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# Throw me a bone

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1.  Junior Member Pro Subscriber
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Reputation ## 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? 2.  Junior Member Pro Subscriber
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Reputation  Originally Posted by tlktomegoose 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?

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. 3.  Junior Member Pro Subscriber
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Reputation That equation gets me close at least. 2.713.
THX 4.  Junior Member Pro Subscriber
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Reputation  Originally Posted by tlktomegoose 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 5.  Junior Member Pro Subscriber
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Reputation  Originally Posted by nwilliams 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
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2.713
What am I messing up? 6.  Junior Member Pro Subscriber
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Reputation  Originally Posted by tlktomegoose for the sq. root of 1.88 I'm getting 1.371
for the sq. root of 1.8 I'm getting +1.342
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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 You're solving for S. When P = 1.88 watts and Q = 1.8 VARs, S is calculated to be approximately 2.60 VAs.  