18awg @ 120vac

[bill@usps06492]

I've been asked if it is okay to wire a line volage(120vac) t-stat with 18awg t-stat wire. The the wire terminates, with other control wires in the same j-box that provides the main power to the furnance, which is also 120vac. The concern is that the insulation on the t-stat wire is not rated high enough to be in the same box as the 12awg that provides the power to the furance. Art310.5 table it shows that the minimum size wire between 0-2000v to be 14awg. So to me it appears that even if the 18awg had the same insulation and could carry the current, the minimum the wire can be is 14awg.....
Has anybody come across a situation like before?
Bill

 

[LarryFine]

It seems that, if you're running this wire in the walls, etc., as you would a regular thermostat run, you are correct. The two solutions are to either use #14 to the thermostat, or use a 24v relay to allow the thermostat to control the 120v without actually seeing it.

 

[tom baker]

The minumum size for a branch circuit is 14 AWG. You can go smaller if its a class 1 2 or 3 Art 725 circuit

 

[ramsy]

I've been asked if it is okay to wire a line volage(120vac) t-stat with 18awg t-stat wire.

I have a 4.5A window AC the uses that #18 at 120vac, but code limits that cord & plug length to 10 feet. Code also shows SFx & TFN insulations are available for #18.