turtle , you say that some times it works sometimes it doesnt if this is the case then both circuits must be linked and they split when the 20amp power draw kicks in ,so the 20 amp will more probibaly be a boiler or air conditioning or something along those lines , thats why it there if the circuits linked when the 20 amp draw came on then it would trip it could possibaly be overcome by a qualified spark but i dont know enough about 120v or the circuits to advise you on how to overcome this, do you have to use both circuts ?