With a clamp on amp meter, I have 8.2 amps at 123.6 volts on one leg and 7.2 amps at 123.9 volts on the 2nd leg. When I touch the volt meter to both legs I get 247.8 volts.
What is the total wattage? I get 3816 watts total if I add the amps together and divide by 247.8 or 1905.5 total if I calculate the watts (v x a) for each leg and add together.
What is the correct way to do this.
The measurement was taken from the two main wires that feed the house panel box from the transformer, so everything that was on in the house was showing in the measurment, there can be an amp difference since single pole breakers only draw from one of the wires.
Best Answer
The correct way to measure AC power is using a wattmeter. This is because you cannot know (or reasonably estimate) the phase angle between voltage and current and measuring amps and volts and simply multiplying them together gives a falsely high reading in all but simple cases of a purely resistive load.
So, if you had two wattmeters you can measure the wattage entering one circuit to neutral and also measure the wattage entering the 2nd circuit with respect to neutral.
If you loads are resistive the powers will be: -
Total power (if resistive load) = 1905 watts.
However, if the load is resistive AND non-linear there will be harmonics in the current waveform that don't contribute to power loss but will register on an RMS measuring current meter so here's another error source.
Best way - use a proper wattmeter.