For such motors, use the nameplate current ratings. High-torque motors (and those built to operate under 1,200 rpm) may have higher FLCs, and the FLC of a multispeed motor varies with its speed.A branch-circuit short-circuit and ground-fault protective device protects the motor against many things but not against overload. If that value isn’t on the nameplate, use 150% of the values found in the NEC tables. If a variable-frequency drive controls the motor, use the maximum operating current that’s marked on the nameplate (motor or control).If you’re calculating separate motor overload protection for torque motors, use the locked rotor current value on the nameplate.But it must use the motor FLA to size separate motor overload protection in accordance with Part III of Art. The NEC does not permit the use of motor nameplate FLA to determine the conductor ampacity the branch-circuit short-circuit, and ground-fault overcurrent device size or the ampere rating of disconnecting switches. The other motor(s) in the group (phase) is the 10-hp 3-phase motor, as indicated by the shaded area. The highest rated motor is the 120V motor at 34 FLC. Which do you use - full-load current (FLC) or full-load amperes (FLA) - for your motor calculations?įig. But there are different types of current in motor applications (see Motor Current Basics on page 80). Table or nameplate? To determine the minimum ampacity of the motor supply conductors, you obviously need to know how much current the motor draws. Notice that it shows the requirements for motor overload protection in Part III and the requirements for short-circuit and ground-fault protection in Parts IV and V. Looking at Figure 430.1 in the NEC will help you keep this distinction clear. Conductors for a single motor must have an ampacity of not less than 125% of the motor full-load current rating.
If you can remember not to confuse motor protection with circuit protection, you’ll eliminate a major point of confusion in motor calculations.įig. To prevent a motor from burning up, you must provide a means of protecting it from overcurrent (overload, short circuit, or ground fault).
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