Electrical wiring question: 2 x 120V lines need to be made into 240V. How?

[Jeff7]

Update
Voltmeter test: Only one of the "incoming" wires shows power, and it's already 240V. I think that the other wire goes off somewhere to a heater that had been removed several years ago.

Part II
Ok, I think it's all finally figured out. There is one wire coming in from the breaker box, providing 240V. It then splits off two ways - one goes directly to the thermostat and then to the heater, the other wire feeds into another thermostat in another room, which controls a different heater.

There were also remnants of the old thermostat's wiring - stranded wire wrapped around bare solid wire, and a 68K 1-or-2 watt resistor wired across somewhere, using a wire that looks to be AWG 16.



I've got two 120V lines coming to an electrical box, and a line going out to a baseboard heater. The two breakers that feed the incoming lines are ganged together.

So what I'm looking at:
- Two cables feeding power, 12 gauge, with a black, white, and ground wire each.
- One cable going out to the heater, with a black, white, and ground wire.

These wires previously connected to a thermostat, but that thermostat broke (it decided to set itself to maximum and stay that way, regardless of what the dial was set to), so it was unceremoniously removed, and the wiring was capped in wire nuts and left unused - until now.

I've got a new thermostat (SKU 442-658, in case that direct link doesn't work), and it's got instructions for either 4 wires or 2 wires. Not counting the ground, I've got 6.


So then, I think that the main thing I really need to do here is to wire the two 120V lines into a single 240V line. With that done, I should be able to use the new thermostat's 4-wire instruction set.


And an FYI, yes, I have done some wiring in the past, but only with 120V lines. I do know how to keep hot, neutral, and ground in their proper places.

 

[Nocturnal]

I think it would be much better if you actually consulted with an electrician instead of taking advice over the internet. Not only that but you're putting yourself in harms way if you really don't know what you're doing.

 

[JohnCU]

Two 120V lines 180 degrees out of phase is 240V, but you can't just wire 2 120V lines into one terminal and get 240V, doesn't work that way.

 

[Greenman]

It could be that you just have two different circuits in one box. Generally, if it's set up to be one 240V circuit it will be run in 3 wire romex.

The wire going to the heater is 120v, you can't get 220/240 to it with only a black, white and ground.

DO NOT CONNECT THE TWO BLACK WIRES IN THE BOX TOGETHER!!

If you could post a pic it would help.

Edit: You're measuring something wrong. You can't have 240 on one wire unless you reside in a commercial building. Or you're not in the usa.

 

[Jeff7]

Check out the Part II update.


I am not measuring anything wrong, I assure you.

Out of Cable #1 come three wires:
Black, white, and ground. Voltmeter connects to the black, and then to the white. Result: 238VAC.

Cable #2: it goes directly to one baseboard heater.

Cable #3: it goes to a thermostat in another room, and then to a baseboard heater.



Pic, as requested

The two capped wires on Cable #1 were the two to which I connected the voltmeter. Voltage measured: 240V.


Apparently these were wired as one would wire a 120V outlet so as to split one incoming line into several. The difference is that here, 12 gauge wire is being used to carry 240V, proivded by two breakers out in the breaker box. (Or maybe it's a dual-pole breaker; all I know is, there are two switches that are linked together. I know enough to leave circuit breakers where they are.)

For those wondering, the drywall in the back is to be disposed of, so I wasn't too concerned about its well-being when removing the electrical box.

 

[Rubycon]

The electrician (?) could've used 12/2NM to wire to a double pole breaker. This would give 240VAC across white and black wires. Sometimes this is done, however the white wire SHOULD be taped with either black or red tape to denote L1/L2 wiring back at the panel rather than L1/L2+N (120V) service.

Also if you have a pair of 120V circuits and the breakers are on opposing positions in the panel you WILL measure 240V across both black lines.

 

[Jeff7]

Originally posted by: Rubycon
The electrician (?) could've used 12/2NM to wire to a double pole breaker. This would give 240VAC across white and black wires. Sometimes this is done, however the white wire SHOULD be taped with either black or red tape to denote L1/L2 wiring back at the panel rather than L1/L2+N (120V) service.

Also if you have a pair of 120V circuits and the breakers are on opposing positions in the panel you WILL measure 240V across both black lines.

