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Old 10-15-2012, 08:15 AM   #1
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Is this a 240V or 120V Circuit?


Hello - I'm trying to learn the terms and come to a full understanding of my 1968 single-wire radiant ceiling. Although the thermostats are all functioning well, I've lost two of my 16 separate zones due to ceiling damage/settling.

The time has come to tackle repairs - I want to thoroughly learn the system before losing any more zones - and repair the lost zones if possible.

System brand is Swan - a now-defunct company from Vancouver, WA

All thermostats are very heavy duty, mechanical SPST.

Here is the wiring inside the thermostat wall boxes. How can I tell if this is a 120V or 240V system? The breaker is 30A, there is no GFI. If necessary, I'm familiar with the procedure to safely open my CB box and inspect connections at the breaker.

Also, I'm unfamiliar with the concept of a "neutral" wire. I know the black is hot, and the copper is ground, but what exactly does the white wire do?



Last edited by imautoparts; 10-15-2012 at 08:28 AM.
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Old 10-15-2012, 08:22 AM   #2
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Is this a 240V or 120V Circuit?


Is the breaker a single pole or 2 pole breaker?
Looks like the drawing you have is 240 v.

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Old 10-15-2012, 08:34 AM   #3
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Is this a 240V or 120V Circuit?


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Is the breaker a single pole or 2 pole breaker?
Looks like the drawing you have is 240 v.
How can I tell? I know it is in a breaker box that controls all my heat and AC. All of them are 30 and 40 amp breakers, and they are twice as wide vs the 20 amp breakers in the adjoining box that feeds the room circuits.

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Old 10-15-2012, 08:40 AM   #4
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Is this a 240V or 120V Circuit?


If they look like that one, it is a 2 pole breaker, and either a MWBC, or a 240 volt circuit.
If it goes to the heat, then it is 240v.
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Old 10-15-2012, 08:51 AM   #5
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Is this a 240V or 120V Circuit?


OK - so I just Googled the "neutral" question and can understand how voltage see-saws back and forth with 120V being fed from both sides, but out of phase.



Would the break in the ceiling grid cause some odd/high voltage readings on the hot black wire when tested to ground? My probes are not of high quality, but I swear I was seeing over 120V when testing to ground off the black wire (after disconnecting the thermostat connecters)

I've ordered a wire toner to trace the grid to the break using the red wire. Can I leave the white wire intact during this testing and still get accurate results?

Last edited by imautoparts; 10-15-2012 at 09:13 AM.
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Old 10-15-2012, 09:41 AM   #6
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Is this a 240V or 120V Circuit?


If your heat is 240 volts and you DO have a break in either wire you would have to disconnect one or the other because the voltage will feed back to the break through the heater element.

Exactly how many volts were you reading?

If 120 volt heat then no, you can leave the white neutral connected.

Last edited by zappa; 10-15-2012 at 09:47 AM.
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Old 10-15-2012, 09:57 AM   #7
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Is this a 240V or 120V Circuit?


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If your heat is 240 volts and you DO have a break in either wire you would have to disconnect one or the other because the voltage will feed back to the break through the heater element.

Exactly how many volts were you reading?

If 120 volt heat then no, you can leave the white neutral connected.
Well, from the prior posts I've determined it is a 240V system. It would be very simple to pull the breaker - will this effectively disconnect the white wire?

The break is in the ceiling - we have some settlement cracking that resulted in losing this zone last year.

The voltage I read was jumping all over the place.... my DVOM is a harbor freight cheapie and I don't particularly trust it. Could a low battery be causing a fluctuating reading?

I'm going to buy a new, higher quality DVOM today. Time to get serious on this. Sorry I'm such a newbie on AC circuits.
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Old 10-15-2012, 10:07 AM   #8
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Is this a 240V or 120V Circuit?


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Well, from the prior posts I've determined it is a 240V system. It would be very simple to pull the breaker - will this effectively disconnect the white wire?

Pulling the breaker would result in no voltage to test for. Disconnect the white wire from the breaker. If no break is found then repeat the test with the white connected and the black removed. If you have a break then one or the other should find it.

The break is in the ceiling - we have some settlement cracking that resulted in losing this zone last year.

Are you sure this has caused a break in the wire? It would have to be some pretty serious damage.

The voltage I read was jumping all over the place.... my DVOM is a harbor freight cheapie and I don't particularly trust it. Could a low battery be causing a fluctuating reading?

Yes, it could.

I'm going to buy a new, higher quality DVOM today. Time to get serious on this. Sorry I'm such a newbie on AC circuits.
BTW...if 240 volt heat you don't have a neutral in the heat circuit.

Last edited by zappa; 10-15-2012 at 10:51 AM.
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Old 10-15-2012, 10:23 AM   #9
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Is this a 240V or 120V Circuit?


A question about your box. Does either the red or white have a romex sheath on it? What did they do with the bare ground? Is the red wire larger than the white? I'm confused about the wiring leaving the box.
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Old 10-15-2012, 10:54 AM   #10
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Is this a 240V or 120V Circuit?


If you really do understand how a split-phase (120/240V) service works with two hots and a neutral, then you're way ahead of most DIYers who post here. And understanding that is 1/2 of the game when doing troubleshooting like this. A few things to consider:

- Just because a wire is white doesn't mean it's actually a neutral. It should mean that (it is a code requirement), but very often 240V circuits are run using 2-conductor romex with both black and white as hots. The white wire is supposed to be re-marked as hot by wrapping it with black tape (or another appropriate color) at both ends, but this is often overlooked. If it is a 240V circuit, then there is no neutral wire - as you should expect.

