOLTAGE
REQUIREMENT. Install a branch circuit disconnect
near the rooftop, in accordance with the N.E.C.,
C.E.C. or local codes.

- and -

Hook-Up
POWER WIRING MUST BE RUN IN CONDUIT. Conduit
must be run through the connector panel below the service
cover and attached to the bottom of the control box.
If low (extra-low in Canada) voltage control wire is run in
conduit with power supply, Class I insulation is required. If
run separate, Class II is required. Low voltage wiring may be
run through the insulated bushing provided in the 7/8 " hole
in the connector panel then routed to the control box.

EQ

http://www.mcquaybiz.com/mcquaybiz/literature/lit_ch_cu/IMOM/IM909-RCS6-7Tons.pdf

What actually happens in the field may well be a different story.

Offhand, I don't know of any NEC requirement for the Class 2 conductors for the outside unit to be sunlight resistant if exposed, nor I am I aware of a requirement for a wet location listing. Can anyone post a code reference?

Maybe it is just a Florida thing but they use sunlight resistant thermostat cable around here.
I assumed it all was. A am not sure about the "wet" listing but since it doesn't have paper packing I am not sure why it wouldn't be. PVC is water tolerant.
The real issue I have heard about is "riser" and "plenum" ratings when that is appropriate.
You might have trouble if you needed plenum, wet and sunlight in one cable.

Not sure about UV ratings or wet environment applications to the example given with an AC unit, but it would seem to apply in some way. But then we could have an entire lengthy separate post / comment thread about the difference between best practice, industry standards, NEC compliance, and what really happens in the field...

With respect to this business of zip-tying Low Voltage to refrigeration lines or the power feed flexible conduit, I would think there may be issues to the whole business of Low Voltage being required to be properly fixed to the building. For example it seems Low Voltage can not be zip-tied to the exterior of conduit or suspended ceiling suppport wires inside the building, so why would it suddenly be OK outside?

We might want to follow this:

http://ecmweb.com/nec/whats_wrong_here/whats_wrong_000409/index1.html

RE potential issue of zip-tying Low Voltage control wire to the power conductor conduit for an AC unit, I take it back.

It appears under NEC2005 section 300.11(2)(B)((2) it is adressed as being OK.

Yet, I still wonder about the control wire being zip-tied to the refrigeration line set as meeting code.

The general consensus is that it is OK to strap the thermo cable to the refrigerant lines around here, at least that is what I see most of the time. They assemble the whole thing on the ground and fish it through the attic as an assembly.

Would 310.8(D) "Locations exposed to direct sunlight" apply? 310.1 Scope doesn't seem to provide an exclusion for Class 2 or any other type of conductor, it seems to be all inclusive.

(NEC2005 reference)

You're right about having a whole separate thread.

300.11(B)(2) doesn't say anything about indoors or outdoors, the practice is permitted in either location if it meets the requirements of this section. Usually, the only place I see class 2 wiring properly secured is when it is tied to the flex at the outside unit, the rest of the cable is just laying on the ceiling panels, draped across trusses, conduits, pipes, etc.

Still wondering, do you think the aspects of 310.8(D) "Locations exposed to direct sunlight" apply to the low voltage control wire strapped to the exterior conduit?

I have looked around and I don't see any thermostat cable around here that isn't "sunlight resistant" but we may have a few guys like George who asked the question years ago.
Once it is the consensus of most AHJs, that will be the only thing the supply houses stock.

Yet, around here, more often than not the "Armaflex type" insulation on the suction line is dry rotted and reduced to black dust after not too many years in the sun (insulation more or less missing). I wonder if at some point UV / sun tolerant insulation or insulation covering is a mechanical code issue, or certainly a voluntary material Spec issue?

(sorry for the side-track, but it is all one system)

I think that has been addressed here in Florida too. I know they don't seem to use the foam rubber type Armaflex anymore. It is more like the material in a swim noodle and does hold up a lot better.
The Armaflex is really just to keep the low side line from sweating in the house anyway from what I hear but it is not my trade either.

I thought the following picture is amusing:

Source: http://en.wikipedia.org/wiki/Smart_meter

IMO, since there is no specific requirement for sunlight resistance that applies to low voltage cables, the practice should be allowed. 310 does not apply to low voltage cables, otherwise, the cables would have a lot more info marked on them and we would also know what type of insulation the conductors have.

If I recall, there is the notion of "protecting from physical abrasion or damage" or something like that, I'm wondering at what point normal landscaping / groundskeeping activities, moving water hoses around in the gardens, etc, could become a potential "cause" for the need to protect these wires in some fashion? Especially the ones zip-tied to suction line insulation that at some point degrades and leaves the wires hanging loose...

But if the low voltages on the conductors are by design (as bounded by the NEC requirements and UL type listings) not capable to ignite or cause physical electric shock, perhaps under the NEC there just isn't any concern for "loss of AC" by damage to the wire?

I'm going to have to retract what I said about Class 2 cables being used outdoors.

I dug out my UL White Book and under the heading "Power-Limited Circuit Cable (QPTZ)" , the basic listing only covers use "within" buildings. Since that means inside, cables outside would need to be marked "wet location" and to me, that would apply even if the cable is installed in a raceway (see def. of wet location).

That said, there are probably millions of installations of CL2 cable exposed to the weather (my air conditioner is one of them) and of the ones I've seen, I've never seen damaged jackets or bare conductors.

I agree that because of the low voltage and energy involved that there is little chance of shock or fire.

Learning about NEC requirments for low voltage instrument and BAS wiring as been a very interesting exercise.