How to Tell if Baseboard Heater is 2 or 4 Wire?

I'm trying to replace the built-in mechanical thermostat on an old Fasco Industries 8- foot, 240 volt electric baseboard heater with a programmable line voltage thermostat, either a Honeywell CT1950 or an Aube Technologies TH106. Problem is I'm supposed to choose between 2 programmable thermostats - the Honeywell CT1950A which is for 2 wires or the Honeywell CT1950B which is for 4 wires, and I have no idea which one I need.
Questions:
1. Is the CT1950A or B just one thermostat that can be wired in 2 different ways, or are we talking 2 different thermostats altogether?
2. In any case, how can I tell if I need a 2-wire thermostat or a 4-wire thermostat? I looked at the existing wiring and noticed that there are only 2 wires coming out of the wall directly from the breaker panel. Then there appears to be 4 wires, all of them colored red, connected to the built-in thermostat mounted on the heater itself - 2 red wires going in and 2 red wires coming out. I have not traced where these wires going in and out of the existing thermostat are coming from.
As you can tell, I am really electrically challenged, so please pardon me if the question sounds stupid. But I really have a problem understanding electrician's English - when they say 2-wire or 4-wire, are they talking about the wires coming out of the wall or the wires coming out of the thermostat?
Any help will again be appreciated. Thanks.
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A bit of background: a line voltage thermostat is one that slips in series with the power feed to the heater and controls the heater by switching the power feed off and on. As contrasted with other thermostats which activate a seperate relay to switch the power feed to the heater.
The CT1950x thermostats are line thermostats, which means that they take power in (via the Lx terminals) and switch the output power on the Tx terminals.
The only difference between the CT1950A and B is that the CT1950A switches only one wire (single pole switch), and the B model switches two wires (double pole switch).
[Ie: electrically, the CT1950A simply connects/disconnects wire T1 from L1. Electric simply connects/disconnects wire T1 from L1 and T2 from L2.]
http://content.honeywell.com/yourhome/ptc-thermostats/CT1950faq.htm
You use single pole thermostats on 120V heaters, because there's only one hot wire present (total of two: neutral plus one hot), and that's the only one you need to switch.
240V has two hot wires (no neutral). Preferably and ideally you should use two pole thermostats for 240V heaters, so that both hot wires are simultaneously detached - otherwise, the element would be live even when the thermostat has the heater turned off. However, most codes do not seem to insist on that - which has always seemed very odd to me.
I always use 2 pole for 220V.

It sounds as if the preexisting thermostat is 2 pole. You will need to replicate how it interconnects.

When they say "4 wire X", they really mean "4 wires connected to X". What that actually means electrically depends on what X _is_.
Poor terminology.
The electrician should _really_ be saying "two pole" (CT1950B) versus "single pole" (CT1950A).

--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
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snipped-for-privacy@nortelnetworks.com (Chris Lewis) wrote in message

Thanks, Chris, your explanation is crystal-clear and most enlightening. Now I can actually visualize a circle where a single pole circuit resembles a dented egg (only one side broken) while a double pole circuit resembles a halved egg (both sides sliced through and broken).
Now here's my problem - I'm stuck with 3 brand-new thermostats that I have already purchased on-line based on my Internet research - a 2-wire Honeywell CT1950A, a 2-wire Ouellet OTH510, and a 2-wire Aube Technologies TH106. It seems that in Canada (I'm actually in the US), they don't make any distinction between single pole and double pole switches for 240-volt baseboard heaters - they use 2-wire thermostats intechangeably for both 2-wire and 4-wire applications.
So I'm focusing on your statement that "preferably and ideally ...use 2 pole thermostats... etc." They sound like permissive, not mandatory, requirements, i.e., I should but I don't have to. I guess my question is - is there a safety issue if 1 wire remains live in a 240-V circuit? Since the 2nd wire is disconnected, is there any chance the live wire could find a way to return to ground and form a complete circuit? Does this constitute, God forbid, a fire hazard? Given the new information that you so kindly provided, I'm thinking that maybe when winter's gone, I'll just go to my breaker panel and turn off the live feed to all baseboard circuits.
It makes me wonder why thermostat manufacturers continue to market 2-wire thermostats as suited for 240 volt applications, instead of reserving 2-wire units only for 120 volts and 4-wire units for 240 volts.
Thanks for any additional input.
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Filipo wrote:

It's covered in Art. 424-19 (NEC).
Basically, in a one-family house, you are permitted to use a single-pole (2W) thermostat and then either the branch circuit (breaker) or the service disconnect (main breaker) serves as the disconnect for the heater.
So, if you were going to work on the heater, you should remember to switch off the breaker first...
Jim
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Chance? Yes, but extremely remote (assumes for example, circuit is otherwise miswired, heater malfunctions). Significant enough to pay a bit extra? That's something only you can answer.

I kill the breakers to all the heaters regardless. Saves money if a T-stat gets accidentally fiddled or fails on. Makes winter startup a distinct process of cleaning them, turning them on, and checking them all at once.



Notice how that's "one family"? It's one of those exemptions specifically intended to lower the cost of wiring in the least risky environment. [Even tho in many cases it makes no difference to the cost of the device, and only a slight difference in labour.]
Notice how that doesn't apply to commercial, industrial, or multi-person dwellings. There's a strong hint there.
Obviously, the NEC has made a tradeoff judgement here. The question is, do you agree with their judgement for _your_ specific circumstances?

Aye, there's the rub ... some people don't. It really shouldn't be hinted that _not_ turning off the breaker before working on a circuit is acceptable practise.
I personally prefer exceeding code as far as is practical. So if I stupidly forget to switch off the breaker, or something else like the thermostat contacts sticking, then I don't find out the hard way.
When the cost difference is minor (or non-existant), as there would be with, say, just one T-stat to buy, there's no contest - 2 pole it is.
If I already had a single-pole T-stat, I likely would decide to use 2 pole. But if I had three? Probably not.
If I was wiring whole subdivisions, I almost certainly wouldn't bother with two pole.
Etc.
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