[jdcaudle]

I understand how single-pole and double-pole thermostats are wired: the single-pole only cuts one of the two line voltage connections to the heater while the double-pole cuts both.

As such, I clearly understand the safety aspect of this - line voltage still reaches the heater when a single-pole is "off".

However, I don't understand why double-pole thermostats have a true "off" setting while single-pole thermostats do not. A single-pole thermostat breaks continuity, turning the heater off. So does a double-pole thermostat. The single-pole thermostat, even turned to its lowest setting, can still close the switch when the temperature drops - why doesn't a double-pole thermostat?

If the double-pole thermostat turned to "off" is disengaged from the temperature-sensitive mechanism, why can't a single-pole thermostat be designed to do the same?

The only reason I can think of is that manufacturers aren't allowed to include an "off" on single-pole thermostats - for safety reasons - because it would mislead people about the voltage present at the heater. Is there a physical reason why double-pole thermostats have a true off but single-pole don't?

Thanks!

 

[Nealtw]

I think your guess is right. it is never really off with one live line.

 

[CodeMatters]

I understand how single-pole and double-pole thermostats are wired: the single-pole only cuts one of the two line voltage connections to the heater while the double-pole cuts both.

As such, I clearly understand the safety aspect of this - line voltage still reaches the heater when a single-pole is "off".

However, I don't understand why double-pole thermostats have a true "off" setting while single-pole thermostats do not. A single-pole thermostat breaks continuity, turning the heater off. So does a double-pole thermostat. The single-pole thermostat, even turned to its lowest setting, can still close the switch when the temperature drops - why doesn't a double-pole thermostat?

If the double-pole thermostat turned to "off" is disengaged from the temperature-sensitive mechanism, why can't a single-pole thermostat be designed to do the same?

The only reason I can think of is that manufacturers aren't allowed to include an "off" on single-pole thermostats - for safety reasons - because it would mislead people about the voltage present at the heater. Is there a physical reason why double-pole thermostats have a true off but single-pole don't?

Thanks!

Correct.

 

[Oldmaster]

You are correct in your logic. Single pole t-stats are no longer legal.

 

[Fishbulb28]

Single pole t-stats are no longer legal.

There is no restriction on single pole thermostats or other control devices. They simply cannot be used as a required disconnect.

 

[Oldmaster]

According to code all thermostat must have a off position that disconnects all phase conductors. What else could you use for heat?

 

[Fishbulb28]

According to code all thermostat must have a off position that disconnects all phase conductors. What else could you use for heat?

There is no such requirement. The thermostat only needs to have an off position and disconnect all ungrounded conductors when the thermostat is also used as the required disconnect.

424.20 Thermostatically Controlled Switching Devices.
(A) Serving as Both Controllers and Disconnecting Means. Thermostatically controlled switching devices and combination thermostats and manually controlled switches shall be permitted to serve as both controllers and disconnecting means, provided they meet all of the following conditions:
(1) Provided with a marked “off” position
(2) Directly open all ungrounded conductors when manually placed in the “off” position
(3) Designed so that the circuit cannot be energized automatically after the device has been manually placed in the “off” position
(4) Located as specified in 424.19
(B) Thermostats That Do Not Directly Interrupt All Ungrounded Conductors. Thermostats that do not directly interrupt all ungrounded conductors and thermostats that operate remote control circuits shall not be required to meet the requirements of 424.20(A). These devices shall not be permitted as the disconnecting means.

 

[Oldmaster]

We were speaking about baseboard heaters. What else could you use for a means of disconnect. I was speaking specifically about his application.

 

[jdcaudle]

Question answered! Thanks everyone.

 

[carmusic]

You are correct in your logic. Single pole t-stats are no longer legal.

all electronic line stat are single pole...