Is this breaker connected to water heater?
1) A double 20 amp breaker is called a 240Volt breaker.
20 amp breaker is actually rated for 16 amps for safety margin.
It is also important that this breaker have 12 gauge wire which is color-coded yellow.
Look at label on side of water heater for wattage of unit.
2) For example 4500 Watt water heater is typical.
4500 watts divided by 240Volts = 18.75 amps which is nearly full capacity for the 20 amp breaker, and is slightly above the safety margin.
http://waterheatertimer.org/B220C.html3) If breaker is getting old, and it works at full load each time water heater is on, it can wear out.
Open circuit breaker box and replace breaker:
http://waterheatertimer.org/How-to-replace-circuit-breaker.html4) You can also move circuit breaker wire to another same-size breaker and see if problem continues.
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