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  1. tlktomegoose is offline Junior Member Pro Subscriber
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    Throw me a bone

    Can someone who has a command on this and the proper calculation enlighten me? What values do I extract in order to calculate burden? TIA

    If a watt-hour meter draws 1.5 watts and 11.3 Vars from a potential transformer, and 1.88 watts and 1.8 Vars from a current transformer, the total apparent burden placed on the current transformer is?

    Correct Answer: 2.60VA

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  3. nwilliams is offline Junior Member Pro Subscriber
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    Quote Originally Posted by tlktomegoose View Post
    Can someone who has a command on this and the proper calculation enlighten me? What values do I extract in order to calculate burden? TIA

    If a watt-hour meter draws 1.5 watts and 11.3 Vars from a potential transformer, and 1.88 watts and 1.8 Vars from a current transformer, the total apparent burden placed on the current transformer is?

    Correct Answer: 2.60VA
    I believe the information from the potential transformer is irrelevant. You can derive the answer of 2.60 VA from the Real Power (watts) and Reactive Power (VARs) drawn from the Current Transformer. It is asking for the Current Transformer Burden after all.

    Since burden is expressed in terms of power and the question wants Apparent Power (VAs), you can calculate the Square Root of (Real Power)^2 + (Reactive Power)^2

    Sqrt( (1.88 watts)^2 + (1.8 VARs)^2 ) = 2.60 VAs

    It’s really just solving for C in the Pythagorean Theorem (A^2 + B^2 = C^2), where A is real power and B is reactive power.

    I could be wrong about not taking into account the potential transformer power. In that case, this method just coincidentally gets the right answer. If I’m wrong and someone else knows the right way, please correct me.

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  5. tlktomegoose is offline Junior Member Pro Subscriber
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    That equation gets me close at least. 2.713.
    THX

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  7. nwilliams is offline Junior Member Pro Subscriber
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    Quote Originally Posted by tlktomegoose View Post
    That equation gets me close at least. 2.713.
    THX
    What result are you getting? I’m getting 2.60276… which is pretty much dead on 2.60 VAs

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  9. tlktomegoose is offline Junior Member Pro Subscriber
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    Quote Originally Posted by nwilliams View Post
    What result are you getting? I’m getting 2.60276… which is pretty much dead on 2.60 VAs
    for the sq. root of 1.88 I'm getting 1.371
    for the sq. root of 1.8 I'm getting +1.342
    ---------
    2.713
    What am I messing up?

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  11. nwilliams is offline Junior Member Pro Subscriber
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    Quote Originally Posted by tlktomegoose View Post
    for the sq. root of 1.88 I'm getting 1.371
    for the sq. root of 1.8 I'm getting +1.342
    ---------
    2.713
    What am I messing up?
    Your problem is you are calculating the sum of square roots. sqrt(1.88) + sqrt(1.8)

    What you need to be doing is calculating the square root of the sum of squares. sqrt( (1.88)2 + (1.8)2 )

    It breaks down like this:

    sqrt( (1.88)2 + (1.8)2 )
    sqrt( 3.5344 + 3.24 )
    sqrt( 6.7744 )
    2.6027

    Click image for larger version. 

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    You're solving for S. When P = 1.88 watts and Q = 1.8 VARs, S is calculated to be approximately 2.60 VAs.

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