The National Electric Code states that a circuit breaker can support only 80% of the breaker rating as a full constant on load. So a 15 amp breaker is rated only for 12 amps continuous load. An air conditioner causes a power surge during startup and for a few micro seconds the current draw can easily exceed the amp capacity of the breaker and then the current draw drops to run mode. Thus if the air conditioner cycles, there may little or no spare amp capacity for the other items that are also running on the circuit.
By using a #12 gage wire extension cord to the external power source, there will be less voltage drop and increased amperage draw and less chance of tripping the breaker at the power source.
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