Thermostat Wiring

[jeff43222]

So I'm playing evening electrician again, and I ran into a puzzler in a thermostat box.

I was called in to replace a 30-year-old thermostat that the HO told me didn't seem to work right. The room, heated with electric baseboard heaters, was either freezing or roasting. She went out to Big Orange (I know, I know ...) and bought a new thermostat (works for either 120V or 240V) for me to install. Here's what I found:

Inside the thermostat box were two 14-2 NM cables. The whites were twisted together, one black was connected to the line terminal, and one was connected to the load terminal. Everything was bonded properly.

The CB protecting the wires was a two-pole 15A breaker. Connected to it were the black and white wires of one 14-2 NM cable. Naturally, this makes me think the baseboard heaters are 240V. But the wiring in the thermostat box sure looked like what I'd expect to find with a 120V circuit.

So I got to wondering if a 240V thermostat is wired with one phase passing through the thermostat and one not (that would explain the wiring in the box). I tend to doubt it, though. Since most people around here heat their homes with gas rather than electricity, I don't have a lot of experience with line-voltage thermostats.

Here's where things get weird. I forgot my meters and only had a voltage detector with me. Both black wires (one line, one load) showed hot. This made no sense, and I figured one of the "hot" wires was actually showing induced voltage. To figure out which was which, I put a light bulb between each hot and the ground wire, and the bulb lit up for each. Now I'm baffled.

What I found in the panel doesn't seem to match up with what I found in the thermostat box. I couldn't find out if the heater was 120V or 240V, and I'm sure not going to guess.

Anyone have any bright ideas?

[stickboy1375]

open the heater up and look at the nameplate...? I've never see 120v baseboard heat. Who ever installed the last T-stat probably only broke 1 leg...

[big vic]

It is common to find 240 volt baseboards hooked up with single pole t-stats

However unless you use a breaker lock, double pole stats should be used with 240 volt heater so both ungrounded conductors are disconnected for servicing.

[mdshunk]

However unless you use a breaker lock, double pole stats should be used with 240 volt heater so both ungrounded conductors are disconnected for servicing.I've even been using double pole stats with the contacts connected in series for 120 volt loads, since they are the only line voltage stats that seem to have a marked 'off' position.

[jeff43222]

One of the local box stores does sell 120V baseboard heaters. If it's a low-wattage heater (e.g., 500W), it doesn't make a whole lot of sense to wire it up for 240V and take up two spaces in the panel if the load could easily be carried with a single 14-2 NM cable at 120V.

I just cracked open the heater and found it rated for 240/208. That matches what I found in the panel.

The thermostat the HO bought, according to the instructions, can be used switch-loop style for a 120V load (line2 and load2 leads tied together and not used), and pass-through style for a 240V load.

What confuses me is why the alleged line and alleged load wires in the thermostat box both lit up the light bulb. Only one of them should have.

[stickboy1375]

I just cracked open the heater and found it rated for 240/208. That matches what I found in the panel.

The thermostat the HO bought, according to the instructions, can be used switch-loop style for a 120V load (line2 and load2 leads tied together and not used), and pass-through style for a 240V load.

What confuses me is why the alleged line and alleged load wires in the thermostat box both lit up the light bulb. Only one of them should have.

Are you talking about a voltage tic? Never trust those... they can be a bit too sensitive at times...

[jeff43222]

No, I know not to trust a voltage tic, as they read induced voltage all the time. Mine was showing both hot, so I used a 60W, 120V light bulb to introduce a high-Z load and find which black wire had the real voltage on it. The bulb lit up each time. ???

[stickboy1375]

I would say it would light up each wire, break the whites and try it, bet it wont after you do that! :)

[jeff43222]

I think you might be on to something there, stickboy. You're right -- I didn't break the whites apart, so with the CB closed, the "load" wire was actually being fed through the heater back to the thermostat box, right? That would explain why the light lit up both times.

But what still doesn't make sense to me is using a single-pole thermostat for a 240V heater. Seems to me that doing that would have one leg of the heater always hot, but since there are no 120V loads, there would be no reference voltage (neutral) that would allow the heater to operate at 120V.

Sounds to me like a good game plan would be to disconnect the whites, check the voltage between each black/white set to find the line side, and wire up the thermostat properly with both poles going through it.

[JohnJ0906]

The reason the bulb lit up was the current flowing through the heating element from the other line (the "white")
L2 of breaker to T-stat box, tied through, to heater L2, through element to L1 of heater back to T-stat -
edit to add-Jeff, you typed faster than me!

[jeff43222]

It is common to find 240 volt baseboards hooked up with single pole t-stats

However unless you use a breaker lock, double pole stats should be used with 240 volt heater so both ungrounded conductors are disconnected for servicing.I'm not sure I follow. Both ungrounded conductors went dead when I opened the CB, as it was a two-pole breaker. I don't see how the thermostat being single-pole or two-pole would make a difference.

[celtic]

I think you might be on to something there, stickboy. You're right -- I didn't break the whites apart, so with the CB closed, the "load" wire was actually being fed through the heater back to the thermostat box, right? That would explain why the light lit up both times.

That is exactly what happened. You read right through the element.


