Quote:
I've seen a device that clamps onto power cables

Those measure the magnetic field surrounding the cable clamped around. What this means is that you can't use it to measure normal power cables, as the two wires in the power cable have opposite magnetic fields that cancel each other out. You can use them only when you can get the clamp around the hot wire by itself. (Or, probably, the neutral wire by itself.)
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Bitt Faulk