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Thread: Interlocking as to not overload a 75kva

  1. #1
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    Interlocking as to not overload a 75kva

    The primary 600V is fused at 90A; there is a load (per phase) of 60A or 173A on the 120/208 side (60x2.88 turns ratio xfmr).
    There's actually 2 of these loads BUT only one is on at a time, 173A on then off and the other load/ room 173A on then off and so on.
    Does a time clock count as an interlock?
    By code I'm sure/ believe that this is allowed as long as it is physically impossible for both loads to be on at once. NEC and CEC feedback appreciated. Thanks.

  2. #2
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    As far as protecting the transformer, the 90 amp OCP protects your transformer from overload well within the 125% as allowed by Art 450.3. I see no requirement for any secondary interlock. With the 125% primary protection you can connect up to 6 secondary loads without concern for the transformer.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  3. #3
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    Quote Originally Posted by justin13me View Post
    The primary 600V is fused at 90A; there is a load (per phase) of 60A or 173A on the 120/208 side (60x2.88 turns ratio xfmr).
    There's actually 2 of these loads BUT only one is on at a time, 173A on then off and the other load/ room 173A on then off and so on.
    Does a time clock count as an interlock?
    By code I'm sure/ believe that this is allowed as long as it is physically impossible for both loads to be on at once. NEC and CEC feedback appreciated. Thanks.
    there is no provision in the code for sizing a xfmr.

    you can make it as small or as large as you like regardless of the load.

    you have to provide appropriate overcurrent protection.

    if you use draw too much current the OCPD will open up.
    Bob

  4. #4
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    I'm trying to use two timers to turn on 2 loads from 1 transformer, one load at a time. One load turns on, stays on for (let's say) 20 hrs and then off and then half an hour later the other load turns on for same amount of time then off and half an hr later other load on etc. The loads on either time clock are actually 35 circuits w 70 electronic ballasts..

  5. #5
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    So I guess the question now is how do I wire this?

  6. #6
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    One cheap time clock controlling a DP interposing relay. The relay controls two contactors large enough for your loads. The NC contact controls Load 1, the NO controls Load 2. Loads will alternate as the time clock cycles.

    I would use a small PLC with an HMI but I like to over complicate and give more options.

    Replace the DP with a TDR(s)to give you the delay you want.
    Tom
    TBLO

  7. #7
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    Quote Originally Posted by ptonsparky View Post
    One cheap time clock controlling a DP interposing relay. The relay controls two contactors large enough for your loads. The NC contact controls Load 1, the NO controls Load 2. Loads will alternate as the time clock cycles.

    I would use a small PLC with an HMI but I like to over complicate and give more options.

    Replace the DP with a TDR(s)to give you the delay you want.
    I thought about the PLC option too, I don't think it's over complicated. I'm only (a little) familiar w Allen Bradley though and concerned it may be an expensive option. Timers and relay suggested sound good too. Thx

  8. #8
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    AB is generally expensive although they have a new small one that is more cost effective. An 810 is what I have sitting in my basement waiting for me to learn how to program it.

    CLICK from AD works well for most my small projects.
    Tom
    TBLO

  9. #9
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    Simple prog:
    sample control.pdf
    Tom
    TBLO

  10. #10
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    Quote Originally Posted by justin13me View Post
    I thought about the PLC option too, I don't think it's over complicated. I'm only (a little) familiar w Allen Bradley though and concerned it may be an expensive option. Timers and relay suggested sound good too. Thx
    Depending on who you talk to, driving two relays with timers or a PLC does not interlock out the loads unless you use:
    • a mechanical relay interlock, or
    • electrically use a NC contact (auxiliary or otherwise)

    ...to completely disable the other relay while the one is in operation.
    I'll never get there. No matter where I go, I'm always here.

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