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Thread: Questions about 240v wall heater wiring

  1. #1

    Questions about 240v wall heater wiring

    I have installed a 240v wall heater in two separate rooms. Each has a single pole wall mount thermostat (from the same manufacturer). The wire is 10-2. I am planning on running them both from a single double pull 20 AMP breaker.
    the stats are:
    each heater is 2400W max which would imply 10 AMPS max each
    each thermostat : 16 max AMPS inductive continuous, 3840 max watts
    I am planning running the 10-2 from the panel to a junction box near the closest heater, then running parallel cable to each thermostat/ heater.
    My questions are:

    Is a 20 AMP dp breaker adequate, or do I need to follow the 80% load rule & go to a 30 AMP?

    Since the wiring to the heaters is single pole, does it matter ,from the junction box, which load wire goes to each thermostat (or to each heater)?

    Below drawing is just a crude sketch - standard 10-2 cable was used




  2. #2
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    Mike, I am not an electrician however if your schematic is correct it looks like you will be having a hot 110v line going to your heaters all the time and never shut off.
    Several years ago I hooked up electric heaters in my old house. The place that sold them to me said to be sure that the common load for each came off of different sides of the breaker. Hope I said that right.
    Common to one heater and the other side if the breaker to the other heater. I don't remember all of what he said now it has been several years. Just don't hook both heaters to the same load.
    I think the term he used was to balance the load on the breaker.

  3. #3
    Dale,

    I hear what your saying dale so like this - please excuse the rather crude sketch:





  4. #4
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    Like Dale, I'm no electrician, but to answer your original question, I think you want to go to a 25 if available, or 30 amp breaker if not. 20 amp would be ok if they never came on at the same time. What are the chances of that?
    Now the wire you are using is 10-2, with ground I hope?? My guess is yes, but if not, stop where you are. On 10-2 w/ ground, your white and black wires will hook up to the breaker, and the ground to the ground bar. Most likely these will go directly to the heater. If the thermostats are 110 volt operated, (most work off of a transformer in the heater and run 24 volt to the thermostat), then the thermostat wiring will run from the heater to the thermostat. If this is the case, then you are going to need 10-3 with ground so that you have a neutral wire for the 110 volt circuit.
    Get with the heater manufacturer and verify the wiring hookup requirements. At a minimum, post the brand and model number here so a Sparky on the list can advise you of the proper hook up. Be safe, not sorry. If you are in anyway unsure, hire an electrician. Jim.
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  5. #5
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    Mike, I got to thinking about my heaters. Like I said it was years ago. I think mine my have been 110v. I don't live in the house anymore so I can't check.
    Just be careful and I agree with Jim. If in doubt hire or at least call an electrician.

  6. #6
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    The 80%/125% rule appears to apply only where there is at least one continuous load on the branch circuit (2011 NEC 210.19.A.1 and 210.20.A) . Where "continuous load" is a load expected to draw its maximum current for 3 hours or more. Your state and local codes may add other requirements. I'm not interested in what the NEC says is the minimum required, I'm interested in what is safe and won't run a risk of tripping breakers all winter long so I personally would run #10 wire and a 30A double pole breaker.

    The wiring diagrams that came with the heaters and thermostats should tell you if you need to supply a neutral and how to connect everything properly. I'm sure the manufacturer can even offer advice on how to size the circuit to supply two heaters.

  7. #7
    Thanks Steve, Dale & Jim,

    I've up loaded PDF's. The "Thermostat" doc shows the wiring for the heater, a King WHF 2424. I may go with the 30amp breaker, since the thermostats are 16A continuous & the heaters max out at 10A each. Or my other option is to run separate circuits to each heater, I guess. With a 20A double pole I was just trying to minimize my total panel wattage. As seen there is no neutral just 2 load wires & ground. The info here is sure appreciated as usual.....

