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Thread: SPDT double-throw switch

  1. #1
    DIY Junior Member Ken Tannenbaum1's Avatar
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    Default SPDT double-throw switch

    I ran a 12/2 cable to use for 240v baseboard heaters in one room and would like to split one of the legs to use ONLY during the summer for an A/C. Can I install an in-line, single-pole, double-throw 20amp
    switch for that purpose? Seems right but want to do the correct thing. It's awfully expensive but should work, right?

    Thanks very much.

  2. #2
    Electrical Contractor jbfan74's Avatar
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    With 12/2 cable, you only have 240 volts and not 120v.
    You do not have neutral wire to use for 120v.

    The only way to do this is buy a pure 240 AC unit.
    Yes I am A Pirate-Jimmy Buffett

  3. #3
    DIY Junior Member Ken Tannenbaum1's Avatar
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    Thanks for the response...Oops, my error...the cable is 12/3, NOT 12/2...it's got a R, B, W and copper wires. Also, nothing's hooked up to the panel yet. Now, just to be sure...at present, the two hots (B+R) run to the 240v heater + the copper ground. There is no neutral necessary from what I gather. Back to me question, can I use the white neutral and split one of the hot legs (B or R) with a SPDT 20amp switch to get to the 110v A/C in the same room? Sorry for the confusion.

  4. #4

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    The right thing to do is pull a separate circuit for the A/C. 12/2 wire isn't that expensive.

    And yes what you are suggesting would work. And SPDT 20-amp switches are more $$ but shouldn't be that expensive as they are commonly used in commercial buildings.

    -rick
    Last edited by drick; 12-27-2010 at 11:15 AM.

  5. #5
    DIY Junior Member Ken Tannenbaum1's Avatar
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    Thanks.....guess my life would have been easier if the box stores carried 12/4....I should have gone to a supplier direct. HD and Lowes also don't carry that switch.
    But you probably knew that. Happy New Year.

  6. #6
    General Engineering Contractor ballvalve's Avatar
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    You did not need the 12 - 3 for the heater as you discovered, 2 runs of 12-2 would have solved your issue much cheaper than 12-4.

    Maybe I'm missing something, but a spdt switch isnt enough, you need a switch with two poles normally off and 2 poles normally on, so that throwing it disconnects the heater completely and engages a 120v circuit to the AC, and the two may never intermix.

    But then you have a 120v circuit on 1/2 of a double breaker.

    The NEC guy will likely be here soon to read you the law.
    Last edited by ballvalve; 12-27-2010 at 12:11 PM.

  7. #7
    DIY Junior Member Ken Tannenbaum1's Avatar
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    Yes, the 12/3 was a waste. About "missing something", you know better than me...that said, based on what you've contributed, I'm going to make another run with 12/2 and not mess with doing it incorrectly. Also, the cost of the 20amp SPDT is probably as much as or more than the 12/2 cable! Anyway, safety first...I live there!
    Thanks.

  8. #8
    Licensed Electrical Contractor Speedy Petey's Avatar
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    Why bother with a switch at all?

  9. #9
    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by Speedy Petey View Post
    Why bother with a switch at all?
    This is my question also. the heat and AC will not be in use at the same time so why not wire the AC to the same circuit as the heat but be sure to use a single receptacle instead of a duplex

  10. #10
    General Engineering Contractor ballvalve's Avatar
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    So the NEC allows you to install a double pole breaker in a panel and label it
    "20 amp 240 volt wall heater, with 120 volt AC receptacle, drawing 20 amps on one leg" ?

    Wonder what the inspector will have to say to that one.

    Wow! That makes me rethink my deep respect for the NEC.

    the heat and AC will not be in use at the same time
    Sure. By that curious assumption, I suppose I can wire my rooftop AC on a 50 amp double pole breaker to my seperate heat pump with seperate thermostat, also with a 50 amp rating, because they will not be on at the same time.

    You guys had too much food at Christmas and got a little light headed?

    Would you take a 120 volt leg off your 240V pool pump to operate a pump house outlet for a electric heater "because they won't be on at the same time"?

    BE WISE! Wire safe circuits.
    Last edited by ballvalve; 12-28-2010 at 02:46 AM.

  11. #11
    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by ballvalve View Post
    So the NEC allows you to install a double pole breaker in a panel and label it "20 amp 240 volt wall heater, with 120 volt AC receptacle, drawing 20 amps on one leg" ? Wonder what the inspector will have to say to that one.
    I donít think that anyone has posted anything that could be closely related to this statement. Did you have to much food at Christmas and get a little light headed and start seeing things?

