Motors & Controls — Master Exam Study Chapter

Learning Objectives

By the end of this chapter, you will be able to:

4.Apply the general requirements of Article 430 to motor branch circuits, feeders, and controllers, including disconnecting means.

5.Correctly size motor branch-circuit conductors at 125% of the full-load current (FLC) and understand when the next-higher standard overcurrent device rule applies.

6.Select and size motor overload protection based on nameplate current and service factor, with the 125% and 140% thresholds.

7.Distinguish between overload protection (Part III) and short-circuit/ground-fault protection (Part IV) and coordinate them properly.

8.Calculate motor feeder conductor and protection sizes using the largest motor rule (430.24 and 430.62).

9.Apply control circuit (Class 1 and Class 2) requirements from Article 725 and the motor control circuit provisions of 430.71 through 430.74.

10.Identify the specific disconnecting means requirements for motors, controllers, and combination installations, including the "in sight from" rule.

11.Navigate the 2023 NEC efficiently to locate motor tables (430.247 through 430.250) and apply adjustment factors correctly.

12.Recognize common exam traps involving voltage drop, duty cycle, and the difference between FLC (table) and nameplate full-load amperes (FLA).

1.1 Scope and General: Article 430

Article 430 governs all electric motor applications, including motor circuits, controllers, and protection. As a Master, you are responsible for the entire installation, not just the motor itself. The NEC treats a motor circuit as a system with distinct parts: branch-circuit conductors, the controller, the disconnecting means, overload protection, and short-circuit/ground-fault protection.

Key distinction: The NEC uses full-load current (FLC) from Tables 430.247 through 430.250 for sizing conductors and branch-circuit protection. The nameplate full-load amperes (FLA) is used only for sizing overload relays (430.32). This is a classic trap — never mix the two.

Duty cycle: For motors used in intermittent, periodic, or varying-duty applications (e.g., crane motors, door operators), 430.22(E) permits the branch-circuit conductors to be sized based on the nameplate current rating and the duty cycle, provided the motor is not continuous-duty. A continuous-duty motor is defined as one that operates for three hours or more without stopping.

1.2 Branch-Circuit Conductors (430.22)

The 125% rule: A single continuous-duty motor branch circuit must have conductors rated at 125% of the motor's FLC (not nameplate). This is the minimum ampacity before any adjustment or correction factors.

Example: A 25 hp, 460 V, three-phase motor has an FLC of 34 A per Table 430.250. Minimum conductor ampacity = 34 × 1.25 = 42.5 A. You would select a conductor with an ampacity of at least 42.5 A after applying temperature correction and bundling adjustment factors (310.15).

Voltage drop: The NEC does not mandate a specific voltage drop percentage, but 210.19(A) Informational Note suggests 3% for branch circuits and 5% total. For long motor runs, you must increase the conductor size to prevent excessive voltage drop, which reduces starting torque and can cause overheating. This is a practical supervision point: always verify the actual length of the run.

Terminal temperature ratings: For equipment rated 100 A or less, use the 60°C column of Table 310.16 unless the terminals are marked otherwise (110.14(C)). For circuits over 100 A, use the 75°C column. This is a frequent inspection failure — conductors sized on the 90°C column but terminated on 75°C-rated lugs.

1.3 Overload Protection (430.31 – 430.40)

Overload protection is designed to protect the motor, the motor control apparatus, and the branch-circuit conductors from excessive heating due to motor overloads and failure to start. It is not short-circuit protection.

Sizing rules (430.32):

Motors with a service factor of 1.15 or greater (or a temperature rise of 40°C or less): overload device shall be sized at no more than 125% of the nameplate FLA.

All other motors: overload device shall be sized at no more than 115% of the nameplate FLA.

Next-higher standard size: If the calculated value does not correspond to a standard heater size, you may round up to the next standard size (430.32(C)). However, if the motor trips on startup, you may increase the overload setting up to a maximum of 140% for motors with a 1.15 service factor and 130% for others (430.32(C) exception).

Exam trap: The 125% and 115% are based on the nameplate, not the table FLC. A motor with a 1.0 service factor and a nameplate of 10 A requires an overload set at no more than 11.5 A. If the motor trips, you can go to 13 A (130%).

Integral thermal protection: Motors with built-in thermal protectors (430.32(G)) are permitted if the protector is approved for the motor and the motor is marked accordingly.

What a Master checks on site: Verify the overload relay heater elements match the motor nameplate. Check that the overload relay is set to the correct current and that the motor's service factor is visible on the nameplate. Ensure the overload is not bypassed or shunted — a common but dangerous field modification.

