I ran into a heating guy thats telling me that a #24 gauge wire can be ran in the same conduit as #14 wire. Can anyone tell me where in the NEC it talks about unlike voltages. Thermostat wire and 120v wiring suppling the furnace is what I'm refering to. Also, does anyone know where the cheapest place to purchase the NEC software. I would love to be able to use the search feature.

Thanks ahead of time.

Kevin, what your heating guy said is a very common practice, but is not considered to meet the electric code.
In practical terms, with household HVAC, you can get away with this violation; that is, it'll work. That doesn't make it right, though.
NEC specifies that when wires of different voltages are run together, the insulation of all wires will meet the rating of the highest voltage present. Since low voltage wire is usually tested/rated for at least 100 volts, and household current isn't much higher than that, there usually isn't a problem- but it doesn't hurt to make sure that the t-stat wire used has (is marked) for a higher voltage.
NEC also specifies that control circuits will be kept in separate raceways than power circuits. This is because current running through a 'power' conductor can induce a voltage in a 'control' conductor, which will fool some types of controls into operating. Once again, since household HVAC usually doesn't use these sort of controls, you can get away with "cheating."
Indeed, running the t-stat wire together with the power wire, at least for the short run from the disconnect to the airconditioner, is probably the most common code violation out there.

As for the NEC on disk, well, the best place is this site. If nothing else, you'll get the latest version- the earlier CD for the 2005 NEC had some glitches.....or get it direct from the NFPA (not reccomended except as a last resort....and I'm a member!)

If you purchase the CD from the NFPA expect to get a solicitation to become a member in the mail every week or so for the rest of your life

Reno- I have to say something here so readers are not misled into think that the issue is voltage. 300.3(C)(1) only addresses voltage. 725.55(A) addresses Classes of circuits and if Kevin is dealing with a Class 2 circuit (I appears that he is) the issue is Class of wiring, not voltage. Most Class 2 cables won't even have a voltage rating identification on the cable. Class 2 wiring is not permitted in a raceway with power wiring even if the insulation rating is rated for the higest voltage present in the raceway.
The NEC does not specify that "control circuits will be kept in separate raceways from power circuits".
Class 1 circuits (control wiring) is permitted in the same raceway with power wiring 725.26

The code does specify that Class 2 wiring is not be in the same raceway with power wiring. 725.55(A)

Finally- I am surprised as a fellow NFPA member that you would not support the worthy efforts of the NFPA.

This looks like what I need to look at to answer my question.

- 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).

B through I doesn't look like it applies to this situation and not sure about J.

They do not want to see a fault on the 120v conductor imposed on a class 2 or 3 circuit conductor - that is why they are not permitted in the same raceway. There is permission to support it to the raceway that is associated with the equipment, one of the few times in the NEC that this is permitted.

PCBelarge

You wrote "There is premission to support it to the raceway that is associated with the equipment". What do you mean by "support".

Are you saying that it's allowed if its dedicated to the equipment such as a furnace or air conditioner? I'm looking at your answer in two different ways.

George. I thank you for your clarification.

As to the NFPA...well, that's another thread altogether. Suffice it to say that I found that my "member discount" and "pre-order" got me the 2005 NEC a month later and $15 pricier than the local supply house made available to everyone else.