11.2 Motor Disconnects, Controllers & Starters

Why Disconnects and Controllers Matter

The Knowledge part of the TX Journeyman exam (59 questions, 130 minutes) tests disconnecting means types, ratings, location rules, and controller/starter terminology under NEC Article 430 Parts IV, VI, and VII. These are definition and rule questions - know the article numbers and the exact percentages.

NEC 430.109 - Disconnecting Means Types

NEC 430.109 lists the permitted types of disconnecting means for a motor circuit:

• 430.109(A) - Listed motor-circuit switch: a switch rated in hp and amperes, designed specifically for motor circuits.
• 430.109(B) - Molded-case circuit breaker: a listed breaker used as the disconnect.
• 430.109(C) - Inverse-time circuit breaker: permitted where suitable for motor circuit duty.
• 430.109(D) - Fusible disconnect switch: a switch plus fuses, rated for motor duty.
• 430.109(E) - Other listed devices: as permitted by the listing.

A general-use snap switch is not permitted as a motor disconnect except for small motors under 430.109(F).

NEC 430.110 - Disconnect Rating

The disconnecting means for a motor circuit must have an ampere rating not less than 115% of the motor FLC (430.110(A)). For a hp-rated disconnect (motor-circuit switch), the hp rating must not be less than the motor hp.

NEC 430.102 - Location of the Disconnect

• 430.102(A) - Controller: a disconnecting means must be located in sight of the controller location.
• 430.102(B) - Motor: a disconnecting means must be located in sight of the motor location. If the disconnect is not in sight, it must be lockable in the open (off) position per 430.102(B) Exception, and the lock must remain in place.

"In sight" is defined in Article 100 as visible and not more than 50 ft from the equipment.

Motor Controllers (NEC 430.87)

A motor controller is the device that starts and stops the motor. NEC 430.87 requires a motor controller to have a horsepower rating not less than the motor horsepower. A controller can be a contactor, a starter, or another listed device rated in hp.

Overload Relays

Overload relays protect the motor from sustained overcurrent above its FLC. Three common types appear on the exam:

Overload relays are distinct from the short-circuit/ground-fault protective device (fuse or breaker, 430.52) - the overload protects the motor, the short-circuit device protects the conductors.

Starters

• Magnetic starter: a contactor (electromagnetically operated switch) paired with an overload relay, controlled by a separate start/stop control circuit. The most common industrial starter.
• Manual starter: an operator handle mechanically closes and opens the contacts; the overload relay still trips the starter on overload.
• Combination starter: a single enclosure containing the short-circuit protective device (fuse or circuit breaker per 430.52), the magnetic contactor/controller, and the overload relay. This is the typical factory-assembled unit and satisfies 430.109, 430.87, and 430.32 in one listed assembly.

AC Motor Starting Methods

The exam may name these methods - know what each does:

• Across-the-line (full-voltage): the motor is connected directly to line voltage. Simplest method, highest starting inrush current.
• Part-winding: the motor has two separate windings; the starter energizes one winding, then both. Reduces inrush. Requires a part-winding-rated motor.
• Wye-delta (Y-Delta): a specially wound motor starts with windings in wye (each winding sees ~58% of line voltage), then transitions to delta for full voltage. Reduces starting current and torque.
• Reduced-voltage (autotransformer, primary reactor): an autotransformer or reactor lowers the starting voltage, then the motor sees full voltage. Used for large motors that cannot tolerate full-voltage starting.

NEC 430.125 - Motor Control Circuits

Motor control circuits are wired per NEC 430.125. Where a control circuit transformer is used, its overcurrent protection follows 430.72. A fault in the control circuit must not create a hazard, and per 430.75 the controller must drop out on loss of control voltage so the motor does not restart unexpectedly when control power is restored.

Worked Example - Disconnect and Controller Sizing

Size the disconnecting means for a 10 hp, 460V, 3-phase continuous-duty motor.

Step 1 - FLC from Table 430.250: 14 A.

Step 2 - Disconnect ampere rating (430.110(A)): 115% of 14 A = 16.1 A. The disconnect is then selected from standard listed ratings, so a 30 A (or larger) motor-circuit switch or molded-case circuit breaker is acceptable - you do not buy a 16.1 A disconnect.

Step 3 - hp rating: the disconnect (if hp-rated) and the controller (430.87) must each carry a hp rating not less than 10 hp.

This is a common two-part question: the 115% gives the minimum ampere rating, and the hp rating must also match.

Overload Protection - 430.32

The overload device protects the motor from sustained overcurrent and is sized from the nameplate FLC, not the Table value. NEC 430.32 sets the overload pickup:

• Motors with a service factor of 1.15 or greater: overload trip at no more than 125% of the nameplate FLC per 430.32(A)(1).
• Motors with a temperature rise marked 40C or less: overload trip at no more than 125% of the nameplate FLC per 430.32(A)(1).
• All other motors: overload trip at no more than 115% of the nameplate FLC per 430.32(B).

The service factor and temperature rise determine which multiplier applies. If neither the 1.15 service factor nor the 40C temperature rise applies, the 115% figure governs. The overload is the one place the nameplate current matters - conductor, disconnect, and short-circuit sizing all use the Table FLC.

Controller Interrupting Rating

A motor controller must be capable of interrupting the locked-rotor (stalled-rotor) current of the motor it controls. The controller hp rating under 430.87 already accounts for this when the motor is within the rated hp. Where a controller is applied above its hp rating through an engineering review, the locked-rotor current must be verified per 430.126. This is rarely a journeyman-level calculation, but the hp-rating requirement is a frequent Knowledge-part question.

Adjustable-Speed Drive Systems

Adjustable-speed (variable-frequency) drives are common, and NEC Article 430 Part X (430.120 through 430.128) applies. Key points: the drive itself is considered the motor controller; the conductors between the drive and the motor must be sized per 430.122; and a separate motor overload is still required unless the drive provides electronic thermal protection listed for the purpose. The drive output conductors carry high-frequency PWM switching, so follow the manufacturer listing on conductor length, type, and shielding - a traps item rather than a deep calculation on the journeyman exam.