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Facts: old Eaton/Cutler Hammer panel with type BR breaker. Double pole 30 amp breaker, 240V, controls two wall heaters and two zone thermostats in two rooms. Heaters and thermostats and my brain got mixed up after 28 years....

Tested 120V at dining room wall thermostat wires in dining room so does it need a 120V or 240V heater for it?

In living room, on same circuit, its thermostat wiring also tested for 120 V. I am asking for advice on what voltage thermostat and heater to install in each room and an explanation of the 120V readings on a 240V circuit....

I vaguely recall that I might have had two underperforming 120V 1500W heaters on this circuit, but that was 28 years ago ........So do I install 120v or 240v thermostats and 120 or 240 v heaters?

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    Wait, is it CH or BR? It can't be both -- CH is 3/4" wide breakers, originally made by Cutler-Hammer, whereas BR are 1" wide breakers originally made by Bryant. – Nate S. Feb 11 at 0:56
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    And you're measuring 120 or 240V in reference to what? 240V circuits normally have two hots, each of which is 120V from ground, so if you're measuring to ground, measuring 120V on a 240V circuit is normal. – Nate S. Feb 11 at 0:57
  • By CH, I was abbreviating Cutler Hammer, not the breaker type BR. Will edit. – DAS Feb 11 at 1:41
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    @Mazura I did not say I didn''t remember how to test 240 V. And line voltage is what was used prior to furnaces and heat pumps. Perhaps you need basic instruction in reading, electrical heating and manners. This is not on a furnace or heat pump, ds. Consider rereading before you throw out insults. – DAS Feb 11 at 3:20
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    @Mazura Line voltage t-stats are as common as dirt in residential. Simple electrical resistance heaters (Cadet in wall heaters, baseboard, etc.), are still very common. Even some old mobile homes with a small furnace were controlled by line voltage tstats. – George Anderson Feb 11 at 4:12
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Ok this is probably a simple issue. You have a double pole circuit but the thermostat is only single pole and you read 120v not 240 as you expected.

With baseboard heaters it is common to only break one leg. With one leg open no current can flow so no heat. The heater is required to be grounded so measuring the voltage with the heater turned on providing heat at either terminal to ground will show ~120vac. With the heater heating the voltage from L1 to L2 (the heater connections) will show 240v.

Now here is what trips up non electricians. With the thermostat off each leg still reads 120v to ground the voltage coming through the heater has no voltage drop because there is no current with a normal high impedance meter. So a 240v heater can be controlled with a 120v line voltage thermostat.

To know if your heater is 120v or 240 measure the voltage L1 to L2 at the heater when the thermostat is turned on (the 2 wires connecting the power to the element) if you read 120 it’s 120 if you read 240 it’s 240.

You have found out you have a 30 amp circuit, this is quite common and #10 wire is normally used. A single pole thermostat can control 120 or 240v if when the thermostat is off you have voltage (120v to ground) at the heater this is another indication it is 240v.

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  • Rereading this now. There are no thermostats at all connected. I just measured voltage at the wall box. As you well know, it is hard to size up the guage of old wires, which are fatter than nowadays. They look like 10 guage and that is indeed what was run to them originally. I was just surprised to find 120V measurements at the wall box wiring. I know for a fact that both heaters are 240V as I purchased and installed them 28 years ago and reexamined them recently. I think a friend erred in wiring the thermostats years ago and I want the correct voltages and backlit ones. – DAS Feb 11 at 20:27
  • Das can you provide us a photo of the wiring? At the thermostat and “wall box” I am guessing the wall box is the connection point to the baseboard unit. There could be 120 at the wall box at all times and 30 years ago this was quite common (still is today in the Pacific Northwest). Although your wiring looks wrong to you it may be ok, but to work with a digital thermostat that lights up we may need to make some changes, not hard I have done this many times we just need to see what you have to help you out. – Ed Beal Feb 11 at 22:57
  • Thank you so much! Was going to start a new thread on thermostat poles.... – DAS Feb 12 at 4:59
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In North America, 240V circuits will measure 120V from ground. It is 2 legs each 120V but of opposite voltages.

Your circuit is good for 7200W (240V * 30A).

Some types of loads, including heaters, must be derated 125% to determine how much can fit on a circuit. E.G. a 2000W heater counts as 2500 watts for circuit capacity.

If all the loads on the circuit are subject to that 125% derate, then they must total <= 5760W, since 5760*125% = 7200.

So you can have 5760W of heaters etc. on the circuit.

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    @DAS no. That's not how 240V works. 240V power is 240V hot-to-hot. There is no neutral. It's 120V hot-to-ground because it's center-grounded (a clever trick). – Harper - Reinstate Monica Feb 11 at 2:24
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    So I understand that I have two branches of 120V each and that I could have up to 5760 watts. So I need two 240v thermostats and 240V heaters? – DAS Feb 11 at 3:43
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    Harp: I agree with the answer but I think an easier way to get there is to take the total capacity of the circuit (7,200 watts) x 80% bc it's a continuous load, we get to the same number (5,760), but your method seems unnecessarily complex. – George Anderson Feb 11 at 4:18
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    @Harper-ReinstateMonica That requirement sounds like a bit of CYA. But consider that the typical HVAC thermostat is a low-voltage easy/safe thing, some extra warning may be warranted. If someone replaces a typical old thermostat with a Nest, they should turn off the HVAC system when they do it, but if they don't there is likely no harm done. That is absolutely not the case here. – manassehkatz-Moving 2 Codidact Feb 11 at 20:40
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    @DAS I misspoke, heater. You can edit details into your question at any time. Look for the words "Share edit ..." directly under your question's keywords. You need to add necessary details if you want us to help you. The details go in the questions not comments (please consider re-taking the tour). This is not a discussion forum, it's a Q&A site. If you prefer a discussion forum, those exist, but the signal/noise ratio is very poor. – Harper - Reinstate Monica Feb 12 at 6:59

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