watt-hour metering

To do it right, you are not missing anything. It takes two current sensors, one on each hot side of the split-phase system.

But wiring all the 'hots' through a single sensor core, as shown in McRascal's diagram (one phase straight, opposite phase reversed) should work as a reasonable 'kinda-sorta' approximation in most cases. It would be just fine if the supply was perfectly clean and balanced, but that isn't the real world. Ordinary impedance and mismatch issues will make it susceptible to interference from other unmonitored loads on the same local transformer, but may be close enough for coarse approximations and monitoring.

On three phase systems, this approximation completely falls apart.

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I am installing a 20A Z-Wave outlet which can monitor power and energy and send that info to my SmartThings which I can view in my iPhone. Just another way to monitor power.

 

Which make/model are you using?

 

Enerwave ZW20RM Wireless Z-wave In-Wall Smart Meter Energy Monitor TR Duplex Receptacle,120VAC, 20Amp