    Quote Originally Posted by Steve Meliza View Post
    The 80%/125% rule appears to apply only where there is at least one continuous load on the branch circuit (2011 NEC 210.19.A.1 and 210.20.A) . Where "continuous load" is a load expected to draw its maximum current for 3 hours or more. Your state and local codes may add other requirements. I'm not interested in what the NEC says is the minimum required, I'm interested in what is safe and won't run a risk of tripping breakers all winter long so I personally would run #10 wire and a 30A double pole breaker.

    The wiring diagrams that came with the heaters and thermostats should tell you if you need to supply a neutral and how to connect everything properly. I'm sure the manufacturer can even offer advice on how to size the circuit to supply two heaters.
    Attached Files Attached Files

  8. #8
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    For one, don't call the unswitched leg a "common" leg, as that implies it is a ground, which it definitely is not.

    It is okay to switch just one leg of a 240V circuit--that opens the circuit to control the appliance, but the breaker must be a bonded double-pole so that when one leg trips, they both trip (or so that both get shut off when working on the circuit).

    In theory, a 20A breaker with 12-2 wire would be sufficient, but if both heaters were on simultaneously, there are more likely to be nuisance trips, which will accelerate the wear on the breaker, leading to more frequent nuisance trips. It's okay to put 10-2 wire on a 20A breaker, but you could use a 30A breaker and that would likely eliminate any nuisance trips.
    Jason

    "Don't get stuck on stupid." --Lt. Gen. Russel Honore


  9. #9
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    It looks like you have a single pole thermostat so you're wire it as you show in the single pole wiring or thermostat PDF that you posted. You can use NM 10/2 for the wiring, just make sure to wrap the white wire with a piece of black electrical tape at each end to indicate that it is not being used as a grounded conductor, but is now an ungrounded conductor (aka hot/live). I see no reason not to use a 30A double pole breaker over a 20A.

  10. #10
    Thanks Steve,

    I have taped the white "black". Do you by chance know if it makes a difference whether the same load, lets say the "white" wire for reference, should run to both thermostats from the j-box, (hence the blacks would wire through directly to the heaters)? Since their going out in parallel not in series, with as 30A breaker I can't see where it would matter. May be I'm just over thinking this thing...



    Quote Originally Posted by Steve Meliza View Post
    It looks like you have a single pole thermostat so you're wire it as you show in the single pole wiring or thermostat PDF that you posted. You can use NM 10/2 for the wiring, just make sure to wrap the white wire with a piece of black electrical tape at each end to indicate that it is not being used as a grounded conductor, but is now an ungrounded conductor (aka hot/live). I see no reason not to use a 30A double pole breaker over a 20A.

  11. #11
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    It won't matter because they are in parallel as you said, but it would be best to be consistent and put the thermostat on the same phase for both heaters.

  12. #12
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    The breaker doesn't care which leg the thermostat is on, because the the load is always across the 240V bus bars, and the current flows through both poles of the breaker no matter which heater is on and no matter which side the thermostats are on.

  13. #13
    Thanks Josiah,

    The wire leads on both the thermostats & heaters are 12 gauge. Does a 30A circuit require 10 gauge even at point of contact with the device? Anyone know?






    Quote Originally Posted by Josiah Bartlett View Post
    The breaker doesn't care which leg the thermostat is on, because the the load is always across the 240V bus bars, and the current flows through both poles of the breaker no matter which heater is on and no matter which side the thermostats are on.

  14. #14
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    I'm not an electrician but I would run the heaters on a 30a breaker even though 20 may be enough. From what I read on electric heaters is that they draw quite a bit of current, usually max current, so with both on your every close to 20 amps if not surpassing it.

  15. #15
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    Quote Originally Posted by Mike Kelsey View Post
    The wire leads on both the thermostats & heaters are 12 gauge. Does a 30A circuit require 10 gauge even at point of contact with the device? Anyone know?
    The #12 wire is good for up to 20A and since a heater and thermostat alone is under 20A they use #12. You will still need #10 wire for the combined load back to the breaker.

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