Let’s stay with this 5 hp 230 volt single phase motor for this discussion as it is what I have already referenced.
This motor straight out of the box with nothing connected to the shaft will need enough current to overcome the static state or in other words to start the shaft turning. I will take approximately six times the rated current to start this motor in motion.
This motor is rated at 28 full-load amps by Table 430.248 of the NEC.
430.22 requires that the conductors supplying this motor to be sized at 125% of the FLA (full-load amps) or 28 x 125% = 35 amps.
If the conductors are in a NM cable then from the 60 degree column a #8 would be required but if installed as one of the conductors outlined in 310.13 and installed in a raceway the from the 75 degree column a #10 will work.
Per Table 430.52 an inverse time circuit breaker is allowed to be sized at 250% of the FLA of the motor and should this not allow the motor to start then this inverse time circuit breaker can be increased to not more than 400% of the FLA.
Under normal circumstances a 70 amp breaker would be used but it could have a 110 amp breaker installed on the same #10AWG conductor.
This motor without any load (straight out of the box with nothing connected) is not the norm for motors as most will start under some sort of load. This load will prolong the time that the startup current will last.
The trip curve for an inverse time circuit breaker is outlined in the attachment below. As can be seen a 20 amp breaker can hold twice its rating for a couple of minutes and even more. This breaker can hold five times its rating for a period of up to close a half a minute.
Cutler Hammer’s trip curve information shows a 20 amp breaker holding under a load of 200 amps for a period of 90 cycles or in time that would equate to 1 ½ seconds.
On a side note I would like to ask who it was that said that breakers are safer than Edison Base fuses?
