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Thread: Safely Maximize Power from a 100amp 3-Phase Delta Panel

  1. #11
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    Thanks guys for all of the advice. I'm sorry if my question isn't entirely clear. I know I have a lot to learn and that is why I am here asking questions.

    I will try one more time to ask the question in a clear manner that has one descrete answer and isn't so fuzzy.

    A building is equipped with a 100amp panel powered with 3-phase delta high leg electricity with AC voltages of 120 - 120 - 208 on each leg when measured w respect to ground. Each leg is restricted by a 100 amp breaker and the transformer supplying the power does not present any limitations. The building is to be equipped with 240v AC resistive loads. Assuming the breakers will trip at 80% capacity, what is that maximum amount of power one can draw with this configuration without tripping the breakers? How would the loads be wired such that they take full advantage of the available power? Assume no voltage drop and a power factor of 1.

  2. #12
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    Quote Originally Posted by EricJ View Post
    Assuming the breakers will trip at 80% capacity,
    Why are we assuming that? It is not reality.

    I also want to ask is your question purely about electrical theory or do you want to meet the NEC code requirements as well?

    Most times load calculations start with the loads not with the supply.

    In other words you add up all your loads and see what the total is and then you make sure the supply exceeds that.
    Last edited by iwire; 03-18-17 at 09:17 AM.

  3. #13
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    Quote Originally Posted by EricJ View Post
    Thanks guys for all of the advice. I'm sorry if my question isn't entirely clear. I know I have a lot to learn and that is why I am here asking questions.

    I will try one more time to ask the question in a clear manner that has one descrete answer and isn't so fuzzy.

    A building is equipped with a 100amp panel powered with 3-phase delta high leg electricity with AC voltages of 120 - 120 - 208 on each leg when measured w respect to ground. Each leg is restricted by a 100 amp breaker and the transformer supplying the power does not present any limitations. The building is to be equipped with 240v AC resistive loads. Assuming the breakers will trip at 80% capacity, what is that maximum amount of power one can draw with this configuration without tripping the breakers? How would the loads be wired such that they take full advantage of the available power? Assume no voltage drop and a power factor of 1.
    If your loads are 240v and the transformer supplies 240V L-L. We don't care what the L-G voltages are, nor does the load.

    Oh, and it will be 80% of what the 100 amp breaker could handle if it were continuous rated and would not tolerate anything more.
    Last edited by ptonsparky; 03-18-17 at 07:53 AM.
    Tom
    TBLO

  4. #14
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    Quote Originally Posted by ptonsparky View Post
    Oh, and it will be 80% of what the 100 amp breaker could handle if it were continuous rated and would not tolerate anything more.
    Hold on, lets be clear only the part of the load that may be continuous gets calculated at 125%.

    So in the case of the main breaker you would have to look at every circuit leaving it and determine which must be calculated at 125%

    A standard breaker will (supposed to) hold 100% load for at least 3 hours.

    And we still do not know if the OP wants to consider the code or just the theory.

  5. #15
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    Quote Originally Posted by iwire View Post
    Hold on, lets be clear only the part of the load that may be continuous gets calculated at 125%.

    So in the case of the main breaker you would have to look at every circuit leaving it and determine which must be calculated at 125%

    A standard breaker will (supposed to) hold 100% load for at least 3 hours.

    And we still do not know if the OP wants to consider the code or just the theory.
    Code, schmode. He is an EIT. I don't think he cares about the NEC yet. Well, not very much anyway.
    Tom
    TBLO

  6. #16
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    Quote Originally Posted by ptonsparky View Post
    Code, schmode. He is an EIT. I don't think he cares about the NEC yet. Well, not very much anyway.
    In that case there is no 80% / 125% to worry about is there?

  7. #17
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    L-L, and L-G or L-N only tell us that you have a Delta configuration. It does not tell us if it is Open or Closed Delta. In this area the Open Deltas were used primarily to supply single phase with a light 3 phase load. The second transformer being smaller than the first. Increased 3 PH load might get the POCO to add the third transformer to make it a Closed. When or if they ever make them all the same size, IDK. Pole mounted transformers for the most part in this area.
    Tom
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  8. #18
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    Quote Originally Posted by iwire View Post
    In that case there is no 80% / 125% to worry about is there?
    Nope.

    Only that 80 amps = 80% of 100 amps and that his theoretical CB will magically trip @ exactly 80+ amp. No more, no less.
    Tom
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  9. #19
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    Quote Originally Posted by ptonsparky View Post
    Nope.

    Only that 80 amps = 80% of 100 amps and that his theoretical CB will magically trip @ exactly 80+ amp. No more, no less.
    Why are you saying a 100 amp breaker will trip at 80?

    What am I missing?

  10. #20
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    If you have a 15kVA balanced 240 volt load you draw 36 amps per line.

    You also figure that as 5 kVA per line, but not at 240 per line, not at 120 per line, not at 208 on the high leg, but at 138 volts per line, that is what the mid point of the delta would be. 208 is not the midpoint, it is a measurement of the distance from one point from the opposite side of the triangle. 120 is only a mid point of one side of the triangle, but does apply for unbalanced 120 volt load calculating.


    But again as OP question is asked a 100 amp three pole main breaker can be loaded to 100 amps per line, but that don't mean you are as likely to have equal current ability per line from the source as you typically have from a wye source with a high leg delta system. Open delta systems don't have equal line current ability, period. Sometimes you do run into a full delta system where they expect higher load on the 120/240 side and put in a bigger transformer for that side, but you can not load the high leg to the same capacity as the other two transformers are not large enough to do so, the 100 amp main will not know any better though.

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