Master Electrician Practice
Chapter VII

Control Devices

Master Electrician Practice study guide with diagrams.

Control Devices

Wyoming Master Electrician Exam — NEC 2023 Study Chapter

Learning Objectives

Upon completing this chapter, you will be able to:

6.Identify the scope and application of Article 430 for motor control circuits and Article 440 for air-conditioning and refrigeration equipment.
7.Apply the correct sizing rules for motor controllers, disconnecting means, and control circuit conductors per NEC 2023.
8.Distinguish between a controller, a control circuit, and a protective device, and apply the correct overcurrent protection for each.
9.Calculate the minimum controller rating for various motor types, including multi-speed and wye-delta starting applications.
10.Navigate the code sections governing control devices in services, feeders, and separately derived systems (transformers and generators).
11.Identify common inspection failures related to control device installation and avoid the classic exam traps.

1.1 Scope and Definitions: The Master’s Framework

For the master electrician, "control devices" is not a single article but a coordinated set of rules spanning Article 430 (Motors, Motor Circuits, and Controllers), Article 440 (Air-Conditioning and Refrigeration Equipment), and Article 409 (Industrial Control Panels). The 2023 NEC maintains the critical distinction between a controller (any device that governs the starting, stopping, or reversing of a motor) and a disconnecting means (a device that isolates the motor and controller from the circuit for maintenance).

Key Definitions (paraphrased from Article 100):

Controller: 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.
Control Circuit: The circuit that carries the electric signals directing the performance of the controller, and which does not carry the main power current.
Motor Control Circuit: The circuit of a control apparatus that carries the electric signals directing the performance of the controller, but does not carry the main power current.

Master’s Note: The single most common code violation on commercial sites is the failure to provide a manual disconnect for the control circuit that is separate from the motor’s line-side disconnect, or the failure to ensure that the control circuit disconnect opens all ungrounded conductors of the control circuit simultaneously.

1.2 Motor Controllers: Sizing and Ratings (NEC 430.82 – 430.84)

Motor Controller Ratings: HP >= Motor HP (430.83) Motor Controller Ratings: HP ≥ Motor HP NEC 430.83(A) & (C) — Master Depth MOTOR NAMEPLATE Type: 3-Phase Induction Voltage: 460 V FLC: 34 A (Table 430.250) HORSEPOWER: 25 HP Code: 430.6(A)(1) — use table FLC, not nameplate, for selection must be ≥ CONTROLLER (CONTACTOR) NEMA Size 3 / 25 HP min. 460 V rated HP RATING: 25 HP MIN Must also interrupt locked-rotor current per 430.83(A) Inverse-time breaker: 430.52 ⚠ UNDERSIZED CONTACTOR Welds closed on start — cannot interrupt locked-rotor current exception DEFINITE-PURPOSE 430.83(C) exception: may be current-rated if listed for motor (e.g., A/C compressor) 34 A FLC minimum Nameplate current + listing restrictions apply SELECTION RULE COMPARISON Standard Controller (430.83A) Definite-Purpose (430.83C) Typical Mistake HP rating ≥ motor HP Current rating ≥ motor current Sizing by FLC alone 25 HP motor → 25 HP controller 34 A motor → 34 A+ controller Undersized contactor welds shut Must break locked-rotor current Listing covers specific motor Ignoring 430.83(A) HP rule Master Electrician Practice — NEC 430.83 Motor Controller Ratings | ICC 701 Wyoming Master Electrician

A motor controller must have a horsepower (hp) rating that is not less than the horsepower rating of the motor, per 430.83(A). This is a strict minimum. For a master supervising a job, you must verify the nameplate of the motor against the controller’s marking.

Exceptions and Special Cases:

Stationary motors rated 1/8 hp or less: A controller can be a general-use snap switch rated at 120 V or 277 V, provided the motor’s full-load current does not exceed 80% of the switch’s ampere rating (430.83(C)(1)).
Automatic controllers: If the controller is automatic (e.g., a thermostat or pressure switch), it must be capable of interrupting the stalled-rotor current of the motor. Additionally, a separate manual controller must be provided for the operator (430.83(C)(2)).
Cord-and-plug connected motors: The receptacle and plug can serve as the controller if the motor is rated 1/3 hp or less, or if the motor is part of an appliance (430.83(C)(3)).

