I want to install a 220v 20 amp air conditioner outlet. Seems like a pretty simple job, but I want to check. The air conditioner is on the floor and position directly above the main panel,

There is already an outlet near there and I can see where the wires go through the bottom wall plate from the basement. I want my new single outlet in the stud bay next to the current outlet duplex.

My plan: cut out the drywall for the new single gang box, drill up through wall plate into the proper stud bay. Use fish tape up from the basement to the new outlet hole. Run 12/2 rommex from the main panel to the new outlet location.

Use a double pole 20 amp breaker in the main service panel (there is plenty of space in the panel I just upgraded it to a 200 amp service, properly grounded, etc).


My question, is it OK to use a plastic old work deep single gang box for this outlet and voltage?

To complete, I would wire outlet with white and black on the hots and ground on the green of the outlet. Black and white go on each of the poles of the double 20 breaker and the ground on the neutral/ground bar. Paint the white black with electrical tape on both ends to indicate they are hot.

Anything I missed? Other than not to lick any hot wires :D

PS. I thought I would use tape to paint whites black in case I decide to go back to 120volts by taking the white off the breaker and putting it on the neutral bar and switching the outlet to a 120 device. I could then remove the tape.

Also how do you test the power on a 220v circuit. I would think if you put the probes in both hots you would have a major problem. Do you touch one probe on one hot and the other on the ground and then repeat on the next hot. Want to be safe when dealing with 220v. I have a simple two probe and light tester. Nothing fancy. I believe it says it is rated for 220v.


<cleaned up> KISS

I would use 10/3
Black and red to the breaker
White and bare or ground to the neutral bar
With a 2 pole 30 amp breaker
No need to paint wires
Plastic box is OK
Checking 220V put meter leads toeach wire to see 220V
Check one at a time to ground to see 120V

Everything you said is fine including taping the whites and the use of 12/2.

You could run 12/3. It's not that long of a run. No reason to run #10. You'll have red/black/white and bare and no confusion. You can just wirenut the white in the box and connect it at the panel.

There is no 220 in the US now, it's a 120/240 service.

Testing:

You have 120 from neutral to any hot and ground to any hot. You'll have 240 from hot to hot.

Thank you! So essentially the same wiring scheme I described but just using the hot wires of a 12/3 cable and capping the white as it won't be used.

What is your thought about the plastic box on this circuit?

Fine. It's still 120 with respect to ground.

Please explain. I don't understand that aspect of this. Why would that matter with respect to a plastic box vs a metal and how could you then have a 240v ground?

The 120/240 V system used in residential is derived by a center tapped transformer. So, the secondary is 240 Center tapped. The center of the transfomer is grounded and it is also your neutral.

So, the legs of the 240 V is just 120 V 180 degrees out of phase.

Metal boxes are good because you can't strip the outlet attaching screws like you can in a plastic box. If something goes wrong inside the box, the leg of the 240 which is 120V with respect to ground shorts to ground and pops the breaker.
If the 240 hot's tough each other, the breaker pops.

Plastic has an insulating value depending on thickness and so does air for that matter. The boxes are likely thick enough to safely take 600 V. The boxes only have to insulate to 120 V with respect to ground and the 240/120 system is 120 with respect to ground.

One line is esentially
L1 = ~170*cos(wt)
L2 = ~-170*cos(wt+PI); note the negative sign

w=1/f

Thus L1+L2 doubles. The cos function goes from +1 to -1. 155 is about the 0 to peak value which is 120*sqrt(2), thus a 120 V sine or cosine wave is 170*2 volts peak to peak.

Hmmmmm... thank you for indulging me. I held on till the second half but appreciate the info so I can learn. Understand the math, don't fully understand its implication on the plastic box v metal.

OK, try this for size. Say I had two conductors ground and hot and both had 0.001 teflon insulation. Since I know teflon has a breakdown of about 1000 V/mil. I applied 50 V between the conductors, they would insulate.

If I applied 10,000 V betwwn those conductors, the insulation would break down and I'd have a short or arc between the air. The insulation would vaporize.

Pretend the conductor is in the plastic box and it's sitting next to a metal grounded stud. Same difference. There is a ground in the cable too and the insulation on the wires prevent it from breaking down the insulation.

The metal box thing is analogous to say a metal washing machine. The washer is grounded, so any insulation breakdown flows through protective ground rather than the neutral, drawing an overcurrent and shutting off the breaker.