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3 phase calculations

Johno12345
it might just be too early in the morning but I'm getting really confused.


On a 3 phase circuit, I measure 6a 6a and 6a on L1 L2 and L3


I want to work out how many watts it is, so I use rapid tables, set it as 3 phase, 400v line to line and it asks for the amps, is that 6, or is it 18?


The results don't make sense. 


If the loading wasn't equal, is it the sum of all three readings, or an average, or, even, do I clamp all three phases together?


so, using the formula P = √3 × pf × I × V which is fine, but again, is I the total amps per phase, 18, or 6? 



Any help is appreciated
  • 0
    think i have got it


    in long hand


    square root of 3 multiplied by the power factor, multiplied by the average of the three phase currents in amps, multiplied by the line-line voltage 


    giving me 4,156w
  • 0
    That would work. An easy way to think of it is as three separate single phase circuits with the voltage as V/√3. For 400V phase-phase that would be 231V phase-neutral so the power on each phase is I x pf x 231. Add the individual powers in each phase together and it should give the same as the formula you originally quoted using the average of the phase currents. (Calculating the power in each phase and adding them is more long winded and I will use the original formula every time, but if you don't understand the basis of the formula it is helpful to look at it from first principles).
  • 0
    that's where I was getting confused


    If I treat it as three separate single-phase circuits then I get


    6a x pf (1) x231 = 1386w x 3


    oh wait, that's right


    That is the way I always used to do it, obviously, I really confused myself this morning, I had a terrible feeling I had done some calculations wrong, but I haven't. 


    I'm making an excel sheet to calculate it out, ill add it here when it's complete.


    I am determined to understand things that I put together as proposals, rather than just blindly trusting people and the results they produce.



    Thanks A
  • 0
    Power Calculator - Copy.xlsx


    Thought it might come in useful to someone in the future
  • Former Community Member
    0 Former Community Member
    Hello,  In any case, you must determine the power factor.

    Maybe this calculator can help you.

    3905407af6495187d07bd7ca8a2b0fde-original-image.png

    The MeteorSPEC can generate a detailed report.

    25e22cc83b0e705e00ee5c9abd6a17e9-original-image.png

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