Study Chapter: Control Devices

NH Master Electrician Exam Preparation (2023 NEC)

Learning Objectives

Upon completing this chapter, you will be able to:

6.Distinguish between "control" and "protection" as defined by the NEC, and apply the correct article to each function.

7.Identify the specific code sections governing the disconnection, control, and overload protection of motors and motor circuits.

8.Apply the requirements for control circuits, including Class 1 and Class 2 power limits, and the location and accessibility of disconnecting means.

9.Evaluate the requirements for emergency stop functions, safety interlocks, and the control of permanently connected appliances and space-heating equipment.

10.Navigate the NEC efficiently to locate control-device requirements during the open-book exam, avoiding common traps related to motor control and disconnection.

1.1 The Hierarchy of Control: Definitions and Scope

For a Master Electrician, the distinction between a controller, a disconnecting means, and a protective device is not academic—it dictates the wiring method, conductor sizing, and device ratings. The NEC defines a Controller as a device or group of devices that serves to govern, in some predetermined manner, the electric power delivered to the apparatus to which it is connected. This is distinct from a Disconnecting Means, which is a device that isolates the circuit from its source of supply. A Protective Device (fuse or circuit breaker) operates automatically to interrupt current.

The primary articles governing these are:

Article 430 (Motors, Motor Circuits, and Controllers): The master reference for motor control.

Article 440 (Air-Conditioning and Refrigerating Equipment): Contains specific allowances that override Article 430 for hermetic refrigerant motor-compressors.

Article 422 (Appliances) and Article 424 (Fixed Electric Space Heating): Govern control and disconnection for these specific loads.

Key Master Concept: A controller is not a disconnecting means unless it is specifically rated for that purpose and meets the lockout/tagout (LOTO) requirements of 430.102 and 110.9 (interrupting rating). A magnetic contactor alone is rarely an acceptable disconnecting means because it is not capable of being locked in the open position.

1.2 Motor Controllers: Location, Rating, and Horsepower

Section 430.83 mandates that a motor controller have a horsepower rating that is not less than the horsepower rating of the motor, at the applied voltage. This is a common field trap: using a 1 HP contactor on a 2 HP motor because the full-load current (FLC) is within the contactor's ampere rating. The NEC requires the horsepower rating to match or exceed the motor's rating, not just the current.

Exception for "Inch" and "Jog" Operations: A controller used only for inching/jogging (momentary operation) is not required to have a horsepower rating equal to the motor, but it must be rated for continuous duty for the maximum current of the motor (430.83(B)).

Location (430.102):

The controller must be located within sight of the motor and the driven machinery, unless the controller is a disconnecting means that is lockable in the open position and the motor has a separate disconnecting means.

Critical Master Point: "Within sight" is defined in Article 100 as visible and not more than 15 m (50 ft) apart. If the controller is not within sight, you must install a disconnecting means within sight of the motor that is capable of being locked in the open position.

Controller Enclosures (430.91): The controller enclosure must be of a type suitable for the location. For example, a NEMA 1 enclosure is not suitable for a wet or dusty location; you must specify a NEMA 3R or NEMA 12, respectively. The master must verify the application of the enclosure, not just the horsepower rating.

1.3 Disconnecting Means for Motors: The "Lockable" Requirement

The disconnecting means for a motor and its controller must:

32.Be a listed, marked switch or circuit breaker (430.109).

33.Be capable of being locked in the open position (110.25). The lock need not be an integral part of the device, but the device must have a hasp or other provision for a lock.

34.Open all ungrounded conductors simultaneously.

35.Have an ampere rating of at least 115% of the motor FLC (430.110(A)).

The "Controller Disconnect" Rule (430.102(A)): A disconnecting means must be provided for the controller. This is often the same switch that feeds the control panel. It must be located within sight of the controller.

The "Motor Disconnect" Rule (430.102(B)): A disconnecting means must be located within sight of the motor and the driven machinery. This is the switch the maintenance mechanic uses to safely change a belt or impeller.

Master Trap: For a motor over 100 HP or 600 V, the disconnecting means is permitted to be a remote-controlled circuit breaker if it is capable of being locked in the open position and the motor is provided with a separate disconnecting means within sight. This is a complex coordination point for industrial installations.

