Baseboard heater, 2 legs of 120v power with a single pole thermostat?

[Jake6879]

I used to do some electrical in my younger days but after a good few years in a desk job I've probably forgotten too much. In helping a buddy I came across something that has me confused. We pulled the baseboard heaters a few weeks ago so they could paint and when I came back to reconnect them and opening the panel, I'm suddenly doubting myself. When we disconnected, I assumed they were wired as 120v and didn't pay much attention to it. When I went to reconnect and took the cover off the panel to trace a couple of circuits, I noticed I have a white wire on the breakers for the heaters. We have two sets of 12-2 romex coming into the heater, I checked and I have 120v to ground on both the black wire and white wire. So far so good. Then I attempt to wire the thermostat and it's only 1 pole.. After pulling apart another heater in the house, it was wired with the black wires being broken by the thermostat and the white running to the other side of the heater, which is why I assumed 120v connection.. so, what gives, have I just forgotten way too much or does this not seem right?

 

[jaylectricity]

Was there 240V between black wires in the heater's junction box?

This may be way out there, but maybe the electrician ran new wires from thermostat to heater and wasn't confident he could use the black and the white and opted to run two cables because that's what he happened to have in his truck.

I would determine which 12-2 was which and abandon one of them. Did you determine if the baseboard heat was indeed 240? If so, obviously you need to get a 2-pole thermostat. I mean, you can have a single pole thermostat, you just need to know that it's not a disconnect.

If it's only 120 than obviously the white neutral can get tied through in the thermostat junction box with the blacks going through the thermostat.

Any chance I'm misreading that there's two 12-2's in the heater when you meant thermostat?

 

[jaylectricity]

Also, if both black wires at the heater are reading 120 to ground, if they aren't reading 240, then they're just supplying the feed twice. It may be the same branch circuit, or they could just happen to be on the same phase in the panel.

Either way, I suggest ditching one of them. Tape them up and coil them up and out of the way inside the junction box.

 

[GoldDigger]

If the heaters are wired between a black wire connected to one breaker and a white wire connected to another breaker, then you have a 240V feed to the heater.
A disconnect would have to interrupt both wires, but a thermostat could get way with interrupting only one.
The problem you are seeing is that whoever installed the wiring in the first place probably pulled a single 2 conductor (plus ground?) length of NM and did not re-identify the white wire as a second hot.
You probably do not have a neutral, just some dangerous white wires.

PS: I read the OP's "interrupting the black wires" as just meaning that it interrupts one black wire, leaving two ends. Just a normal switch leg. Not that there are two sets of black wires connected to the same thermostat pole. We will see what his answer is.

 

[jaylectricity]

One last theory, that the feed comes into the baseboard, then a switch leg is run to the thermostat.

 

[Sierrasparky]

One heater may feed another.
Or it was a single pole thermostat on a 240v heater. Pretty commonplace in the old days.

 

[Jake6879]

One heater may feed another.
Or it was a single pole thermostat on a 240v heater. Pretty commonplace in the old days.

I may not have been as clear as I could have been. I'm fairly sure the second set of 12-2 does continue on to feed another heater, I will go over there and verify.. I think I'm rounding the corner on remembering what I'm looking at but I still need adult supervision, lol.. After some reading tonight, I am reminded that the black and white 120v legs would be out of phase and as such could be connected to opposite ends of the device to give 240v. It seems clear now but late at night and after a long time away from the trades, It made zero sense to me..

 

[Sierrasparky]

i would bet that you have a single pole t-stat powering a 240v device. Not legit today as a disco unless the breaker is within site of the heater.

Please note many t-stats do not have a positive OFF. So the power may not be off.

 

[growler]

After pulling apart another heater in the house, it was wired with the black wires being broken by the thermostat and the white running to the other side of the heater, which is why I assumed 120v connection.. so, what gives, have I just forgotten way too much or does this not seem right?

I would think that this is also a 240V heater. It should be marked or you can just measure the voltage between white and black.


This is often confusing for people. They see a white and black and think it's 120V. On newer installations we are required to remark the white wire but it wasn't done that way years ago.


You can break either leg (phase) you wish with the thermostat (white or black wire ).

The thing to remember is that turning off the thermostat does not turn power off to the heater and for safety the breaker needs to be off.

 

[GoldDigger]

The thing to remember is that turning off the thermostat does not turn power off to the heater and for safety the breaker needs to be off.

Assuming of course that it is a single two pole breaker.

Who knows, it might be two single pole breakers without even a handle tie, let alone a common trip.
Current rules require a common trip for a 240V load and a common trip or handle tie for 120V loads on an MWBC.

 

[Sierrasparky]

If you install a 2 pole thermostat with a positive off that can suffice as the disco

 

[hurk27]

Since you clarified that the other NM feeds another baseboard heater then you need to see if it has its own thermostat, if it does then wire the two black together and the two whites together in both of the NM cables, connect a pigtail from the white to the one end of the heater then pigtail the black to the line side of the thermostat which the load side will go to the other end of the heating element, if the other baseboard doesn't have it's own thermostat then put both heaters on the load side of the thermostat and it will control both heaters as long as the thermostat is rated for the current of both heaters.

Some things to make sure of for safety:

Make sure both baseboards are rated for 240 volts, putting 240 volts to a 120 volt heater is a fire waiting to happen, there should be a label under the front cover that gives the ratings and wattage of the heater, also they should be connected to a double pole breaker not two single pole breakers, also make sure the thermostat is rated for the current of both heaters if it will switch both, also the thermostat has to be line voltage rated do not try to use a low voltage stat on a line voltage heater, had a friend almost burn down his house trying to do this.

 

[growler]

We have two sets of 12-2 romex coming into the heater.

One heater may feed another.
Or it was a single pole thermostat on a 240v heater. Pretty commonplace in the old days.

It's a little late in the game for this advice but if you are not sure of what you are disconnecting it's a good idea to mark everything when taking it apart so that you can reconnect the same as it was.

It probably does feed another heater but now you will have to check to make sure.

 

[hurk27]

Assuming of course that it is a single two pole breaker.

Who knows, it might be two single pole breakers without even a handle tie, let alone a common trip.
Current rules require a common trip for a 240V load and a common trip or handle tie for 120V loads on an MWBC.

Where does the code require a common trip for a baseboard heater? this would mean fuses would no longer be allowed.

The only place I know of in the NEC that requires a common trip is 450.5 for autotransformer's where using a Zig-Zag to create a derived neutral

Don't get me wrong but I don't see for this application that the NEC requires a common trip?

Here's the handbook commentary to 210.10 about disconnects:

Circuit breakers or switches that are used as the disconnecting
means for a branch circuit must open all poles simultaneously
using only the manual operation of the
disconnecting means. Therefore, if switches and fuses are
used and one fuse blows, or if circuit breakers (two singlepole
circuit breakers with a handle tie) are used and one
breaker trips, one pole could possibly remain closed. The
intention is not to require a common trip of fuses or circuit
breakers but rather to disconnect (manually) the ungrounded
conductors of the branch circuit with one manual operation.See 240.15(B) for information on the use of identified handle
ties with single-pole circuit breakers.

The above in red spells it out very nicely.

 

[GoldDigger]

Agreed....

(Although Article 647 also requires common trip for a balanced 60V technical power system.)

Common trip is certainly a good idea for 240V loads, but is not generally required.