For the Master Exam: Do not confuse the controller rating with the branch-circuit short-circuit and ground-fault protective device (BCSCGFPD) sizing. The controller is sized by horsepower, while the protective device is sized by full-load current (FLC) from Table 430.247 (DC), 430.248 (single-phase AC), or 430.250 (three-phase AC). The FLC tables are used for conductor sizing and protection, not the nameplate current, unless the nameplate is higher.

1.3 Control Circuit Conductors and Protection (430.71 – 430.74)

Control-Circuit Conductors: Protection & Disconnect (430.72) Control-Circuit Conductors: Protection & Disconnect NEC 430.72, 430.73, 430.74 — Master Depth (2023 NEC / NFPA 70) Motor Branch Disconnect 430.102 Branch OCPD (430.52) Inverse-time Contactor M M 3-Phase L1 L2 L3 480V 3-Phase Supply Control Transformer 480V → 120V 430.72(C) Tap Primary OCPD Leaves enclosure → needs own OCPD 430.72(B) Push Button Station (remote) Pilot Device (limit switch) Coil NEC 430.73 — Physical Protection Control-circuit conductors must be protected against physical damage — raceway or equivalent required. NEC 430.74 — Disconnect All Sources When the motor disconnect opens, the control circuit must drop ALL sources of supply. A separate control supply left live while motor is locked out = VIOLATION ⚠ Transformer Sizing — Sum of Coil Burdens VA_total = Σ (contactor coil VA + pilot device VA) Size transformer so voltage does not sag when multiple contactors pull in simultaneously. Contactor A: 12 VA Contactor B: 12 VA Pilot: 5 VA Total: 29 VA → 50 VA min Master Electrician Practice — NEC 430.72, 430.73, 430.74 Control-Circuit Protection & Disconnect

This is a high-yield area for the Wyoming Master exam. Control circuits are classified into two types:

33.Class 1 Control Circuits: These are the standard motor control circuits operating at 600 V or less. They must be protected against overcurrent.
34.Class 2 and Class 3 Control Circuits: These are power-limited circuits (typically from a listed transformer or power supply) and follow the rules of Article 725.

Conductor Sizing (430.72(A)): Control circuit conductors must be sized at not less than 1/3 of the rating of the motor’s branch-circuit protective device, but never smaller than 14 AWG for copper (unless specifically permitted for electronic circuits). If the control circuit is tapped from the motor branch circuit, the tap must be protected per the 1/3 rule.

Overcurrent Protection (430.72(B)): The control circuit must be protected against overcurrent. If the control device (e.g., a relay or contactor coil) is rated for it, you may use a supplementary protector. The critical rule is that the control circuit cannot be connected to the load side of the motor’s branch-circuit short-circuit protective device unless the control circuit has its own overcurrent protection, or the combined rating of the control circuit and the motor does not exceed the branch circuit device rating.

The "Single-Phase" Trap: If a control circuit is connected across two phases of a 3-phase motor circuit (e.g., L1 and L2), and the motor is controlled by a magnetic starter, the control circuit must be arranged so that a ground fault in the control circuit does not cause the motor to start unintentionally. This is the classic "control transformer" issue — the master must ensure a control power transformer (CPT) is used to derive a grounded control voltage, or the control circuit must be two-wire with a grounded conductor.

1.4 Disconnecting Means for Controllers (430.102)

Motor Disconnects: In-Sight Rule & 115% Rating (430.102) Motor Disconnects: In-Sight Rule & 115% Rating (430.102) 2023 NEC / NFPA 70 — Wyoming Master Electrician (ICC 701) — Open Book · Chapter 7 Control Devices ROOM A — CONTROLLER AREA ROOM B — MOTOR AREA CONTROLLER (Starter / Contactor) NEC 430.81 DISC. 1 In sight of controller 430.102(A) From panelboard M 25 HP MOTOR FLC = 34 A @ 460 V DISC. 2 In sight of motor & driven machinery 430.102(B) Distance between rooms: 45 ft (13.7 m) — less than 50 ft "In sight" = visible & ≤ 50 ft (Art. 100) EXCEPTION — 430.102(B) Ex: If motor cannot be seen from disconnect location, a lockable disconnecting means satisfies rule Worker can lock it open before service RATING — NEC 430.110(A) Motor FLC (Table 430.250): 34 A Minimum = FLC × 115% = 34 × 1.15 = 39.1 A minimum Listed motor-circuit switch, hp-rated (430.109) ✓ Master Electrician Practice — NEC 430.102 & 430.110: Motor disconnecting means, in-sight rule, and 115% rating Wyoming Master (ICC 701) · 2023 NEC

The controller must have a disconnecting means that is located in sight from the controller location. "In sight" is defined as visible and not more than 15 m (50 ft) from the equipment.