- You can troubleshoot this without a circuit tracer. All you need is a non-contact voltage tester and a bit of creativity. The voltage tester works by picking up the electromagnetic field from a nearby energized conductor. If you take the white wire off of the breaker and connect it to the neutral bar in your panel, you will have temporarily converted your 240V heating circuit into a 120V circuit. That will make it so that one end of the ceiling grid is hot, and the other is a neutral potential (instead of being connected to the other hot leg as usual). Now you have 120V on one side of the break in the heating wire, and 0V on the other side of the break. A non-contact voltage tester should pick up the field from the hot side, and give no response on the neutral side. Use it to trace the wire in the ceiling until you find the break.

- A low battery in a cheap DMM can cause weird voltage readings. Test it on some known voltage sources and compare. As you should expect, it should be impossible to get a reading higher than 120V nominal (up to about 130V could be considered normal) between any conductor and ground in a split-phase 120/240V system.
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Old 10-15-2012, 11:43 AM   #11
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Is this a 240V or 120V Circuit?


There are some problems that are both code violations and make it hard to measure voltates etc.

A wire (such as the white wire in the Romex cable you are referring to) may not exit the box and make a connection inside the wall behind.

Individual wires (such as the fat red wire) may not run loose inside the wall; they must be part of a cable or enclosed in a conduit.

You would like to be able to measure voltage between the white wire and other places at that box where the thermostat is but you can't do it readily with the white wire immediately exiting the box. (Well you could put a needle or common pin or safety pin on the end of your meter probe and puncture the white insulation to make contact with the white wire conductor.)
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Old 10-15-2012, 11:44 AM   #12
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Is this a 240V or 120V Circuit?


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Originally Posted by imautoparts View Post
I've ordered a wire toner to trace the grid to the break using the red wire. Can I leave the white wire intact during this testing and still get accurate results?
Sorry, I read your words but was thinking non-contact voltage tracer. Yeah, no voltage on the wires using a toner. With either device you may still have to try twice. If you pick the unbroken wire first and it's in the same sheath you could detect no break because it has reached the heating element and is making the return trip.
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Old 10-15-2012, 12:04 PM   #13
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Is this a 240V or 120V Circuit?


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...very often 240V circuits are run using 2-conductor romex with both black and white as hots. The white wire is supposed to be re-marked as hot by wrapping it with black tape (or another appropriate color) at both ends, but this is often overlooked.
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Originally Posted by zappa View Post
Sorry, I read your words but was thinking non-contact voltage tracer. Yeah, no voltage on the wires using a toner. With either device you may still have to try twice. If you pick the unbroken wire first and it's in the same sheath you could detect no break because it has reached the heating element and is making the return trip.
Regarding the "detect no break", it is my impression this is a single-wire system. It does not have multiple wires in the ceiling heat cable - it just goes up into the grid, then back down; like old-timey Christmas tree lights. That's why you can't use a thermal imager to diagnose these.

Regarding the use of standard Romex for 240, I think you guys hit the nail on the head. I bet a dollar to a donut when I open the CB box I'll find the special marking indicating both wires are "hot".


As far as code goes, I agree that it can not be proper that the white wire is used to go up into the grid without a wire nut in the t-stat box. There are no other junction boxes in this circuit. I'm not going to demo the wall though, unless I find the break is in that stretch of wire before it hits the ceiling.

As these thermostat boxes are quite small they don't have the standard cutouts - the romex comes in through the bottom side and the red and white wire exit at the rear top of the box with a little clamp assembly keeping them secure at the back.

The ground wire is grounded to the thermostat box on the inside.

So anyway, thanks for all this info - I'm off to the store to get a new battery and leads for my DVOM, and to shop for a non-contact tester. I've located an eBay auction for a pretty good wire/cable toner kit; so within a few hours I should have that on the way.

I absolutely love this forum - I went from zero knowledge to a rational understanding of this system in under a week! One last question: Would the use of conduit for the ceiling feeds have been code way back in '68? (Indiana)

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Old 10-15-2012, 01:12 PM   #14
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Is this a 240V or 120V Circuit?


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Regarding the "detect no break", it is my impression this is a single-wire system. It does not have multiple wires in the ceiling heat cable - it just goes up into the grid, then back down; like old-timey Christmas tree lights. That's why you can't use a thermal imager to diagnose these.
Ahhhh.....I understand. All along I was thinking that you had regular old baseboard heat and were trying to find a break in the romex. Ceiling grid, settling damage, it makes sense now.
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Old 10-15-2012, 02:24 PM   #15
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Is this a 240V or 120V Circuit?


From the two systems like this that I've seen in person (and I didn't do any actual work on either of them), it seems typical for the conductors to leave the thermostat box as free individual conductors, not in a cable or conduit, and then make their way separately to the opposite ends of the ceiling heating wire. The connections to the heating wire are embedded in plaster, not in a box. This would not be code compliant nowadays, of course. Remember though: the entire heating wire, including all connections, must be embedded in plaster. The plaster is necessary to spread and conduct heat, and without it the wire and any connections will overheat and could start a fire.

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