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

  1. #1
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    Safely Maximize Power from a 100amp 3-Phase Delta Panel

    What is the maximum amount of power that can be safely loaded on a 100amp 3 phase delta panel assuming all loads are 240v?

    120 - 120 - 208.

    Here's what I was thinking:

    120v * 100a * 80% = 9600w
    120v * 100a * 80% = 9600w
    208v * 100a * 80% = 16640w

    Total = 35,840w

    How would the loads be distributed?

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    This is the second thread that I remember when the OP gave his occupation as EIT.

    What is an EIT?
    Once in a while you get shown the light
    In the strangest of places if you look at it right. Robert Hunter

  3. #3
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    Quote Originally Posted by ActionDave View Post
    This is the second thread that I remember when the OP gave his occupation as EIT.

    What is an EIT?
    Engineer In Training?

    https://www.google.com/search?q=eit+...utf-8&oe=utf-8
    I suggest that we put it back, approve, and explain the workings of a three phase 4-wire high leg delta.

    If it were not for the center tapped neutral, the pro-forma delta and wye energy calculations for balanced loading would give the same result.
    But in this case the phase relationships are not right and the current in the two ends of the center-tapped winding are not in phase with the line to neutral voltage.
    Last edited by GoldDigger; 03-17-17 at 05:52 AM.

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    Quote Originally Posted by EricJ View Post
    What is the maximum amount of power that can be safely loaded on a 100amp 3 phase delta panel assuming all loads are 240v?

    120 - 120 - 208.

    Here's what I was thinking:

    120v * 100a * 80% = 9600w
    120v * 100a * 80% = 9600w
    208v * 100a * 80% = 16640w

    Total = 35,840w

    How would the loads be distributed?
    The panel can handle 100 amps per leg regardless of conditions below.

    The source however is a different story. Full delta source with three equal sized coils can handle same current per leg.

    Open delta or mismatched units in full delta are going to vary in capacity per leg, depending on what is used for the source.

  5. #5
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    EIT is the certification you receive after college and before becomeing a PE.

    I am helping a client explore the feasibility of a project and also want to understand how 3phase delta works a bit better.

    Quote Originally Posted by kwired View Post
    The panel can handle 100 amps per leg regardless of conditions below.

    The source however is a different story. Full delta source with three equal sized coils can handle same current per leg.

    Open delta or mismatched units in full delta are going to vary in capacity per leg, depending on what is used for the source.
    Assuming leg c is the delta leg, when a load is placed between a - c does leg c deliver more of the power because it has a higher potential?

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    EricJ:

    Look at the question you asked. Then look at your answer. Then ask yourself how does your answer correspond to the question. The proposed answer does not correspond to the question.

    What is the maximum amount of power that can be safely loaded on a 100amp 3 phase delta panel assuming all loads are 240v?
    Your question stated all 240 V loads. A 3 phase delta panel alone does not define what kind of delta you have. The delta source could be anyone of several types. But this really does not matter relative to the question ask. Note: your proposed answer implied that you were thinking of a high leg delta, but that was not stated in the question and does not matter for the question ask.

    That you have specified a current limitation as part of the question, then this means your load must be pure resistance for maximum power.

    What does safely mean? Here one must make some assumption. The logical assumption for this question is that the panel and power source can safely tolerate a 100 A load per leg on a continuous basis,

    From a visualization perspective or even a practical resistive load, you can consider the load to be a wye. For a balanced resistive wye load the voltage from a line to the mid point of the resistors is 240/1.732 = 139 V. It is easy to see that this voltage is in phase with the line current. Therefore power can be calculated from V*I.

    Thus, 1/3 of the total maximum power is 139*100 = 13,900 W, and total power is 41,700 W.

    .

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    Gar

    Oops that was a mistake. I am specifically referring to a high leg delta configuration.

    By safely I really mean without overloading any breakers or fuses and having enough head room so that plugging in a small appliance or stero or something of the sort doesn't trip a main breaker or fuse.

    I think my question was worded poorly. I guess what I'm wondering is if it is possible to distribute 240v loads on a high leg delta configuration in such a way that the current from all three legs are balanced thus utilizing the full power capacity of the panel? Or will the current on the high leg always be less?

  8. #8
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    Quote Originally Posted by EricJ View Post
    Gar

    Oops that was a mistake. I am specifically referring to a high leg delta configuration.

    By safely I really mean without overloading any breakers or fuses and having enough head room so that plugging in a small appliance or stero or something of the sort doesn't trip a main breaker or fuse.

    I think my question was worded poorly. I guess what I'm wondering is if it is possible to distribute 240v loads on a high leg delta configuration in such a way that the current from all three legs are balanced thus utilizing the full power capacity of the panel? Or will the current on the high leg always be less?
    If you balance the line to line loads evenly across all three phases, it will result in equal currents in all three ungrounded wires.
    The fact that you are looking at a high leg delta does not change that in the least.

    Your problem is that if you put balanced line to line resistive loads on a wye source you can calculate the total power in two different ways:

    You can multiply the load (single line to line) currents by the line to line voltage and add up the three terms.
    Or you can multiply the line current (resulting from two line to line loads, out of phase) by the line to neutral voltage and add them up.

    The two results must be exactly the same, since the source cannot tell whether the line currents result from a balanced delta load or a balanced wye load.

    Your problem is that you are trying to make a similar calculation using the line to neutral voltages in a high leg delta. That simple current times line to neutral voltage calculation does NOT work in this case since the current in the A and C wires is NOT in phase with the A or C to N voltage.

    The fact that the B leg is carrying the same current as the A and C legs but has a higher L-N voltage does NOT mean that the B leg is somehow supplying more power. If you do the calculation correctly, the sum of the three voltage x current x power factor products will be equal to the delta calculation of the simple application of the three phase power formula.

    As an example of why your calculation does not work, look for a moment at a corner grounded delta with a grounded B leg.
    It is not true that all of a sudden the B leg does not provide any power. And the line currents in A and C are NOT in phase with the A-B or A-C voltages.

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    The question is still fuzzy. If all of the loads are 240v, then you are not utilizing the grounded center-tap. I assume, then, that you are picturing the normal result of a 12/240 high-leg panel where every third breaker space is skipped, because the loads are 120v. That wouldn't occur with 240v loading, so this wouldn't be an issue.

  10. #10
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    EricJ:

    You need to try to figure out what question you really want to ask and precisely define the conditions. So far your comments make no sense.

    Possibly you may want to ask why there are single phase systems with a center tapped secondary that have a single added transformer that turns the system into a wild leg three phase system? We have many of these in our area. The answer has nothing to do with panel and breaker ratings.

    Viewed from a transformer capability perspective you generally do not have equal line current capability. But when you get to an individual panel where there are many customers on the same two transformers, then it is possible to view the panel capability of a single panel as having equal current on each leg. This can not be true for all customers simultaneously, or the total load of all customers has to be limited to the current capability of the smaller transformer (logically the one supplying the wild leg).

    As your question was originally proposed we have to assume that when you speak of a maximum current that this means a precise value for calculation purposes. Below that value is OK, and above the value is an overload. But in the real world for actual breaker panels, and transformers there is no precise threshold. Rather the limits are defined by ratings, temperature, materials, and expected lifetime. Thus, your small added loads are not significant until they become larger.

    A very important part of your future education is learning how to define and ask the correct question. A project or customer may not send you in the correct direction.

    .

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