Critical 2023 NEC Update: The disconnecting means for the controller must disconnect the controller and the motor. However, 430.102(A) requires the disconnect to be in sight from the controller. 430.102(B) requires a disconnect in sight from the motor. If the motor and controller are not in sight of each other, you need two disconnects.

Master’s Supervision Point: On a commercial rooftop, the master must verify that the disconnect for the motor is not just in sight of the motor, but also that the controller (often inside the unit) has its own disconnect or is within sight of the unit-mounted disconnect. A lockable disconnect handle is required if the disconnect is not in sight of the controller.

1.5 Overload Protection and the "Service Factor" Rule (430.32)

Overload relays (heaters) are the primary control devices protecting the motor from running overcurrent. The master must size these based on the nameplate full-load current, not the NEC table current.

Motors with a service factor of 1.15 or greater, or a temperature rise of 40°C or less: The overload device must be set at not more than 125% of the motor nameplate current rating (430.32(A)(1)).
All other motors: The overload device must be set at not more than 115% of the nameplate current (430.32(A)(2)).
Maximum permissible setting: If the 125% or 115% value does not allow the motor to start (due to high inertia loads), the next higher standard size is permitted, but cannot exceed 140% of the nameplate current for motors with a service factor of 1.15 or higher, and 130% for other motors (430.32(C)).

Exam Trap: The exam will give you a motor nameplate of 18 A, a service factor of 1.0, and a Table 430.250 FLC of 22 A. The overloads are sized at 115% of the nameplate (18 A × 1.15 = 20.7 A). The branch circuit conductors are sized at 125% of the table FLC (22 A × 1.25 = 27.5 A → 10 AWG). Do not mix these values.

1.6 Air-Conditioning and Refrigeration Equipment (Article 440)

For the master exam, Article 440 is a specialized subset of motor rules. The key is understanding the "combination load" — the compressor motor and the condenser fan motor.

440.12 – Disconnecting Means: The disconnect for a hermetic refrigerant motor-compressor must be rated at not less than 115% of the nameplate rated-load current or the branch-circuit selection current, whichever is greater. For a unit with multiple motors (e.g., compressor and fan), the disconnect must be rated for the sum of all currents.

440.22 – Short-Circuit and Ground-Fault Protection: The BCSCGFPD for a hermetic compressor is based on the branch-circuit selection current (BCSC) which is typically 115% of the rated-load current. The maximum permitted rating is 175% of the BCSC, with a next-higher-standard-size allowance up to 225% if the 175% value fails to start the motor.

Master’s Field Check: For a packaged rooftop unit, the single disconnect on the unit serves as the disconnect for the controller and the motor. The master must verify that the disconnect is a motor-circuit switch rated for the locked-rotor current of the compressor, not just a general-use switch.

1.7 Control Devices in Services and Separately Derived Systems

Control devices are not limited to motor starters. The master must understand how control and protection devices interact with the service and SDS.

Services (Article 230): The service disconnecting means is the ultimate control device. Each service disconnect must be a suitable enclosure, and the number of disconnects is limited to six (230.71). For a master supervising a job, the critical issue is that the service disconnect must be externally operable and must be capable of being locked in the open position.

Generators and Transformers (Article 700, 701, 705, and 450): A separately derived system (SDS) — such as a step-down transformer or a standby generator with a transfer switch — requires a grounding electrode conductor and a bonding jumper at the SDS. The control device here is the overcurrent device on the secondary side.

Transformer (450.3): Primary protection can be set at 125% of the primary FLC (or next higher standard size). If the primary protection is set at 125% and the secondary is not protected, the secondary conductors must terminate in a single overcurrent device.
Generator (445.12): Generators must be protected from overload by a device that opens all ungrounded conductors. The master must ensure that the generator’s rated current is not exceeded.

Feeder Sizing for Control Panels (409.20): For an industrial control panel, the feeder conductors must be sized at 125% of the continuous load plus 100% of the non-continuous load. The master must sum the nameplate ratings of all motors and control transformers inside the panel.

