Last night my main breaker tripped (100A). I reset it and it tripped again 10 minutes later. I suspect it is the water heater because it's old (like 15 yrs). But it trips the MAIN breaker, not the 30A breaker in the inside panel that runs only to the water heater. Left the water heater off all last night-no problem. Turned it on this morning and 10 minutes later... Same thing. Main trips, NOT the breaker between that goes to the water heater. Do I need a new water heater, main breaker, something else?

Hi nu13, I'm no expert but I am curious, Have you added anything new to the system, like space heaters, since the main started knocking? There should be an expert along anytime for a more informed answer. Good luck

Have the Main Breaker checked, and have the amps the Main is carrying.

Could be a defective main, or a loose connection, causing this to trip when the high amp load of the water heater is applied.