1.4 Overload Protection vs. Short-Circuit Protection

This is the most misunderstood area for journeymen transitioning to master. The NEC requires two distinct levels of protection for a motor circuit:

42.Overload Protection (Article 430, Part III): Protects the motor, the motor control apparatus, and the branch-circuit conductors against excessive heating due to motor overloads and failure to start. This is typically provided by overload relays (heaters) in the motor starter.

Sizing: The overload device must be sized at not more than 115% of the motor's nameplate full-load current rating for motors with a service factor of 1.15 or greater, or a temperature rise of 40°C or less. For all other motors, it must not exceed 125% of the nameplate rating (430.32(A)(1)).

Master Exception: If the motor is not able to start due to the load (e.g., a high-inertia fan), the overload device is permitted to be sized up to 140% of the nameplate rating (430.32(C)). This requires a field judgment call and documentation.

45.Short-Circuit and Ground-Fault Protection (Article 430, Part IV): Protects the branch-circuit conductors, the controller, and the motor against high-level faults.

Sizing: The rating of the branch-circuit protective device (fuse or breaker) must not exceed the values in Table 430.52. For example, a standard squirrel-cage motor (Design B) with a non-time-delay fuse is limited to 300% of the motor FLC; with an inverse-time breaker, it is limited to 250%.

Master Trap: The FLC for sizing the short-circuit device comes from Tables 430.247 through 430.250, not the motor nameplate. The nameplate is used for overload sizing and conductor sizing (if the nameplate current is higher). The tables are used for branch-circuit protection and conductor sizing (if the table current is higher).

1.5 Control Circuits: Class 1 and Class 2

Control circuits (e.g., start/stop pushbuttons, relay coils, PLC inputs) are classified based on their power source.

Class 1 Control Circuits (Article 725, Part II): These are control circuits where the power is not limited by a Class 2 or Class 3 power source. They are typically supplied from a separate transformer winding or a tapped winding of the motor controller.

Conductor Protection: Class 1 conductors must be protected against overcurrent. If the control circuit is tapped from the motor branch circuit, the overcurrent device for the branch circuit protects the control circuit, but only if the control conductor ampacity is sufficient. If not, a supplementary overcurrent device is required (430.72(A)).

Master Point: Table 430.72(B) specifies the maximum rating of the overcurrent device protecting the control circuit conductors, based on the control conductor ampacity. For example, if you use an 18 AWG wire for a control circuit tapped from a 100 A motor branch circuit, you must install a fuse not exceeding 7 A in the control circuit tap.

Class 2 Control Circuits (Article 725, Part III): These are limited-power circuits (typically 24 VDC or 24 VAC from a listed Class 2 power supply or transformer). They are safer and allow for smaller conductors without the same overcurrent protection requirements.

Master Application: When using a PLC or a 24 VDC relay system, verify the power supply is listed as a Class 2 power unit. If you build a control panel with a 120V-to-24V transformer that is not listed as a Class 2 power source, the entire circuit is treated as Class 1, and you must apply the wiring methods of Chapter 3 (e.g., conduit or MC cable) rather than the flexible wiring methods permitted for Class 2.

1.6 Emergency Stop and Safety Interlocks

Section 430.105 requires that a motor be provided with a means to stop it. The controller must be capable of stopping the motor.

Emergency Stop (E-Stop): The NEC does not explicitly require E-Stops for general motor installations; that is a function of NFPA 79 (Electrical Standard for Industrial Machinery). However, when an E-Stop is installed, it must be a "red mushroom head on a yellow background" per NFPA 79, and it must be a hard-wired device that removes power from the motor controller, not a software command to a PLC.

Master Supervision Point: On a site inspection, verify that the E-Stop circuit is not routed through a PLC input module. The E-Stop must directly de-energize the master control relay or the motor starter coil. If the E-Stop is a PLC input, a PLC processor failure will render the E-Stop inoperable, which is a violation of NFPA 79 and a serious safety hazard.

1.7 Control of Specific Loads: Appliances and Heat

Appliances (Article 422):

422.31: A readily accessible disconnecting means is required for each appliance. For a permanently connected appliance rated over 300 VA, the disconnect must be within sight of the appliance or be capable of being locked in the open position.

