9.3 Motor Controllers, Circuits & Disconnects (Article 430 Parts II, VII, IX)

Motor Controllers — 430.82 (Part II)

A motor controller is the device that starts and stops the motor and interrupts the stalled (locked-rotor) current. Per 430.82, a controller must be rated in horsepower and voltage for the motor it serves — the horsepower rating, not just the ampere rating, is what makes a switching device suitable as a motor controller. A general-use switch rated only in amperes is not automatically a motor controller.

A controller can be a magnetic contactor, a manual motor starter, a circuit breaker rated in hp, an ASD (variable-frequency drive), or a listed combination starter. The controller must be located so the operator can see the motor or driven machinery when starting, unless the controller is lockable and the location is justified under the 430.102 exceptions.

Disconnecting Means — 430.102 and 430.110

Two-Disconnect Rule (430.102)

430.102(A) — A disconnecting means is required for each motor controller, and it must be within sight of the controller. "Within sight," per Article 100, means visible and not more than 50 ft away.

430.102(B) — A separate disconnect must be located in sight from the motor and the driven machinery. The controller disconnect can serve as the motor disconnect if it is itself in sight of the motor [430.102(B)(2)]. If locating the motor disconnect in sight is impracticable or would introduce a new hazard, the controller disconnect can substitute if it is lockable in the open position per 110.25.

Rating (430.110)

430.110(A): the disconnect ampere rating must be not less than 115% of the motor's Table FLC. For our running 10 hp, 460V example (Table FLC 14 A), the disconnect must be rated at least 14 × 1.15 = 16.1 A → use a 30 A or 60 A standard disconnect (the next size up; 15 A is too small). The interrupting capacity must also be sufficient for the available fault current at the disconnect location.

Type (430.109)

430.109 lists the seven permitted disconnect types (motor-circuit switch, circuit breaker, molded-case switch, etc.), with exceptions for certain cord-and-plug connections and limited-duty applications.

Control Circuits — 430.72 Through 430.75

Control-circuit conductors (the wiring between the pilot device and the contactor coil) are generally protected at their ampacity under 430.72, but with several exceptions:

• 430.72(C)(1) — conductors supplied by a Class 2 or Class 3 transformer (energy-limited) need no separate OCPD.
• 430.72(C)(2) — conductors not more than 3 ft long and protected by the branch-circuit OCPD on the line side.
• 430.72(C)(3) — conductors inside a listed motor controller enclosure.

430.75 adds that control-circuit conductors crossing a controller enclosure boundary must be protected unless the run is short and contained.

Part IX — Motors Over 600 V

Part IX applies the same concepts at medium voltage with tighter requirements: controllers must be rated for the voltage class, disconnects must be lockable, and the controller must bear a warning label stating where its disconnect is located when the disconnect is not in sight. The Virginia journeyman exam rarely goes deep into Part IX, but you should recognize that medium-voltage motor work has additional clearance, guarding, and disconnect-visibility rules.

Adjustable-Speed Drive Systems — Part X (430.122–430.128)

ASDs (VFDs) get their own Part X because the drive does work the motor circuit normally distributes across several devices: it acts as controller, as adjustable-voltage source, and — when so marked — as overload protection (430.124).

Conductors (430.122)

• 430.122(A) — Supply (input) conductors: ampacity ≥ 125% of the drive's rated input current (not motor FLC). The drive's input current is on its nameplate and can differ substantially from motor FLC because the drive draws on a power-factor-corrected front end.
• 430.122(B) — Output (motor) conductors: ampacity ≥ 125% of motor Table FLC, unless the drive is marked "Suitable for Output Motor Conductor Protection," in which case use the larger of 125% of motor FLC or the minimum conductor size marked on the drive.

Disconnect (430.128)

The ASD disconnect must be rated at least 115% of the drive's rated input current and may be located at the drive's incoming line.

Branch-Circuit OCPD (430.130)

Branch-circuit short-circuit/ground-fault protection is sized per 430.52(C)(1) using motor Table FLC, but the manufacturer's maximum OCPD rating cannot be exceeded even if the Code would allow a larger device. Always check the drive nameplate for the max OCPD.

Overtemperature (430.126)

Because a fan-cooled motor at reduced speed gets less cooling, 430.126 requires the ASD to provide motor overtemperature protection when the motor is not rated to operate at nameplate current over the full speed range.

Worked Scenario — ASD-Fed 10 hp, 460V Motor

• Motor Table FLC (430.250) = 14 A.
• Drive rated input current (nameplate) = 17 A.
• Supply conductors (430.122(A)): 17 × 1.25 = 21.25 A → #12 AWG copper (75°C, 20 A is just short — step to #10 AWG at 35 A for headroom).
• Output conductors (430.122(B)): 14 × 1.25 = 17.5 A → #12 AWG copper.
• Disconnect (430.128): 17 × 1.15 = 19.55 A → 30 A standard disconnect.
• Branch OCPD (430.130): 14 × 2.50 = 35 A or the manufacturer's marked max, whichever is smaller.

Notice how the drive's input current — not motor FLC — drives the supply conductor and disconnect sizing. That is the key Part X distinction.