Control Devices

Control Devices

Kentucky Master Electrician Exam Preparation — 2023 NEC

Learning Objectives

Upon completing this chapter, you will be able to:

1.1 Scope and Definitions — Where Control Devices Live in the NEC

Control devices are not governed by a single article. The master electrician must navigate across several articles, primarily Article 430 (Motors, Motor Circuits, and Controllers) , with critical cross-references to Article 409 (Industrial Control Panels) , Article 725 (Class 1, Class 2, and Class 3 Remote-Control, Signaling, and Power-Limited Circuits) , and Article 240 (Overcurrent Protection) .

For the exam, understand these definitions:

Exam Trap: Many candidates confuse the controller disconnecting means with the motor disconnecting means. They are separate requirements (430.102 and 430.103). The controller must have a disconnecting means within sight, and the motor must have its own disconnecting means within sight of the motor.

1.2 Control Circuit Conductors — Ampacity and Protection (430.72)

This is a high-yield area for the Kentucky Master exam. Control circuits are often tapped from the motor branch circuit. The rules depend on the voltage and the overcurrent protection arrangement.

1.2.1 Control Circuit Tapped from the Motor Branch Circuit

When a control circuit is tapped from the motor branch circuit (e.g., a 120-volt coil powered from a 480-volt 3-phase system via a control transformer), the overcurrent protection for the control circuit conductors must comply with 430.72(B) .

The general rule: Control circuit conductors must be protected against overcurrent. The protection is permitted to be the branch circuit overcurrent device if the control circuit conductors have an ampacity that is not less than the branch circuit overcurrent device rating.

The Critical Exceptions (Memorize These):

Master-Level Insight: For a control transformer, the primary side is the tap. If the transformer is rated less than 100 VA, you can use the 15-ampere rule on the primary. The secondary side is treated as a new circuit and must be protected per its own ampacity, typically with a fuse or supplementary protector.

1.2.2 Control Circuit Conductors — Minimum Size

Per 430.72(A) , control circuit conductors must be at least 14 AWG (or 18 AWG if stranded and installed in a raceway, per the exceptions). For the exam, default to 14 AWG for any control circuit you are sizing.

1.2.3 Control Circuits Not Tapped from the Branch Circuit

If the control circuit is supplied from a separate source (e.g., a separate 120-volt circuit from a panelboard), it falls under Article 725 (Class 1 circuits) for conductor sizing and protection. Class 1 control circuits must be protected at their ampacity, and the conductors must be sized per Table 310.16 (or 310.17 for free air).

1.3 Controller Ratings — Horsepower and Voltage (430.83)

The controller must have a horsepower rating that is not less than the horsepower rating of the motor it controls. This is the fundamental rule of 430.83(A) .

Exceptions and Nuances for the Master Exam:

Exam Trap: Do not confuse the controller rating with the disconnecting means rating. A disconnecting means must be rated in horsepower (or amperes for certain exceptions), but it does not need to interrupt the motor starting current — it only needs to interrupt the locked-rotor current. The controller must be able to make and break the motor's locked-rotor current.

1.4 Disconnecting Means for Controllers and Motors (430.102, 430.103)

The master electrician must ensure that both the controller and the motor have disconnecting means that are:

1.4.1 Controller Disconnecting Means (430.102)

The controller disconnecting means must be located within sight of the controller. This is a strict requirement. If the controller is in a panelboard and the motor is remote, the disconnecting means for the controller must still be within sight of the controller.

Exception: If the controller is in an industrial installation with a written safety procedure (lockout/tagout), the disconnecting means is permitted to be out of sight, but it must be capable of being locked in the open position.

1.4.2 Motor Disconnecting Means (430.102(B))

The motor disconnecting means must be within sight of the motor and the driven machinery. This is a common inspection point. If the motor is out of sight from the disconnecting means, the disconnecting means must be capable of being locked in the open position, and a written procedure must be in place.

Master-Level Insight: For a motor that is part of a commercial HVAC unit on a roof, the disconnect at the unit satisfies the motor disconnect requirement. The controller (contactor) inside the unit is also within sight of the disconnect, satisfying 430.102(A).

1.5 Control Devices in 3-Phase Systems — Voltage and Phase Considerations

For the master exam, you must understand how control devices interact with 3-phase systems.