Master Trap: A cord-and-plug connected appliance is considered to have a disconnecting means if the receptacle is accessible. For a hard-wired commercial dishwasher, you must provide a separate disconnect switch, not just a circuit breaker in a remote panel.

Fixed Electric Space Heating (Article 424):

424.19: A disconnecting means is required for each fixed heater. It must be within sight of the heater, or be lockable, and must open all ungrounded conductors.

Master Trap: For a central electric furnace, the branch-circuit breaker in the panelboard can serve as the disconnecting means if it is capable of being locked in the open position and is within sight of the furnace. If the panel is not within sight, a separate disconnect is required at the furnace.

1.8 Code Navigation: Where to Find It

Concept	NEC 2023 Location
Definitions (Controller, Disconnecting Means)	Article 100
General Requirements for Disconnecting Means	Article 110, Part II (110.25 for lockable)
Motor Controllers (Rating, Location)	430.83, 430.91, 430.102
Motor Disconnecting Means	430.102, 430.109, 430.110
Motor Overload Protection	430.32, 430.36
Motor Short-Circuit Protection	430.52, Table 430.52
Motor FLC Tables (for SC protection)	Tables 430.247 through 430.250
Control Circuits (Class 1, Class 2)	Article 725, Parts II and III
Control Circuit Overcurrent Protection	430.72, Table 430.72(B)
Air-Conditioning Equipment	Article 440 (specifically 440.14 for disconnects)
Appliances (Disconnect)	422.31
Fixed Space Heat (Disconnect)	424.19
Lockable Disconnecting Means	110.25

1.9 Inspection and Supervision Points

As a Master supervising an installation, you must verify the following on site:

76.Horsepower Rating: Check the nameplate on the motor starter/contactor. Does it exceed the motor's HP rating? A 5 HP contactor on a 7.5 HP motor is a violation, regardless of the load current.

77.Lockable Disconnect: Attempt to place a padlock on the motor disconnect switch. If the switch does not have a hasp, it is non-compliant.

78.Overload Heater Sizing: Compare the heater element size (often stamped on the relay) to the motor nameplate FLC. Calculate the 115%/125% rule to ensure the heaters are correct.

79.Control Transformer Fusing: Open the control panel. Is the secondary of the control transformer fused? If the control wire is 18 AWG, the fuse must be sized per Table 430.72(B), not just a "whatever is in the drawer" fuse.

80.E-Stop Wiring: Trace the E-Stop wires. Do they go directly to the starter coil or a master relay? If they go to a PLC input, flag it as a violation.

1.10 Common Exam Traps

Trap 1: Using Nameplate Current for Fuse Sizing. The exam will give you a motor nameplate FLC of 10 A and a Table 430.250 FLC of 12 A. The branch-circuit fuse is sized from the table (12 A × 300% = 36 A, next standard size down is 35 A). The overload relay is sized from the nameplate (10 A × 125% = 12.5 A).

Trap 2: Confusing the Controller Disconnect with the Motor Disconnect. The controller disconnect (430.102(A)) is for the electrician to work on the starter. The motor disconnect (430.102(B)) is for the mechanic to work on the machine. Both are required if the controller is not within sight of the motor.

Trap 3: Ignoring the "Within Sight" Distance. "Within sight" is strictly defined as 50 ft. If the disconnect is 60 ft away, it is not within sight, and it must be lockable.

Trap 4: Applying Motor Rules to HVAC Equipment. For a hermetic compressor, you must use the equipment nameplate current and the specific rules of Article 440, which allow the branch-circuit selection current to be used for conductor sizing, not the motor table FLC.

Trap 5: Assuming a Contactor is a Disconnect. A magnetic contactor without a manual handle and lockout provision is a controller, not a disconnecting means. The exam will test your understanding that a control circuit failure (loss of coil voltage) does not satisfy the requirement for a positive, lockable disconnect.

Summary

Master-level control device knowledge requires moving beyond the simple "switch and motor" model. You must understand the interplay between horsepower ratings, lockable disconnects, overload protection (nameplate-based), and short-circuit protection (table-based). The distinction between Class 1 and Class 2 control circuits dictates the wiring methods and overcurrent protection for the entire control panel. By mastering the specific sections of Article 430 and its companion articles (440, 422, 424, and 725), you will be prepared to supervise installations that are both code-compliant and safe for the personnel who operate and maintain the equipment.

Control Devices