
What will each wattmeter indicate?
Three coils with a resistance of 4 ohms and impedance of 13 ohms each are connected in wye to a 120/208 Volt panel. What will each wattmeter indicate if three singlephase wattmeters are used to measure the power?
The correct answer is 341W according to my answer sheet but I seem to keep getting 311W and I don't know what Im doing wrong. Any help would be great!

Originally Posted by
Stephen30002
Three coils with a resistance of 4 ohms and impedance of 13 ohms each are connected in wye to a 120/208 Volt panel. What will each wattmeter indicate if three singlephase wattmeters are used to measure the power?
The correct answer is 341W according to my answer sheet but I seem to keep getting 311W and I don't know what Im doing wrong. Any help would be great!
how did you come up with 311W? i'm having trouble with this question, not sure where to start.

I get same as you...
Originally Posted by
Stephen30002
Three coils with a resistance of 4 ohms and impedance of 13 ohms each are connected in wye to a 120/208 Volt panel. What will each wattmeter indicate if three singlephase wattmeters are used to measure the power?
The correct answer is 341W according to my answer sheet but I seem to keep getting 311W and I don't know what Im doing wrong. Any help would be great!
I get the same answer as you and verified it through calculator on this site: http://www.rapidtables.com/calc/elec...calculator.htm
Impedance Z = SQRT(4*4 + 13*13) = 8.823 Ohms
Angle Theta = ATAN (Impedance/Resistance) = ATAN (13/4) = 72.9 degrees
Total Power VA = V*V/Z = (120 * 120) / 8.823 = 1058.82 VA
Real Power Watts = Total Power * Cos Theta = 1058.82 * Cos(72.9) = 311 watts
Beats me how someone got 345 watts.

Originally Posted by
baldscreen
I get the same answer as you and verified it through calculator on this site:
http://www.rapidtables.com/calc/elec...calculator.htm
Impedance Z = SQRT(4*4 + 13*13) = 8.823 Ohms
Angle Theta = ATAN (Impedance/Resistance) = ATAN (13/4) = 72.9 degrees
Total Power VA = V*V/Z = (120 * 120) / 8.823 = 1058.82 VA
Real Power Watts = Total Power * Cos Theta = 1058.82 * Cos(72.9) = 311 watts
Beats me how someone got 345 watts.
Impedance should be 13 ohms instead of 8.8

I stand corrected...more or less.
Originally Posted by
Allenxu
Impedance should be 13 ohms instead of 8.8
You are correct on the impedance. I was thinking of it as reactance ohms.
So, I'll try it again:
COS(Theta) = (Resistance/Impedance) = 4/13
Total Power VA = V*V/Z = (120 * 120) / 13 = 1107.69 VA
Real Power Watts = Total Power * Cos Theta = 1107.69*4/13 = 340.8 watts
Closer, but not quite the 345 watts given as the answer.

either luck or ???
PF = R/Z = 4/13 = 0.3077
E = I*R , 120 = I*13 , I = 120/13 = 9.23A
P = I*E , P = 9.23*120 = 1107.7VA
0.3077(PF) = W/1107.7VA , W = 1107.7*0.3077 = 340.84
340.84(W)/9.23A = 36.93V
36.93V/4OHMS = 9.23A
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