15.4 Motor Controllers, Disconnecting Means, Drives & Motor Control Centers (Article 430 Parts VII–XI)
Key Takeaways
A motor disconnecting means must be within sight of the motor and its driven machinery, unless the controller disconnect is lockable in the open position and a disconnect at the motor is impracticable or the site is an industrial establishment with written safety procedures (430.102(B)).
Motor disconnecting means must have an ampere rating of at least 115% of the motor's full-load current for motors rated 1000 V or less (430.110(A)).
For a stationary motor of 2 hp or less at 300 V or less, a general-use switch rated at least twice the motor full-load current may serve as the disconnecting means (430.109(C)(2)).
Conductors supplying adjustable-speed drive equipment must have an ampacity of at least 125% of the drive's rated input current (430.122(A)).
The 2026 NEC 430.98(A) requires a motor control center supplied by a feeder to be marked with the location of the means to disconnect all power to it.
Motor controllers (Article 430, Part VII)
A controller is any switch or device used to start and stop a motor by making and breaking the motor circuit current. It may be a magnetic starter, manual motor starter, drive, or, for small motors, a switch or breaker.
| Rule | Requirement |
|---|---|
| Rating (430.83(A)) | Horsepower rating at the application voltage not lower than the motor's; a circuit breaker rated in amperes may also serve as a controller |
| Small motors (430.83(C)) | For stationary motors of 2 hp or less at 300 V or less: a general-use switch rated at least twice the motor full-load current, or an AC general-use snap switch where the motor full-load current does not exceed 80% of the switch rating |
| Voltage (430.83(E)) | Slash-rated controllers (for example 480Y/277 V) only on solidly grounded systems within both values |
| Conductors opened (430.84) | A controller need not open all conductors to the motor, unless it also serves as the disconnecting means |
| One controller per motor (430.87) | Each motor needs its own controller, with limited exceptions for groups of motors at 1000 V or less driving parts of a single machine, or in a single room within sight of the controller |
Disconnecting means (Article 430, Part IX)
Location (430.102)
- Controller disconnect (A): an individual disconnecting means must be provided ahead of each controller and within sight from the controller location.
- Motor disconnect (B): a disconnecting means must also be located within sight from the motor location and the driven machinery. The controller disconnect may serve as the motor disconnect if it is within sight of the motor.
- Exception: the motor disconnect is not required if the controller disconnect is lockable in the open position (110.25) and either a disconnect at the motor is impracticable or would introduce additional or increased hazards, or the installation is in an industrial establishment with written safety procedures where only qualified persons service the equipment.
"Within sight" means visible and not more than 50 ft away (110.29).
Operation and accessibility (430.103 to 430.107)
- The disconnecting means must open all ungrounded supply conductors simultaneously and be designed so it cannot close automatically.
- It must plainly indicate whether it is open (off) or closed (on).
- At least one of the disconnecting means must be readily accessible.
Type (430.109)
Permitted types include a listed motor-circuit switch rated in horsepower, a listed molded-case circuit breaker, a listed molded-case switch, an instantaneous-trip circuit breaker that is part of a listed combination motor controller, a listed self-protected combination motor controller, a listed manual motor controller marked "Suitable as Motor Disconnect" (on the load side of the final branch-circuit short-circuit protective device), and listed system isolation equipment. Special cases:
| Motor | Permitted disconnecting means |
|---|---|
| Stationary, ⅛ hp or less | The branch-circuit OCPD |
| Stationary, 2 hp or less, 300 V or less | A general-use switch rated at least twice the full-load current, or an AC general-use snap switch where the full-load current is not more than 80% of its rating |
| Cord-and-plug connected | A horsepower-rated attachment plug and receptacle, or for portable motors of ⅓ hp or less, an attachment plug and receptacle |
Rating (430.110)
For motors of 1000 V or less, the disconnecting means must have an ampere rating of at least 115% of the motor's Table full-load current. Where a disconnect serves several motors or other loads, its rating is found by adding the full-load currents and the locked-rotor currents of all loads and converting to an equivalent horsepower using Table 430.251(A) or (B).
Example: a 25 hp, 460 V, three-phase motor has a Table 430.250 full-load current of 34 A. The disconnect must be rated at least A and at least 25 hp at 460 V, so a 60 A, 25 hp or larger switch is typical.
Control circuits (Article 430, Part VI)
- Overcurrent protection (430.72): motor control circuit conductors tapped from the branch circuit need protection sized from Table 430.72(B); control transformers need protection under 430.72(C).
- Arrangement (430.74): an accidental ground in the control circuit must not start the motor or bypass shutdown devices.
- Disconnection (430.75): control circuits must be disconnected from all sources by the disconnecting means; where a control transformer is in the controller enclosure, it must be on the load side of the disconnect.
Adjustable-speed drives (Article 430, Part X)
| Rule | Requirement |
|---|---|
| Conductors (430.122(A)) | Ampacity at least 125% of the rated input current of the power conversion equipment |
| Bypass device (430.122(B)) | Where a bypass is provided, conductors are sized for the larger of 125% of the drive input current or 125% of the motor full-load current |
| Overload protection (430.124) | Provided by the drive if it is marked as providing it; otherwise separate overload protection is required |
| Disconnecting means (430.128) | Permitted in the incoming line to the conversion equipment, rated at least 115% of its rated input current |
Motor control centers (Article 430, Part VIII)
Motor control centers must be protected under Article 240, meet working space rules, and be marked with their ratings. The 2026 NEC 430.98(A) requires an MCC supplied by a feeder to be marked with the location of the means to disconnect all power to the MCC, so a worker can find the upstream disconnect quickly.
Worked example: a rooftop exhaust fan
A 3 hp, 208 V, three-phase exhaust fan on a roof is controlled by a starter in an electrical room two floors below.
- Table FLC: Table 430.250 gives 10.6 A for 3 hp at 208 V.
- Controller disconnect: within sight of the starter in the electrical room.
- Motor disconnect: the electrical room is not within sight of the fan, so a disconnect is needed at the fan, unless the controller disconnect is lockable and the conditions of the exception apply. A disconnect at a rooftop fan is practical, so install one.
- Rating: at least A, and horsepower rated for 3 hp at 208 V, such as a 30 A, 3-pole horsepower-rated switch.
A 15 hp, 460 V motor has a Table 430.250 full-load current of 21 A. What is the minimum ampere rating of its disconnecting means?
21.0 A
26.3 A
36.8 A
24.2 A
A motor's controller is in a locked electrical room 120 ft from the motor, out of sight. The controller disconnect can be locked in the open position. When may the separate disconnect at the motor be omitted?
Whenever the controller disconnect is lockable in the open position
When the motor is smaller than 10 hp and is fed from a lockable breaker
Where a motor disconnect is impracticable or hazardous, or at certain industrial sites
Never; every motor needs its own disconnect within sight
An adjustable-speed drive has a rated input current of 40 A. What minimum ampacity is required for the conductors supplying it, with no bypass?
40 A
50 A
46 A
70 A
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