1.4 Short-Circuit and Ground-Fault Protection (430.51 – 430.58)

This protection (the "fuse or breaker" in the branch circuit) is designed to protect the conductors and equipment from high-level faults. It is not overload protection.

Sizing rule (430.52): The maximum rating of the branch-circuit short-circuit and ground-fault protective device (SCGFP) shall not exceed:

Non-time-delay fuses: 300% of FLC

Dual-element (time-delay) fuses: 175% of FLC

Inverse-time circuit breakers: 250% of FLC

Next-higher standard size: If the calculated value does not correspond to a standard ampere rating (240.6), you may round up to the next standard size. This is the only place in the motor rules where you can exceed the calculated percentage.

Example: A 50 hp, 460 V motor has an FLC of 65 A. With an inverse-time breaker: 65 × 2.5 = 162.5 A. The next standard size is 175 A. You may use a 175 A breaker. However, if the motor's starting current (inrush) causes nuisance trips, you may increase the breaker up to 400% of FLC (430.52(C)(1) Exception No. 2) for inverse-time breakers, but only if the motor is marked with a code letter showing a locked-rotor current that requires it.

Exam trap: The "next-higher standard size" rule applies to the branch-circuit SCGFP, not to the feeder protection (430.62). For feeders, you must use the actual calculated value and cannot round up arbitrarily.

Combination motor controllers: A motor controller with a disconnecting means and SCGFP in one enclosure (a "combination starter") is common in commercial work. The SCGFP must be sized per 430.52, and the controller must be marked with the maximum fuse or breaker size.

1.5 Feeder Conductors and Protection (430.24, 430.62)

Feeder conductors supplying two or more motors must have an ampacity of at least 125% of the FLC of the largest motor plus the sum of the FLCs of all other motors on the same feeder.

Formula: Feeder ampacity = (1.25 × FLC of largest motor) + Σ(FLC of all other motors).

Example: A feeder supplies three motors: 10 hp (14 A), 15 hp (21 A), and 25 hp (34 A). Feeder ampacity = (1.25 × 34) + 14 + 21 = 42.5 + 35 = 77.5 A. You would select conductors rated at least 77.5 A.

Feeder SCGFP (430.62): The feeder protective device shall be sized based on the largest branch-circuit protective device (calculated per 430.52) plus the sum of the FLCs of the other motors.

Formula: Feeder SCGFP = (largest branch-circuit SCGFP rating) + Σ(FLC of other motors).

Critical rule: The feeder SCGFP cannot exceed this calculated value. You may not round up to the next standard size for feeders. If the calculated value is not a standard size, you must use the next lower standard size. This is the opposite of the branch-circuit rule and a major exam trap.

Example: Using the three motors above, if the largest motor (25 hp) has a branch-circuit breaker of 34 × 2.5 = 85 A (next standard 90 A), the feeder SCGFP = 90 + 14 + 21 = 125 A. You would use a 125 A breaker. If the calculation yielded 127 A, you would have to drop to 125 A.

1.6 Motor Controllers and Disconnecting Means (430.81 – 430.91)

Controller requirements: Each motor must have a controller capable of starting and stopping the motor and interrupting the locked-rotor current of the motor (430.83). A controller rated for the motor's horsepower is required. For stationary motors of 1/8 hp or less, a general-use snap switch is permitted if it is rated for the motor.

Disconnecting means (430.101 – 430.113):

A disconnecting means must be provided for the motor, the controller, and the motor branch circuit.

The disconnect must be "in sight from" the motor and the controller (430.102). "In sight" means visible and not more than 15 m (50 ft) apart.

The motor disconnect must open all ungrounded conductors simultaneously.

For a motor and controller located in sight of each other, one disconnect can serve both, provided it is in sight of both.

If the motor is not in sight from the controller, a separate disconnect must be installed at the motor (430.102(B)).

What a Master checks on site: Confirm the disconnect is within 50 ft and visible. Verify that the disconnect is a horsepower-rated switch, not a general-use switch, unless the motor is 1/8 hp or less. Check that the disconnect is capable of being locked in the open position (430.109) — this is a safety requirement for maintenance personnel.

Control circuits (430.71 – 430.74): Control circuits (e.g., start/stop pushbuttons, PLC outputs) are covered by Article 725. If the control circuit is Class 1 (power-limited or not power-limited), the conductors must be protected per 430.72. If the control circuit is Class 2 (e.g., 24 V DC from a listed power supply), it must comply with 725.130. A common violation is running Class 1 control wiring in the same raceway as power conductors without proper insulation ratings.

1.7 Three-Phase Systems and Services

As a Master, you must understand the service and feeder calculations that feed motor loads.

Service sizing (230.42): The service conductors must have an ampacity of at least the calculated load per Article 220. For motor loads, this means applying the demand factors of 430.26 (if applicable) or the standard motor feeder rules.