    Quote Originally Posted by ballvalve View Post
    Wow! That makes me rethink my deep respect for the NEC.
    I would suppose that you was into the eggnog just a little to much as this is one of your jokes is it not.



    Quote Originally Posted by ballvalve View Post
    Sure. By that curious assumption, I suppose I can wire my rooftop AC on a 50 amp double pole breaker to my seperate heat pump with seperate thermostat, also with a 50 amp rating, because they will not be on at the same time.
    Do you not know what a heat pump is? A heat pump is a dual action air conditioner. In one mode it cools the inside and heats the outside and in the reverse mode it heats the inside and cools the outside. What you purpose is having two air conditioners on one circuit.

    Quote Originally Posted by ballvalve View Post
    You guys had too much food at Christmas and got a little light headed?
    Someone sure did, see above.

    Quote Originally Posted by ballvalve View Post
    Would you take a 120 volt leg off your 240V pool pump to operate a pump house outlet for a electric heater "because they won't be on at the same time"?
    Now are you making another joke? This does not even come close to heat and AC now does it/

    Quote Originally Posted by ballvalve View Post
    BE WISE! Wire safe circuits.
    Finally after many attempts you make a smart post. I know that this took some effort on your part. I am so proud of your effort.

    210.4(C) Line-to-Neutral Loads. Multiwire branch circuits shall supply only line-to-neutral loads.
    Exception No. 2: Where all ungrounded conductors of the multiwire branch circuit are opened simultaneously by the branch-circuit overcurrent device.

    Ever see a receptacle that had both a 240 volt and 120 volt receptacle on the same yoke?
    http://www.leviton.com/OA_HTML/ibeCC...minisite=10026

  12. #12
    Licensed Electrical Contractor Speedy Petey's Avatar
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    Quote Originally Posted by ballvalve View Post
    So the NEC allows you to install a double pole breaker in a panel and label it
    "20 amp 240 volt wall heater, with 120 volt AC receptacle, drawing 20 amps on one leg" ?

    Wonder what the inspector will have to say to that one.

    Wow! That makes me rethink my deep respect for the NEC.



    Sure. By that curious assumption, I suppose I can wire my rooftop AC on a 50 amp double pole breaker to my seperate heat pump with seperate thermostat, also with a 50 amp rating, because they will not be on at the same time.

    You guys had too much food at Christmas and got a little light headed?

    Would you take a 120 volt leg off your 240V pool pump to operate a pump house outlet for a electric heater "because they won't be on at the same time"?

    BE WISE! Wire safe circuits.
    This whole post is a joke. Even the last line, since it is obvious you don't even know what a "safe" circuit is.

  13. #13
    General Engineering Contractor ballvalve's Avatar
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    Ever see a receptacle that had both a 240 volt and 120 volt receptacle on the same yoke?
    http://www.leviton.com/OA_HTML/ibeCC...minisite=10026
    Yes, but that is not how his system will work.

    Well at least you guys are starting to appreciate some irony and humor. But I don't think you get the point, or do not want to.

    This OP was smart enough to know that if his AC and bases board heater were on at the same time, he would gravely overload one half of his 20 amp breaker. The base heat is a dedicated, permanently installed appliance controlled by a thermostat and it does not lock out in summer.

    The AC might be permanently installed in a hole in the wall like mine, and is then also a permanently installed appliance. It is also not locked out in winter.

    I can't make anything of the NEC nonsense, but even a layman would not want his electrician to wire up a circuit to save 4$, and then guarantee it will be overloaded under some very possible circumstances.

    You would hardwire a base board heater and then pull a jumper to a dedicated A/C outlet?. I just don't think so. Or hope so.

  14. #14
    Electrical Contractor/Instructor jwelectric's Avatar
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    First why would anyone need a lock out between the two?

    Second one can’t overload half of a two pole breaker. The breaker is either overloaded or it isn’t.

    Third what damage would be done should the breaker be overloaded? Would it not trip and prevent and damage?

  15. #15
    Licensed Electrical Contractor Speedy Petey's Avatar
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    Quote Originally Posted by ballvalve View Post



    I can't make anything of the NEC nonsense, but even a layman would not want his electrician to wire up a circuit to save 4$, and then guarantee it will be overloaded under some very possible circumstances.
    Disagree. Try "extremely unlikely situation".
    Besides, like JW said, SO the breker gets overloaded because in the middle of summer someone spilled a bucket of ice on the heat thermostat. It trips, you reset it, you clear the ice off the t-stat and go back to your party.

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