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Thread: Load Switching double throw disc.

  1. #1
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    Load Switching double throw disc.

    I wasn't sure what to title this thread.

    Lets say you have two loads, a table saw and a ban saw coming from two different circuits, both 208-1. Lets also say you want to devise a way so that these loads never run simultaneously. Is there a good way to do this without using a bunch of relays and contactors?

    Do you think it would be possible/permissible to use a non-fused double throw disconnect assuming the over current protection for each circuit is sized properly and the disconnect ampacity rating is greater than the load, then size the branch circuit wiring for the larger load.

    This has nothing to do with load calculations or service sizing.

  2. #2
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    A DPDT switch of adequate voltage and current rating and switching only one wire to each motor will logically accomplish the goal. Also you can do this with a contactor with isolated contacts, one NO, and the other NC. Add two more isolated contacts and you can switch both lines to each motor. Just draw out the circuits to see how it works.

    ,

  3. #3
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    I cannot think of anything more complicated than doing this 'your way' with switches.

    You can buy a single 4-pole contactor with 2 'open' contacts and 2 'closed contacts.
    Coil off = load 1, Coil on = load 2. Use a 208V coil and you can get the power from one of the circuits.

    Or
    You use two 2-pole contactors and a small double throw switch to control them.
    Just because you can, doesn't mean you should.

  4. #4
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    Why would you say the double throw was complicated? Just asking so I know.

    I would think the double throw method would be simpler but more expensive.

  5. #5
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    Quote Originally Posted by Npstewart View Post
    Why would you say the double throw was complicated? Just asking so I know.

    I would think the double throw method would be simpler but more expensive.
    Unless I'm missing something here, your way seems pretty simple and straight forward to me and maybe just as inexpensive in this size range.

  6. #6
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    Quote Originally Posted by Npstewart View Post
    Why would you say the double throw was complicated? Just asking so I know.

    I would think the double throw method would be simpler but more expensive.
    You said you had two different circuits supplying two different loads. I interpret this to be 4 incoming wires and 4 outgoing wires. A double throw switch is 4 incoming wires and only 2 outgoing wires.

    How many amps are you looking at switching?
    Just because you can, doesn't mean you should.

  7. #7
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    If you have disconnects, or the right type of CBS, you could get kirk key interlocks. Pricey, but foolproof.

  8. #8
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    Quote Originally Posted by jim dungar View Post
    You said you had two different circuits supplying two different loads. I interpret this to be 4 incoming wires and 4 outgoing wires. A double throw switch is 4 incoming wires and only 2 outgoing wires.

    How many amps are you looking at switching?
    I too was a little unclear on the two separate power sources AND two separate loads that can never run at the same time. Can they run at the same time as long as each one is on a different power source? If not, why not? Is this a power issue or is it a safety issue?

    Or did you really mean that at all?

    A double throw safety switch is simple enough to switch loads so that your power source only feeds one or the other but not both at the same time. The only caveat is to look at the HP rating of the switch, not the Amp rating, they may be different. But then again if you NEVER EVER switch under load, which seems unlikely, then it really wouldn't matter as long as you have the amps covered.
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  9. #9
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    I think I understand why you would say it would be more complicated now, its because of the two loads and two sources.

    Its kind of a long story why I was asking. We have a wood shop, and if you exceed a certain amount of exhausted cfm then the shop has to comply with nfpa 664. I was trying to come up with a way to have all the machines in the shop that we need, but make it impossible to exceed the cfm by not allowing simultaneous use of the two largest machines. I guess if I drew a wiring diagram of the double throw disconnect method I would have realized the problem.

  10. #10
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    Quote Originally Posted by Jraef View Post
    I too was a little unclear on the two separate power sources AND two separate loads that can never run at the same time. Can they run at the same time as long as each one is on a different power source? If not, why not? Is this a power issue or is it a safety issue?

    Or did you really mean that at all?

    A double throw safety switch is simple enough to switch loads so that your power source only feeds one or the other but not both at the same time. The only caveat is to look at the HP rating of the switch, not the Amp rating, they may be different. But then again if you NEVER EVER switch under load, which seems unlikely, then it really wouldn't matter as long as you have the amps covered.
    Now I see why Jim made his comments. I took the OP to mean one source, two loads. My bad.

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