Is there a practical way to sense 60Hz AC power in a pair of wires? Measuring the exact power or even current isn't necessary; it would be sufficient to know whether the load is pulling more than, say 5 amps, or not. For example, suppose you have a well pump wired directly into the house distribution panel, and you only want to monitor when it turns on and off.
This would be a piece of cake if it were possible to separate the conductors and get a regular AC current probe around one of them, but cutting into the romex is unsatisfactory. Finding a place where the conductors are already separate is an obvious angle of attack, but let's leave taking the face off the distribution panel for last resort.
Ideally there are already commercial sensors for this, but it's proving to be an awkward thing to google for. Seems like 8 or 10 amps through two conductors 3/8" or so apart should give off enough stray field to at least tell the difference between on and off.
Answer
I agree that the stray magnetic field from your power cables seems like it would be enough to at least discriminate between "at least 5 amps on" or "0 amps off". Perhaps you could build up a system based on one of the magnetic field sensors listed at Open Circuits and press it against the power cord at some convenient point.
As Joby Taffey pointed out, most equipment plugs into an electric socket, and so inserting a meter between the equipment and the electric socket is common:
These things are generally calibrated to linearly measure current, so they are total overkill for merely discriminating between "on" and "off". And they won't work as-is for your application, since there is no socket between your well pump and the distribution panel. But perhaps using one of these off-the-shelf devices, hacking it open and pressing the magnetic sensor against the weak magnetic field that leaks out of your romex cable, will be able to discriminate between "on" and "off" and take much less time and effort than building up something from scratch.
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