Calculating neutral current with assymetric load waveform

[Electric-Light]

Under conventional definition, the neutral carries the difference in current. If you have a 1kW light bulb on each side, the neutral current is essentially zero.

What about given:
A = 1kW space heater, 120v 8.33Arms. PF = 1.0. waveform is sinusoidal.
B = 1kVA 600W in CFLs and computers. 120v 8.33Arms, PF = 0.6. phase shift = 0 deg. Waveform is what you normally see with bulk-storage capacitor/rectifier input load type.

I don't know how you'd calculate it, but I doubt its zero.

Red shows A side current waveform. Blue shows B side current waveform.

 

[gar]

110406-20215 EDT

First your blue waveform has to have a much higher peak value than the red for the values you specified. Possibly 3 times the red peak. Then you need to invert the blue waveform. Next point by point add the two waveforms. This will result in a neutral waveform with negative spikes at the k*90 deg points.

After you get the composite waveform, then perform the square of the instantaneous values. The new waveform is all positive or 0 values. Determine the area under the curve for one complete cycle. Divide the area by the period of the one cycle. Now you have the mean value of the square of the waveform.

Take the square-root of that mean and you have the RMS current.

Generally you learn how to do this in a course in calculus, or at least get exposed to the concept.

A resistor with an attached thermocouple will do all this for you automatically. Its called a hot wire ammeter.

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