Actually, the black wire has red tape around it, if that matters.

And indeed the wire does have (very weakly raised) lettering which says "AWG 12 Type NM-B."

 

[Rubycon]

Generally the white is taped black or red to denote it is bonded to a hot (ref ground) bus instead of a neutral to prevent accidents.

NM-B is the most common household wire used in the past 30 years or so. The outside version is called UF.

 

[sao123]

A properly wired 240V circuit (residential) should use 10/3 or 12/3 wire on a 2 pole breaker. It has 4 wires inside... black, red, white & ground.

 

[Rubycon]

Originally posted by: sao123
A properly wired 240V circuit (residential) should use 10/3 or 12/3 wire on a 2 pole breaker. It has 4 wires inside... black, red, white & ground.

Three wires (plus ground) are only needed if an appliance needs 240VAC split phase power. (such as a stove or dryer that may have a 120VAC motor or timer circuit, for example.)

It's fine to wire a resistive load (water heater, for example) with 10/2 NM-B wire.

Also most NM is good up to 600V so a 15A circuit for an air conditioner or a 1hp compressor can most certainly be wired with 14/2 NM-B.

Of course what determines what must be used is your local electrical codes. Adhering to these is important to pass inspections for insurance regulations, for example.

 

[Greenman]

Originally posted by: Jeff7
Check out the Part II update.


I am not measuring anything wrong, I assure you.

Out of Cable #1 come three wires:
Black, white, and ground. Voltmeter connects to the black, and then to the white. Result: 238VAC.

Cable #2: it goes directly to one baseboard heater.

Cable #3: it goes to a thermostat in another room, and then to a baseboard heater.



Pic, as requested

The two capped wires on Cable #1 were the two to which I connected the voltmeter. Voltage measured: 240V.


Apparently these were wired as one would wire a 120V outlet so as to split one incoming line into several. The difference is that here, 12 gauge wire is being used to carry 240V, proivded by two breakers out in the breaker box. (Or maybe it's a dual-pole breaker; all I know is, there are two switches that are linked together. I know enough to leave circuit breakers where they are.)

For those wondering, the drywall in the back is to be disposed of, so I wasn't too concerned about its well-being when removing the electrical box.

Got it. I thought you were getting 240v from 1 wire to ground, which can't happen on a single phase circuit. When you state 1 wire you mean 1 piece of romex.

Are the baseboard heaters 120v?

 

[Jeff7]

Originally posted by: Greenman
Are the baseboard heaters 120v?

The baseboard heaters are 240V; one of them is rated 1500 watts, and the other one is likely the same.

 

[Paperdoc]

OK, after reading through this, you have a junction point - should be a box, but pic looks like just a hole in the wall for now. It has 3 cables in it, and each has black, white and bare (ground) wires. One comes from the breaker panel and its black and white are fed through a pair of ganged breakers conected to opposite sides of a 120 / 240 v supply system. Across those two wires you have 240v. The second cable goes directly to a baseboard heater rated for 240v, 1500W, and you want to hook up a new thermostat to control it. The third cable goes to a separate thermostat and its baseboard heater, of similar power rating.

First, the total power consumption of the two heaters is 3000 W at 240v, OK for a 15A circuit.

IF the cables all had been 12/3 (black, white, red and bare) instead of 12/2, it might be easier to understand. Then the two halves of the supply would be on black and red. White would be neutral as usual, but it would be useless - connected to nothing! So it was done without the red. As one other said, it would be ideal to wrap each white wire in each box with red tape to show clearly it is NOT neutral, but a hot lead.

But, understanding all that, what you have is ONE feed cable (not two as in your first post) from the breaker panel being split into two loads. For each load there is a line (one black and one white wire) going to a thermostat, and that device's output goes to its own baseboard heater. On each thermostat you probably have two input terminals (for black and white from the 240v supply) and two output terminals (for black and white out to the baseboard heater). So, you split the feed: feed black > 2 blacks - one to the remote thermostat/heater, and one to the input black terminal on your new thermostat. Same split from feed white to two whites, etc. All the bare (ground) leads get twisted together and fastened down to the box ground screw. Last item is simply to connect the (currently uncontrolled) baseboard heater to the two output terminals of your new thermostat.