Originally Posted by boman47k
I never understood that. My thinking has been that a motor/compressor will pull more amps than they are rated when startingup. 12 g wire =20 amps. 10 g wire =30 amps. Right?
The compressors and motors will only draw the extra current at start up. The wire is sized or the continous running load, and the breaker is allowed to be larger so that it will not neusance trip on start up.
The confusion is what you are protecting against. On most circuits the breaker serves two functions. It protects against short circuits, and against overloads.
On motor circuits the breaker is only protecting against short circuit. The motor over loads (on hvac equipment these are usually built into the motor) protect against overload.
Since a motor is on a dedicated circuit, and the built in overloads serve that function, there is nothing else on the circuit that would cause the need for overload protection of the wire.
If you look at the trip curves for different size breakers you will see that the difference in the time it takes for the breaker to clear a fault on short circuit for two breakers that are close to eachother in amp rating (say for example: 20 or 30 amp breaker) is minimal. So allowing the breaker to be up to 200 percent of the actual motor FLA is not a bad thing at all.
It is important to understand that the available fault current on a short circuit is limited by the size of the transformer and the size and length of the wires between the transformer and the short, NOT the size of the breaker. The size of the breaker will only limit the lenght of time before the fault is cleared.