Motor circuits

Motors are the largest category of load in industry and a major one in commercial buildings: pumps, fans, compressors, conveyors, elevators and machine tools. Motor circuits follow their own code rules because motors behave unlike other loads. They draw five to eight times their running current when starting, they can be overloaded mechanically for long periods without drawing a fault-level current, and their nameplate ratings vary between manufacturers. The code therefore separates protection into distinct functions, each sized differently. This lesson covers motor full-load currents, conductor sizing, overload protection, short-circuit and ground-fault protection, feeders for several motors, disconnecting means, control circuits and circuits for variable-frequency drives.

Motor circuits are covered by Article 430 of the NEC, with full-load currents in Tables 430.247 to 430.250 and maximum overcurrent device settings in Table 430.52. Figure 430.1 maps the parts of the article to the parts of a motor circuit.

The parts of a motor circuit

A motor branch circuit has separate elements, each with its own job:

• Branch circuit short-circuit and ground-fault protection (fuses or a circuit breaker): protects the conductors and equipment against short circuits and ground faults, but must be large enough to let the motor start.
• Disconnecting means: allows the motor and controller to be isolated for maintenance.
• Controller (starter or drive): starts and stops the motor.
• Motor overload protection: protects the motor and conductors against sustained overcurrent from mechanical overload or failure to start, usually thermal or electronic overload relays in the starter.
• Branch circuit conductors, sized for continuous duty.
• Control circuit and its protection.

Because the short-circuit device must pass the high starting current, it cannot also protect against overload; the overload relay handles that, and the two are coordinated.

Full-load current: tables or nameplate?

A key principle is which current to use for which purpose:

• For conductors, short-circuit and ground-fault protection and disconnecting means, use the full-load current from Tables 430.247 to 430.250 (430.6(A)(1)), not the nameplate value, so that the circuit will suit any motor of that horsepower that might later be installed.
• For overload protection, use the actual nameplate full-load current (FLA), because overloads protect this particular motor.

Branch circuit conductors

Motors may run continuously at full load, and their current varies with load and voltage, so branch circuit conductors for a single motor are sized at 125% of the motor's table full-load current.

430.22 requires conductors supplying a single continuous-duty motor to have an ampacity of not less than 125% of the motor's full-load current from the tables, with Table 430.22(E) giving other percentages for short-time, intermittent, periodic and varying duty.

Overload protection

Overload devices, usually thermal or electronic relays in the motor starter, trip when current stays above their setting for long enough, with a time-current characteristic that allows starting current briefly but trips on sustained overload or a locked rotor. They are sized from the nameplate current and the motor's service factor or temperature rise.

430.32 requires overload protection for continuous-duty motors over 1 hp, set at no more than 125% of nameplate full-load current for motors with a service factor of 1.15 or more or a marked temperature rise of 40 °C or less, and 115% for all others. 430.32(C) allows higher settings (up to 140% and 130%) if the motor will not start or carry its load.

Short-circuit and ground-fault protection

The branch circuit protective device must carry the motor's starting current (typically 6 to 8 times full-load current for a few seconds) without tripping, while still opening quickly for a short circuit or ground fault. The code therefore allows ratings well above the conductor ampacity, set as a percentage of table full-load current depending on the type of device:

430.52 requires the device not to exceed the Table 430.52 percentages; where the value does not correspond to a standard size, the next higher standard size may be used (430.52(C)(1) Exception No. 1), and higher values are permitted where the motor will not start.

Feeders supplying several motors

A feeder supplying several motors is sized for the largest motor at 125% plus the sum of the others at 100%, plus any other loads, because it is unlikely that all motors are overloaded at once. The feeder's overcurrent device must allow the largest motor to start while the others run: its rating may not exceed the largest branch circuit device rating plus the sum of the full-load currents of the other motors.

430.24 sets the feeder conductor ampacity and 430.62 the maximum feeder overcurrent device rating; if the result is not a standard size, the next lower standard size is used.

Disconnecting means

Every motor and controller needs a disconnecting means that opens all ungrounded conductors, is rated in horsepower (or is a suitable circuit breaker or molded-case switch), and is located where the person working on the motor can see it, so they know it remains open. Where the disconnect is not within sight, it must be lockable in the open position, and specific conditions apply. Lockout and tagout procedures (CSA Z460 in Canada, OSHA 1910.147 in the US) govern how disconnects are used to make equipment safe for work.

Part IX of Article 430 requires a disconnecting means in sight from the controller (430.102(A)) and in sight from the motor and driven machinery (430.102(B)), where "in sight" means visible and not more than 50 ft away (Article 100). The motor disconnect may be omitted if the controller disconnect is lockable and other conditions are met.

Control circuits

Motor starters are operated by control circuits: start and stop pushbuttons, limit and pressure switches, interlocks and the coil of the contactor, often supplied at 120 V (or 24 V) from a control transformer in the starter. Control circuits need their own overcurrent protection, sized for the small conductors used (often 14 or 16 AWG), and should be designed so that a ground fault cannot start the motor or prevent it from stopping: the coil is connected to the grounded side, and the switches to the ungrounded side. A typical three-wire control with a holding (seal-in) contact also provides undervoltage protection: after a power loss, the motor does not restart until the start button is pressed again, which protects workers from unexpected restarts.

Variable-frequency drives

Variable-frequency drives (VFDs) supply motors through a rectifier and inverter (ELE-370). The conductors to the drive are sized for the drive's rated input current, which can exceed the motor's current because of harmonics and drive losses, and the drive often provides electronic motor overload protection. Output conductors carry high-frequency switching voltages, so cable type, length limits, shielding and grounding follow the manufacturer's instructions, and motors must be suitable for inverter duty.

Part X of Article 430 covers adjustable-speed drive systems: conductors supplying a power conversion unit are sized at not less than 125% of its rated input current (430.122(A)), and the drive may provide the motor overload protection if it is identified for that purpose.

Verifying a motor installation

Before a new motor is put into service, the electrician checks that the installation matches the design and the code, and that the motor will run as intended. A typical commissioning sequence is:

• Compare the motor nameplate (voltage, full-load current, service factor, code letter, temperature rise) with the drawings and with the settings of the overload relay; the relay is set from the nameplate current, not the table value.
• Confirm the short-circuit protective device type and rating, and that the starter, disconnect and drive are rated for the available fault current marked on the equipment.
• Test insulation resistance of the motor windings and feeder conductors with an insulation tester at a voltage suited to the equipment, and record the readings so later tests can show deterioration.
• Check torque on terminations, the equipment grounding conductor and bonding of the motor frame.
• Verify the control circuit: the stop button, overload contact and any safety interlocks must each drop out the contactor.
• Bump the motor briefly to confirm rotation before coupling it to the load; reverse any two supply conductors if it turns the wrong way.
• Measure running current on each phase under load and compare with the nameplate; a current imbalance of more than a few percent points to voltage unbalance or a winding problem.

Lesson 4 widens the view to overcurrent protection and fault current throughout the system, and Lesson 8 covers industrial control panels and machine safety.