1.8 Overcurrent Protection Coordination (240.12 and 240.87)

While not strictly a "control device," the master must understand the coordination of overcurrent devices to supervise the installation of control panels and motor control centers (MCCs).

Selective Coordination: For life safety systems (Article 700), the overcurrent devices must be selectively coordinated — meaning the fault on a downstream branch circuit will not cause the upstream feeder device to open. This requires a time-current study.
Arc-Flash Reduction (240.87): For services rated 1200 A or more, the master must verify the presence of one of the permitted arc-flash reduction methods (e.g., zone-selective interlocking, differential relaying, or a maintenance switch). This is a supervisory requirement that is often overlooked on large commercial projects.

1.9 Code Navigation — Where to Find It

ConceptNEC 2023 Reference
Motor controller rating430.82, 430.83
Control circuit conductors430.71, 430.72
Disconnect for controller430.102(A)
Disconnect for motor430.102(B)
Overload sizing (service factor)430.32(A), (C)
Full-load current tables (3-phase)Table 430.250
Full-load current tables (single-phase)Table 430.248
Hermetic compressor (A/C)440.12, 440.22
Industrial control panelsArticle 409
Transformer protection (SDS)450.3
Generator protection445.12
Selective coordination (life safety)700.28, 240.12
Arc-flash reduction (1200 A+)240.87
Service disconnects (max six)230.71
"In sight" definitionArticle 100

1.10 Inspection and Supervision Points

As a master electrician signing off on work, you must physically verify the following on every motor and control installation:

76.Controller Horsepower Rating: Check the nameplate on the starter. If the motor is 10 hp, the starter must be marked "10 hp" or higher. A 7.5 hp starter on a 10 hp motor is a violation.
77.Overload Heater Availability: Verify that the overload relay heaters are installed and are the correct size per the motor nameplate. Do not rely on the "adjustable dial" setting alone — the physical heater element must match.
78.Control Circuit Fusing: Open the control panel. The control circuit transformer (CPT) secondary must have a fuse or supplementary protector. If the control circuit is tapped directly from the line side of the starter, verify the 1/3 rule for conductor sizing.
79.Disconnect Location: Measure the distance from the disconnect to the motor. If it exceeds 50 ft (15 m), it is not "in sight." Also, verify the disconnect is capable of being locked in the open position.
80.Grounding of the Control Circuit: If using a control transformer, verify that the secondary is grounded (bonded) at one point. If the control circuit is ungrounded, ensure the starter coil is not connected between two phases in a way that a ground fault could cause a false start.

1.11 Common Exam Traps

Trap 1: Using nameplate current for conductor sizing. Conductors are sized from the table FLC, not the nameplate. Overloads are sized from the nameplate, not the table.
Trap 2: Forgetting the 115% vs. 125% overload rule. The higher percentage (125%) only applies to motors with a service factor of 1.15 or higher or a 40°C temperature rise.
Trap 3: Sizing the disconnect for the motor instead of the controller. The disconnect must be rated for the locked-rotor current of the motor, which is typically 6 to 8 times the FLC. A general-use switch is not always sufficient.
Trap 4: Assuming a snap switch can control a 2 hp motor. A general-use snap switch can only control a motor rated 1/8 hp or less, unless it is specifically marked with a horsepower rating.
Trap 5: Ignoring the "in sight" rule for the controller. The disconnect must be in sight of the controller. If the controller is inside a panel and the disconnect is on the wall outside the room, you must provide a lockable disconnect inside the panel or a remote-operated circuit breaker.
Trap 6: Applying motor rules to A/C units. For a hermetic compressor, you use the branch-circuit selection current (usually 115% of rated-load current) for sizing the disconnect and the branch circuit, not the standard motor FLC tables.

Summary

The master electrician’s role in control devices is to bridge the gap between the schematic diagram and the physical installation. You must ensure that the controller is properly rated for the motor, that the control circuit is independently protected, and that the disconnecting means is correctly located and rated. The 2023 NEC places significant emphasis on the separation of the control circuit from the power circuit, and the master must verify that all field wiring complies with the tap rules of 430.72. By mastering the relationship between Table 430.250, the nameplate data, and the specific requirements of Articles 430, 440, and 409, you will be prepared to sign off on complex commercial and industrial installations with confidence.

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