But what still doesn't make sense to me is using a single-pole thermostat for a 240V heater. Seems to me that doing that would have one leg of the heater always hot, but since there are no 120V loads, there would be no reference voltage (neutral) that would allow the heater to operate at 120V.
Right again....one leg of a 240v will not do "anything"...it needs the other leg.


Sounds to me like a good game plan would be to disconnect the whites, check the voltage between each black/white set to find the line side, and wire up the thermostat properly with both poles going through it.
There's nothing wrong with doing something right ;)

[stickboy1375]

A 240v load won't operate on 120v, it's the same as a 120v load won't operate without a neutral... Even a 240v Tstat will only break 1 leg in operation, but when you move it to the OFF position, then it will break both legs for servicing....

[celtic]

It is common to find 240 volt baseboards hooked up with single pole t-stats

However unless you use a breaker lock, double pole stats should be used with 240 volt heater so both ungrounded conductors are disconnected for servicing.

Is a t-stat intended to be a disconnect?

[stickboy1375]

If it says OFF you better believe it...

[jeff43222]

I know the 240V heater wouldn't operate at 120V, but I guess I just prefer that all ungrounded conductors be dead when you want a device to be off. Probably because of all the switched neutrals I've run into in old houses. Off means off, and to me that means both legs are off.

[stickboy1375]

I know the 240V heater wouldn't operate at 120V, but I guess I just prefer that all ungrounded conductors be dead when you want a device to be off. Probably because of all the switched neutrals I've run into in old houses. Off means off, and to me that means both legs are off.


Exactly, thats why when the Tstat is labeled OFF, its OFF...

[JohnJ0906]

I agree, but I still don't trust thermostats if I'm working on them. I turn the breaker off.

[jeff43222]

I always turn off the breaker, too. I just don't like the idea of a live 120V conductor connected to an "off" heater.

[celtic]

Exactly, thats why when the Tstat is labeled OFF, its OFF...

...unless it's busted ;) in the closed position...

[stickboy1375]

...unless it's busted ;) in the closed position...


I know... watch everyone go try and work on one tomorrow get belted... :)

[jeff43222]

Thanks for all the help. I think a solution has been found!

[celtic]

Thanks for all the help. I think a solution has been found!

Gonna change it out to gas heat?

[jeff43222]

I would, but I'm not licensed to do mechanical work. Or plumbing/gas. And I suspect the condo association might not be too keen on the idea.

Glad I live in a house where there is no association to deal with. I have gas heat, and not just after I eat chili. :D

[iwire]

FYI the NEC does not require the thermostat to break both legs.

Never assume it does.

[hardworkingstiff]

When a single-pole T-stat breaks one of the ungrounded conductors to a heating element, you don't really have 120-volts to the heating element. There is a voltage on the heating element that is 120-volts to ground, but there is 0-volts across the heating element.

Yea, I know it's just a technicality.

[wag]

The 120v lamp lights up on each black wire because the 120v white wire feeds through the LOW resistance of the heater. If you disconnect the 2 spliced white wires and use the test lamp to ground it will light for only one white wire and one black wire. These are the home runs to the panel. The other white and black wire are the load wires. A 1000 watt, 240 volt heater has a resistance of only 58 ohms. A test lamp doesn't draw much current so there is very little voltage drop across the heater and the lamp doesn't appear dim on the load wire.

[kbsparky]

FYI the NEC does not require the thermostat to break both legs.

Never assume it does.I thought it did if you intended to use the thermostat as one of the required disconnecting means.

:confused:

[celtic]

FYI the NEC does not require the thermostat to break both legs.

I thought it did if you intended to use the thermostat as one of the required disconnecting means.

:confused: Have you read:
424.20(A) & (B)?

[kbsparky]

Have you read:
424.20(A) & (B)?Yes, that was the basis of my response.

IF you want to use a thermostat as a disconnecting means, then all ungrounded conductors must be opened when placed in the "off" position.

In regard to Bob's statement, and for clarification, the NEC does not require the thermostat to break both lines during the normal course of operation. Only when switched "off" and used to disconnect power from the heater.

[LarryFine]

A 240v load won't operate on 120v, it's the same as a 120v load won't operate without a neutral... Even a 240v Tstat will only break 1 leg in operation, but when you move it to the OFF position, then it will break both legs for servicing.... This is correct. Besides, even a T-stat that did operate both legs, would likely never break both legs at the same instant.

That's one reason 2-pole breakers have a common trip. Even two identical fuses in series will rarely both blow under overload or short conditions.


A 240v heater will operate at 1/4 power on 120v. There are 120v baseboards, but you have to go out of your way to find them.

[stickboy1375]

This is correct. Besides, even a T-stat that did operate both legs, would likely never break both legs at the same instant.

That's one reason 2-pole breakers have a common trip. Even two identical fuses in series will rarely both blow under overload or short conditions.


A 240v heater will operate at 1/4 power on 120v. There are 120v baseboards, but you have to go out of your way to find them.

How come it would be 1/4 power and not 1/2?

[LarryFine]

How come it would be 1/4 power and not 1/2? Because when you halve the voltage across a given resistance, the current also halves. Since power is voltage times current, half of a half is one quarter (0.5 * 0.5 = 0.25).

This is also one reason it's such a good idea to run a convertible hottub on 240: you get 6Kw (if that's the rating) of heat instead of 1.5Kw. You get faster heating and you can heat while the jets (hi-speed) are running.