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Yes you can and it is not legal. One or two people who attempt this have been prosecuted as you can't really hide it. All you need do is place a few coils of wire under the power cables and magnetic induction will power up your coils. Vary your needs with the number and size of your coils. However, it only works during the same time your power cables do and most companies have a helicopter flying over the power lines every now and then to check for problems and your coils, even if buried underground, will stand out like a sore thumb. As this is a theoretical question you have what you need to know and the answer is based on things I've been told - not practical experience.