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.