1.5.1 Control Transformers

A control transformer is used to step down 480 V 3-phase to 120 V for the control circuit. The primary is connected to two phases (line-to-line). The secondary is typically a single-phase 120 V circuit.

Key Code Points:

Exam Trap: A control transformer with a 120 V secondary that is not grounded is a code violation. The 120 V control circuit must have a grounded neutral conductor to provide a low-impedance path for fault current.

1.5.2 Control Relays and Contactors in 3-Phase Motors

For a 3-phase motor, the controller (contactor) must open all ungrounded conductors. A 3-pole contactor is required. The control circuit (coil) is typically 120 V, energized from one phase and the grounded neutral.

Inspection Point: Verify that the control circuit is not connected to a grounded phase conductor. In a corner-grounded delta system (rare but exists in older commercial buildings), the control circuit must be connected to the ungrounded phases only.

1.6 Overcurrent Protection Coordination for Control Devices

The master electrician must understand that control devices are part of a coordinated protection scheme.

1.6.1 Short-Circuit Current Rating (SCCR)

Per Article 409 (Industrial Control Panels), the entire control panel must have a Short-Circuit Current Rating (SCCR) . This is the maximum fault current the panel can withstand without catastrophic failure. The SCCR is determined by the lowest rated component in the panel.

Master-Level Insight: If you install a control transformer with a low SCCR (e.g., 5 kA) in a panel that is on a service with a 65 kA available fault current, the panel SCCR is only 5 kA unless you add current-limiting fuses. This is a common design error and a frequent exam topic.

1.6.2 Coordination with Motor Branch Circuit Protection

The motor branch circuit overcurrent device (fuses or breaker) is sized per 430.52 . It protects the branch circuit conductors and the motor against short circuits and ground faults, not overloads. The overload relays (heaters) in the controller protect the motor against overload.

Coordination Requirement: The overload relays must be sized per 430.32(A)(1) — not more than 125% of the motor full-load current for motors with a service factor of 1.15 or more. The branch circuit device must be sized to allow the motor to start without nuisance tripping, but must not exceed the maximum permitted by Table 430.52 (typically 250% for inverse-time breakers, 300% for fuses).

1.7 Control Devices in Separately Derived Systems and Generators

When a control circuit is supplied from a generator or a separately derived system (e.g., an on-site transformer), the master must ensure the control circuit is properly grounded and bonded.

1.7.1 Generator Supplied Control Circuits

If a generator provides backup power to a motor, the control circuit must be able to operate from both the normal and the emergency source. This often requires a transfer switch that also transfers the control circuit.

Code Point: Per 700.10(B)(1) , the emergency system wiring must be separate from all other wiring. Control circuits for emergency motors must be routed separately from normal control circuits.

1.7.2 Grounding of Control Transformers on Separately Derived Systems

Per 250.30(A) , the secondary of a control transformer (if it is a separately derived system) must have:

Inspection Point: For a control transformer inside a motor control center, the secondary neutral is typically bonded to the enclosure, and the enclosure is grounded via the equipment grounding conductor. This is acceptable if the transformer is not required to have a separate grounding electrode (see 250.30(A)(4) for exceptions).

1.8 Commercial and Industrial Installations — Special Considerations

1.8.1 Multiple Motors on One Branch Circuit (430.87)

In commercial installations, you may have multiple motors on a single branch circuit (e.g., a conveyor system with several small motors). Each motor must have its own controller, but they can share a branch circuit if:

1.8.2 Control Devices in Hazardous Locations

If the control device is in a Class I, Division 1 location (e.g., a paint booth), the controller must be rated for the location. This is per Article 500 . The master must ensure that the control transformer and relays are in an explosion-proof enclosure or are intrinsically safe.

1.9 Code Navigation — Quick Reference

1.10 Inspection and Supervision Points

As a master electrician, you are responsible for the final sign-off. On site, verify the following:

1.11 Common Exam Traps

Chapter Summary

Control devices are the interface between the operator and the motor. For the Kentucky Master exam, you must know the specific requirements of Article 430 for controllers, disconnecting means, and control circuits. The key to success is understanding the hierarchy: the branch circuit protects the conductors, the overload relays protect the motor, and the controller makes and breaks the circuit. The disconnecting means provides a safe way to isolate the equipment for maintenance. By mastering the exceptions in 430.72 and 430.83, and by understanding the grounding requirements for control transformers, you will be well-prepared for both the exam and the responsibilities of a master electrician.