So, what is a main breaker anyway? You'd think that it is something special having a name like that. Actually, the main breaker is the breaker that the feeder wire connects to. This breaker is a two-pole breaker that is connected to 240 volts to power your home. The main breaker acts as the disconnecting means to the entire power load of your breaker box. With it off, there is no power being fed to the buss bar that feeds the branch circuit breakers.
The main breaker is usually located atop the rest of the breakers in a panel, but sometimes the breaker panel is installed upside down. In this case, the main would be at the bottom. Some main breakers are bolted in place and others are snapped into place just like the branch circuit breakers.
Main breakers come in different sizes, but 100 and 200-amp breakers are generally used in residential installations. In older homes, you may still have a 60-amp service and I personally have seen a 30-amp, 120-volt main service. If you think you are short on power, think about that installation!
Main breakers are designed to trip if the amperage rating exceeds the value labeled on the breaker. For instance, if you have a 100-amp main breaker in place, if the amperage exceeds 100 amps, the breaker will trip.
There are a number of reasons why breakers trip. It may be lightning strikes, power surges from the utility company, or you may have overloaded your electrical panel.
If your electrical panel's main breaker does trip, turn off the branch circuit breakers before resetting the main breaker. Then, one at a time, start turning on breakers again. The idea here is not to dump the entire power load on the main breaker all at once. If the breaker trips again, you may be well advised to call in a professional.