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 Section 28 of the Canadian Electrical Code, with full-load currents in Tables 44 (three-phase) and 45 (single-phase) and maximum overcurrent device settings in Table 29.

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 Table 44 or 45 (Rule 28-010), 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.

Rule 28-106 requires conductors supplying a single motor to have an ampacity of not less than 125% of the full-load current rating of the motor, with different percentages for motors on short-time or intermittent 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.

Rules 28-300 to 28-314 require overload protection for most motors. Rule 28-306 sets the maximum rating or setting at 125% of nameplate full-load current for motors with a service factor of 1.15 or more or a marked temperature rise not exceeding 40 °C, and 115% for all others, with higher settings (up to 140% and 130%) permitted if the motor will not otherwise 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:

Rule 28-200 requires each motor branch circuit to be protected by overcurrent devices with ratings not exceeding the values of Table 29; where the value does not correspond to a standard rating, the next higher standard rating may be used. 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.

Rule 28-108 sets the feeder conductor ampacity and Rule 28-204 the maximum feeder overcurrent device rating; if the result is not a standard rating, the next lower standard rating 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.

Rules 28-600 to 28-604 require a disconnecting means for each motor and controller, within sight of and within 9 m of the motor and the driven machinery, or capable of being locked in the open position where these conditions cannot be 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.

Section 28 also covers adjustable-speed drive systems; conductors supplying a drive are sized from the drive's rated input current rather than the motor's, at 125% for continuous duty.

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.