Can i wire three way switch using 12-2 romex?

fiddling fingers have caused you a problem
best get in an electrician to check the wiring and fix it properly

Hi - Romex is a brand name for NM type cables, and is acceptable wiring method in the 2012 National Electrical Code for use anywhere that is not a wet location in residential buildings; unless your state, county or town specifically prohibits its use.

When you say "12/2" or "12/3" all you are telling us is that you have some type of cable that consists of 2 insulated wires and a second cable type that consists of 3 insulated wires (and possibly a ground wire - but you didn't say "12/2 with Ground" as it would be indicated on the packaging. This same "12/2" or "12/3" designation is used to describe BX, AC, MC, NM, UF, and other cables, too. Since I do not know what cable type you have, I can't say that you can use it or not with any certainty. I can tell you that based on the 12/2 or 12/3 info you provided, it can be used on 20 amp circuits without a problem.

Please provide the cable type or description for a better answer.

Copy following link for switch wiring diagrams:
http://waterheatertimer.org/How-to-wire-switches.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

I think you mean that you need to wire a set of three way switches. That is when you have a switch at two locations to run the same load (lights for example). Without being able to draw a picture, I will try to explain this the best I can. You have probably 120 volts coming in on a 14-2 romex for a 15 amp circuit or a 12-3 romex for a 20 amp circuit consisting of a black, white, and a bare wire. You run that set of wires to one of the two switches. Then from that switch you need to run a type of 3-wire cable from that switch to the other. For example:14-3 romex for a 15amp. circuit or 12-3 romex for a 20 amp circuit. Do not reduce wire size if you start with 12 gauge wire coming in from the breaker. Then you again run a piece of 12-2 or 14-2 (depending again on the amp circuit you are running) from the second switch to the load or lights that the switches are serving. Now for terminating the wires. In both switch boxes you just connect the white wires together with a wire nut. The bare wires you also connect together with a wire nut, but also twist a short piece of bare wire in the wire nut. That tail will serve as your jumper to the green screw on the switch. Then all you should have left in each box is two black wires and a red wire. These three wires all go to the switch. On the switch you should have three screws on the sides. One is darker than the other two. That darker screw gets the black from the 14-2 or 12-2. Then all thats left is the black and the red that come from the 14-3 or 12-3. Those two go to the remaining two screws on the switch. It does not matter which one either goes on. That should do it now you just wire the light like you would any other.
Hope this helps. Any other questions feel free to email me.
P.S. Don't forget to turn off the power!!

If all you are using is household AC, Use this link:

http://en.allexperts.com/q/Electrical-Wiring-Home-1734/Wire-gauge-size.htm

I like to authenticate my answers, (hence the links)
Good luck...

OK, your new switch will require the use of a neutral wire. So you will have to do what is called a pigtail. You will have to twist another white wire in with your other white wires to attach your neutrals to your switch. Note: white wires are called neutral not common. Hook the wires that were originally hooked up to your LINE to your HOT (Black) on your new timer, and the what was originally LOAD up to the new timer LOAD. SO as far as wiring all that will change is your white wires that need to attach to your timer NEUTRAL.

Hi .. with only bone switch for this combo unit, the connections would be like this .. all ahites together with the romex white , all colors, red, black, blue with the romex black, all greens together IF you had more than one switch , then the red, black, blue would be connected separately to one switch eack and separately to a 4 wire romex from the switch to the combo unit.

Hi, this is very easy for you to do.The 12/2 Romex wire coming in is the power which you know. What you do need to know, is the voltage coming in? Sometimes they will use 12/2 Romex with ground for 220, 240, operation. You need to know if the T-stat is Line voltage 220, or 240 volt. You have to have the same voltage for both for it to control. Reguardless of the voltage, lets say it is in the 220 and up range. You take one of the 12/2 wires, white or black and hook it to the black or red on the T-stat and the white to the other one, red or black. Thats it, you are done! If the T-stat is 2-pole I am sure it is a high, Line votage T-stat. Best of Luck,
Shastalaker7

12 gauge wire sounds under sized for the application. And 20 amp 240 v breakers might be sufficient if you where wiring the heaters separately. But be sure that your wiring is not romex - because romex is an insulator and is not permitted in conduit. I would suggest that you up size your wire to 10 guage and your breaker to 30 amp as well. Then double check your heater wiring connections to insure that they're wired the same throughout.

The black and white in the fan box are the main power and neutral for normal (low speed) of the fan. The two reds are the signal wires. It is basically a switch loop. You will actually need a total of 4 conductors (plus a ground) in order to run the high and low speed of the fan. The four conductors consist of 1) power from one switch to power up the fan, 2) neutral, 3) signal going to second switch, 4) signal going back to fan from second switch in order to run high speed. Without four conductors you can still run the high speed only by tying the two red wires in the fan together. This would be like always having the second swith in the on position. Hope this helps.