Separately derived systems (Article 250.30): Transformers and generators that supply motor loads are separately derived systems. The grounded conductor (neutral) must be bonded to the grounding electrode system at the source (transformer or generator) or at the first disconnecting means. The system must have a grounding electrode conductor sized per Table 250.66.

Generator applications (Article 445): Generators used as a backup or prime power source for motor loads must have overcurrent protection per 445.12. The generator's rated output current is used for sizing conductors, not the motor FLC. If a generator supplies a motor directly, the generator must be capable of handling the motor's starting current (locked-rotor current), which can be 6–8 times the FLC.

Exam trap: A generator rated 100 kW at 480 V three-phase has a rated current of approximately 120 A. The feeder to a motor control center must be sized for the generator's rated current, not the sum of the motor FLCs, unless the generator is specifically rated for motor starting.

1.8 Commercial and Industrial Installations

Motor control centers (MCCs): An MCC is a factory-assembled assembly of motor starters, feeders, and disconnects. Each starter unit must have its own branch-circuit protection and overload relays. The MCC bus is a feeder, and the feeder SCGFP must comply with 430.62.

Variable frequency drives (VFDs): VFDs are covered by Article 430.120 through 430.132. The VFD is considered the controller and the motor disconnect. The input conductors to the VFD must be sized per the VFD's input current rating (not the motor FLC). The output conductors from the VFD to the motor must be sized per 430.122, which requires the conductors to have an ampacity of at least 125% of the motor FLC. The VFD must have its own short-circuit protection per the manufacturer's instructions.

What a Master checks on site: Verify that the VFD's input and output conductors are correctly sized. Check that the VFD is not used as a motor disconnect unless it has a marked disconnecting means. Confirm that the motor overload protection is provided by the VFD's electronic overload function, which must be set per the motor nameplate.

1.9 Code Navigation: Where to Find It

Topic	NEC 2023 Location
Motor branch-circuit conductors	430.22
Motor feeder conductors	430.24
Overload protection	430.31 – 430.40
Short-circuit/ground-fault protection	430.51 – 430.58
Motor FLC tables (single-phase)	Table 430.248
Motor FLC tables (three-phase)	Table 430.250
Motor controllers	430.81 – 430.91
Disconnecting means	430.101 – 430.113
Control circuits	430.71 – 430.74, Article 725
VFDs	430.120 – 430.132
Standard fuse/breaker sizes	240.6
Conductor ampacity tables	Table 310.16
Temperature limitations	110.14(C)
Separately derived systems	250.30
Generators	Article 445
Transformers	Article 450

1.10 Inspection and Supervision Points

85.Nameplate vs. Table: Always verify the motor nameplate for FLA, service factor, and temperature rise. The table FLC is for conductor sizing; the nameplate FLA is for overloads.

86.Overload heater size: Check that the heater elements in the starter match the motor nameplate. A mismatch is a top-five inspection finding.

87.Disconnect location: Measure the distance from the disconnect to the motor. If it exceeds 50 ft or is not visible, it is a violation.

88.Feeder SCGFP: Do not allow rounding up on feeder protection. If the calculated value is 127 A, the breaker must be 125 A, not 150 A.

89.Terminal temperature: Verify that conductors are sized using the correct temperature column for the terminals (60°C for ≤100 A, 75°C for >100 A).

90.Control wiring: Ensure Class 1 and Class 2 wiring are separated or properly insulated. Check that control transformers are protected per 430.72(C).

91.VFD installations: Confirm the input conductors are sized for the VFD's input rating, not the motor's FLC. Verify that the motor thermal protection is enabled in the VFD parameters.

1.11 Common Exam Traps

Using nameplate FLA instead of table FLC for conductor sizing. The tables are the law for conductors.

Using table FLC instead of nameplate FLA for overload sizing. The nameplate is the law for overloads.

Rounding up on feeder SCGFP. Only branch circuits allow rounding up.

Forgetting the 125% factor for the largest motor in a feeder calculation.

Applying the 125% factor to the sum of all motors on a feeder, not just the largest.

Confusing the "in sight from" rule — it applies to the motor and its disconnect, and the controller and its disconnect.

Sizing conductors for a VFD based on motor FLC instead of the VFD's input current rating.

Ignoring the 60°C terminal rating for small motors and controllers.

This chapter provides the core theory and code references you need for the Motors & Controls section of the Arkansas Master exam. Practice navigating the 2023 NEC quickly — your open book is only as good as your ability to find the right section under time pressure. Focus on the tables and the exceptions, as these are where the exam differentiates a journeyman from a Master.

Motors & Controls