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  1. #1
    Join Date
    Apr 2002
    Location
    Western piedmont of Carolinas
    Posts
    1,577
    I'm working on a hot bar w/2 4kw heating elements @208 3phase. By my calculation, 4k x 2= 8kw / 208 = 38amps. Does this sound right? It came from the factory w/a 30 amp breaker.

  2. #2
    Join Date
    Jul 2004
    Posts
    33
    Three phase amps, resistive loads only, amps = Watts divided by volts divided by the square root of 3.

    8000/208/1.73 = 22.2 Amps.


    Getting to divide by the square root of 3 is the advantage of 3-phase power!

  3. #3
    Join Date
    Apr 2002
    Location
    Western piedmont of Carolinas
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    1,577
    Thanks for the help. This is a resistive load, but what about motor loads?

  4. #4
    Join Date
    Jul 2000
    Location
    Dallas,Texas
    Posts
    4,953
    Are these 3 heaters or single ?

    It makes a big difference.

  5. #5
    Join Date
    Jul 2004
    Posts
    33
    Motors are inductive loads. This is where this mechanical engineer bails out!

    Any EE's out there???

  6. #6
    Join Date
    Sep 2002
    Location
    South Dakota
    Posts
    6,579


    Three Phase Watts on a motor is equal to:

    Ave Volts X Ave Amps X %Efficiency X Power Factor X 1.73




    The %Eff listed on the motor will be close enough.
    The power factor listed on the motor will also be close enough.
    The 1.73 is the square root of 3

    You will have to measure the voltages across each leg and use the average.

    You will have to measure the amperage on each leg and use the average.

    Remember, the wattage that you obtain will only be the motor's wattage under the conditions when you took the readings. For example, if you take the readings while the motor is not fully loaded, you will get a lower reading than when the motor is fully loaded. A motor operating a blower in an HVAC system is a good example. If the CFM changes, so will the power consumption (wattage)

    By the way, if you divide the motor's calculated wattage by 746, you will get the motor's actual operating horsepower as opposed to the motor's rated horsepower listed on the name plate. This actual HP is called "Brake Horsepower" and is the actual amount of work the motor is doing.

    Norm

  7. #7
    Join Date
    Jun 2001
    Location
    Northwest Indiana
    Posts
    1,302
    Originally posted by NormChris


    Three Phase Watts on a motor is equal to:

    Ave Volts X Ave Amps X %Efficiency X Power Factor X 1.73




    The %Eff listed on the motor will be close enough.
    The power factor listed on the motor will also be close enough.
    The 1.73 is the square root of 3

    You will have to measure the voltages across each leg and use the average.

    You will have to measure the amperage on each leg and use the average.

    Remember, the wattage that you obtain will only be the motor's wattage under the conditions when you took the readings. For example, if you take the readings while the motor is not fully loaded, you will get a lower reading than when the motor is fully loaded. A motor operating a blower in an HVAC system is a good example. If the CFM changes, so will the power consumption (wattage)

    By the way, if you divide the motor's calculated wattage by 746, you will get the motor's actual operating horsepower as opposed to the motor's rated horsepower listed on the name plate. This actual HP is called "Brake Horsepower" and is the actual amount of work the motor is doing.

    Norm
    Great Post Norm,
    Bernie
    If you cant fix it right, try again.

  8. #8
    Join Date
    Apr 2002
    Location
    Western piedmont of Carolinas
    Posts
    1,577
    Thankfully I only need the resistive load, but I'm , going to print this for future reference.

    Thanks again.

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