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what could be the cause of this? The output power i get from my calculation after measuring AC current with a clamp meter and multiplying by 220v is not always close to the amount of load plugged

  • Is your loads pure 220V or are they a mix of 110V and 220V. Also you may want to look at the wiki for AC Power, not every thing is pure values. – spicetraders Sep 4 '16 at 0:33
  • Get a Kill-a-Watt which is a $20 voltage measuring device, and measure each of your loads individually. Look closely at Watts, VA and power factor, As Ecnerwal discusses. – Harper - Reinstate Monica Sep 4 '16 at 17:28
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Welcome to Power Factor. Amps times volts only equals watts for loads that have a power factor of 1.0, such as a pure resistive heater or incandescent light.

Typical loads such as motors LED or florescent lights and most computer power supplies (though some newer ones are well-adjusted and may hit 0.99) have current that is somewhat out of phase with voltage. Some items are actually rated in VA (volt-amperes) rather than Watts, and that commonly reflects a non-unity power factor.

You may also have the simpler situation where you are reading a label with the maximum load and the actual load may be less than the maximum; or the "maximum load" may ignore a starting surge for a motor that may briefly be much higher than the label states for running load.

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  • Note that with electronic loads, their low PF isn't just a function of I and V being out of phase -- without PFC, they draw spikes of current, leading to their low effective PF despite the spikes being in phase with the mains voltage waveform. – ThreePhaseEel Sep 4 '16 at 2:16
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What kind of load? Many devices, motors in particular, draw different amounts of power when starting vs. once they are rumning, and circuits have to be rated for the higher number. Many others cycle between different modes: most household appliances would be examples of that. And if course circuits are rated for maximum permitted draw, so adding up the circuit breakers should result in z larger number than what is actually being used.

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