Wiring for tstat when line is at the heater.

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  #1  
Old 12-10-08, 11:22 PM
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Wiring for tstat when line is at the heater.

Hey all,

Thanks in advance for your help. I am fairly new to electrical but have done some residential lighting branch circuit wiring, but this is my first time dealing with heat and thermostats. In my family room there are two heaters, both 240-volt Cadet ZA-202 wall heaters rated 2000 watts@240-volts. There was a mechanical-style thermostat installed to control both heaters, but the confusing part is how it's connected. SP is the service panel, S is the single-pole thermostat, and H1 and H2 are the heaters:



The circuit uses 10 gauge NM-B 2-wire with both conductors used as ungrounded "hot" conductors and each conductor protected by a 20A CB. Not having a grounded neutral conductor is messing with my head, but that aside, I don't understand how this works. In this scenario, my logic tells me that the thermostat would have no control over the heaters. The heaters would be receiving uninterrupted current from the service panel because the tstat is not in-line with the heaters. I guess I am not understanding the flow of electric current.

I'd like to install a programmable double-pole thermostat to control the circuit, but as far as I can tell, that isn't possible without the line coming into the thermostat, which would dictate either re-wiring or moving the thermostat. I read once somewhere that double-pole thermostats are a code requirement for 240-volt circuits in the U.S. Anyone know if that's true? I live in Oregon.

Thanks,
William
 
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Old 12-11-08, 06:37 PM
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Originally Posted by wmcglass View Post
The circuit uses 10 gauge NM-B 2-wire with both conductors used as ungrounded "hot" conductors and each conductor protected by a 20A CB. Not having a grounded neutral conductor is messing with my head
The service to your house is 240V single split phase. You want to use the full 240 volts for you heater so no need to split with a neutral. In your situation just think of one wire as being the neutral and wire accordingly. (Keep in mind though that both wires are hot when the breaker is on).

Originally Posted by wmcglass View Post
but that aside, I don't understand how this works. In this scenario, my logic tells me that the thermostat would have no control over the heaters. The heaters would be receiving uninterrupted current from the service panel because the tstat is not in-line with the heaters. I guess I am not understanding the flow of electric current.
You need to have another look as your wiring diagram is wrong and the heaters would be powered 24/7 in that arrangement not to mention the tstat would cause a short circuit across the 2 wires. One wire from the service panel goes to the tstat first, then on to the heaters. The other goes from the service panel directly to the heaters. This is no different the wiring a light switch to switch a light, except here you light switch is the tstat and your light is the heaters.


Originally Posted by wmcglass View Post
I'd like to install a programmable double-pole thermostat to control the circuit, but as far as I can tell, that isn't possible without the line coming into the thermostat, which would dictate either re-wiring or moving the thermostat. I read once somewhere that double-pole thermostats are a code requirement for 240-volt circuits in the U.S. Anyone know if that's true? I live in Oregon.
Its NEC code 424.19. You need a disconnecting means. A two pole tstat would work, but not in your case because of the wiring. I would do the following; The code allows you to install a two pole switch with a marked off position at the baseboard (per 424.19(C)) as your disconnecting means. Then you could go ahead with the single pole tstat and that would be legal. Also my experience has been it is difficult to find 2 pole programmable tstats (I have no idea why, but it has been the case).
 

Last edited by doug321; 12-11-08 at 07:05 PM.
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Old 12-12-08, 02:43 PM
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This article might help you unmess your head.
 
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Old 12-13-08, 12:39 PM
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Thank you both tremendously for your help! I haven't read through the article yet, but will certainly make a point to. I went down to Jerry's (a local Oregon hardware store) and got a voltage detector pen so I could come to a better conclusion about how it's wired. Here is how it is actually wired:



So it would appear that the thermostat is only switching one leg of the circuit as Doug suggested. I'll look into installing a double-pole tstat at the first heater. I, too, had a hard time finding a programmable double-pole thermostat. It seems that the selection in the line-voltage arena is slim all around. The one that I found is the Lux ELV4. They don't have it listed on their website, but it was available at Jerry's and Amazon.com.


Thanks again!
William
 
 

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