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Calculating 208V power

  1. Jul 27, 2016 #1
    Need some help here, as things get a little blurry with the phase angles and everything.
    But here's my question:

    I have a 208V/30A 2 pole breaker, feeding 10A to each phase. i need to calculate power.
    I have 3 methods and need to know which one is correct.

    1.) 208V x 10A = 2080 Watts (total power used by both phases)

    2.) (208V x 10A) for 1 phase + (208V x 10A) for the other phase = 4160 Watts for both phases

    3.) (120V x 10A) for 1 phase + (120V x 10A) for the other phase = 2400 Watts for both phases

    OMIT POWER FACTOR AND ANYTHING ELSE PLEASE. I would like to eliminate confusion, because coefficients are easy to understand. and please just answer my question directly.
  2. jcsd
  3. Jul 27, 2016 #2


    Staff: Mentor

    You have either a single phase system (2 wires) or a three phase system (3 wires). There is no such thing as a two phase system.

    Method 1 is the way to calculate power for single phase systems.
  4. Jul 27, 2016 #3
    I was little confused by your statement "there is no such thing as a two phase system"

    But i think i get what you mean. Even though the circuit has 2 hots, it is still considered single phase.

    Is that correct?
  5. Jul 27, 2016 #4


    Staff: Mentor

    Yes, that's correct.
  6. Jul 27, 2016 #5


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    It depends. If you have a resistor across the 208 volt pair then it is 10 A x 208 V. But, you may have a resistor from each hot to ground. This is not the same on a 208 wye system which I suspect you have.
  7. Jul 27, 2016 #6


    Staff: Mentor

    Ay here we go again. Soon someone will mention neutral and cause @sophiec@hoophy to melt down. :oldsmile:
  8. Jul 27, 2016 #7


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    Good one! :smile:
  9. Jul 28, 2016 #8
  10. Jul 28, 2016 #9


    Staff: Mentor

    True, but a motor is not a power distribution system.

    The key idea is that the currents do not disappear. Current supplied by the source must be returned to the source. That is Kirchoff's Current Law applied to a system.

    In a single phase system, we have two wires. Current comes from the source through one wire, and returns via a second wire. Two wires, one phase.

    In a three phase system, the currents in the three phases are shifted 120 degrees relative to each other, so that when they are combined, the three add up to zero. Thus no return wire is needed. Three phases, three wires.

    Clever people can invent hundreds of other combinations of wires and phase shifts and give them many names. (Earth returns excluded.) Here are just a handful of them.

    1-Phase, 2-Wire 120 V with neutral 120 – US 3Y-208
    1-Phase, 2-Wire 230 V with neutral 230 – EU, Others 3Y-400
    1-Phase, 2-Wire 208 V (No neutral) – 208 US 3D-240
    1-Phase, 2-Wire 240 V (No neutral) – 240 US 3D-240
    1-Phase, 3-Wire 120/240 V 120 240 US 3Y-208
    3-Phase, 3-Wire 208 V Delta (No neutral) – 208 US 3D-240
    3-Phase, 3-Wire 230 V Delta (No neutral) – 230 Norway 3D-240
    3-Phase, 3-Wire 400 V Delta (No neutral) – 400 EU, Others 3D-400
    3-Phase, 3-Wire 480 V Delta (No neutral) – 480 US 3D-480
    3-Phase, 3-Wire 600 V Delta (No neutral) – 600 US, Canada none1
    3-Phase, 4-Wire 208Y/120 V 120 208 US 3Y-208, 3D-240
    3-Phase, 4-Wire 400Y/230 V 230 400 EU, Others 3Y-400, 3D-400
    3-Phase, 4-Wire 415Y/240 V 230 415 Australia 3Y-400, 3D-400
    3-Phase, 4-Wire 480Y/277 V 277 480 US 3Y-480, 3D-480
    3-Phase, 4-Wire 600Y/347 V 347 600 US, Canada 3Y-600
    3-Phase 4-Wire Delta 120/208/240
    Wild Phase 120,
    208 240 US 3D-240
    3-Phase 4-Wire Delta 240/415/480
    Wild Phase 240,
    415 480 US 3D-480
    3-Phase Corner-rounded Delta 208/240 – 240 US 3D-240
    3-Phase Corner-Grounded Delta 415/480 – 480 US 3D-480

    Some are called two phase, so in that respect you are correct But in the end, they must reduce to one of the two above. Either there is a wire for return current (sometimes called the neutral :oldsurprised: ), or the currents add to zero.

  11. Jul 28, 2016 #10
    Don't let anyone in the center of Philadelphia know their two phase system produced by the Scott T transformer connection doesn't exist.
    They may become confused as well.

    Always enjoying the forum.

  12. Jul 28, 2016 #11


    Staff: Mentor

    OK, if they're still using that, I must be wrong about two phase. The Scott T transformer was invented in 1890.
  13. Jul 28, 2016 #12


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    2 phase has existed in the past and may well exist today but I cannot be sure. I never knew until a few years ago that there was such a thing as true 2 phase.
  14. Jul 28, 2016 #13


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    Staff: Mentor

    The typical US residential service is often called "split phase".
  15. Aug 2, 2016 #14
    LOL this thread is so funny. Im glad i was able to stir up an old debate.

    And thank you so much for everyone's inputs.
  16. Aug 3, 2016 #15

    jim hardy

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    A question well stated is half answered
    and yours falls just short of well stated.

    What is that 208 volt breaker's feed coming from ? And what kind of a load is it connected to ?

    If its feed comes from a residential type distribution transformer , which is 230 volt single phase centertapped,
    and it's reading just 208 today instead of 230 for some reason you didn't tell us about ,
    then the best answer from the three you proposed is (1) .
    If it's fed from a 208/120 three phase distribution transformer , like they use for motels and such
    where 208 volts is phase to phase voltage for big loads like airconditioners
    and 120 volts is phase to neutral voltage for lights and receptacles,
    then the best answer will depend on what the breaker feeds.
    It will be either:
    (1), if the breaker connects to 208 volt loads
    (3), if the breaker connects to 120 volt loads.

    (2) is out, barring something really weird...

    So - is this a single phase panel where we'd expect to find not 208 volts but 230 ?
    Or is is a 3 phase panel where we'd expect 208 volts ?

    In other words,
    Does all the current entering through one pole return through the other pole or is there a neutral you didn't mention ?
    Had you said 230 volts not 208 i'd have assumed single phase residential...

    Lastly, if you want to calculate power, you CANNOT omit power factor.
    To get Watts, you must multiply the product Volts X Amps by Power Factor.

    old jim
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