There are three currents that must be determined before designing and selecting the elements to make up circuits supplying power to motors. The first current that must be found is the full-load amps (FLA) from Table 430.248 for single-phase motors and Table 430.250 for three-phase motors. This current rating in amps, per 430.6(A)(1), is used to size all the elements of the circuit except the overload protection. The second current to be determined is the nameplate amps on the motor per 430.6(A)(2). This current rating in amps is used to size the overloads (OLs) to protect the motor windings and conductors.  The third current, per 430.7(A)(9), is the locked-rotor current (LRC), in amps (Design Letters), from Tables 430.251(A) and (B). The overcurrent protection device shall be sized large enough to hold this current rating (LRC) in amps and permit the motor to start and run. When using the code letter to determine the locked-rotor current (starting current), see Table 430.7(B).

 

Electrical systems containing AC and DC motors must have their circuits and elements designed per Article 430. AC and DC motors are available in various types and sizes. To protect such motors and circuits and still allow them to operate, the conductors, controllers, starters, protection devices, and disconnecting means must be designed and installed properly.