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Thread: SPDT double-throw switch

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  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.

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