According to :
I have a digital thermostat controlling three baseboard heaters in my
finished basement, and I'm looking to upgrade it to a programmable one.
I'm uncertain whether I should go with a single-pole unit or a
double-pole unit.

Here is a photograph of the existing Cadet C2002 thermostat:
http://snice.net/thermostat.jpg

Retailers list this unit as single-pole, but wiring descriptions for
single-pole thermostat's done seem to jive in terms of number of wires
and color of wires. I'm looking to replace this with a Honeywell
single-pole unit:

http://content.honeywell.com/yourhom...ats/CT1950.htm

... and just wondering if the CT1950A (two-wire) can replace my unit,
even though it has 3 wires?

Also, I get this vague sense of "you can get away with single-pole, but
you might want to go double-pole.." If safety or robustness is an
issue, I'll just go double.

As I understand it, both the NEC and the CEC permit single-pole thermostats
for 240V heaters, despite the fact that switching only one pole means
that the wiring in the heater remains live w.r.t. ground.

Two pole is safer (the guts of the heaters are, theoretically at least, dead
when the thermostat is off), but you'll find that particularly with electronic
controls, they usually cost MUCH more and are harder to find.

Tho, that CT1950B seems to be a dual version, if your local has it in
stock and it doesn't cost much more, I'd recommend it.

Your existing thermostat probably has three wires so that it can maintain
power inside itself (it gets power from both hots) while the heater
is switched off. The CT1950 models probably get power by "leaking" a small amount
of current through the heater element.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.