Motors & Controls

Motors & Controls — Master Exam Study Chapter

Learning Objectives

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

1.1 The Motor Circuit Hierarchy: From Service to Load

A master electrician must visualize every motor circuit as a series of distinct zones, each with its own code requirements. The zones are: feeder, branch circuit, motor controller, motor disconnecting means, and the motor itself. Article 430 governs all of these, but the definitions and general requirements in Article 100 apply.

The critical distinction for the exam: branch circuit for a motor is the circuit between the final overcurrent device (OCPD) and the motor, including the controller and disconnecting means. The feeder supplies multiple branch circuits or a single branch circuit from the service or separately derived system.

1.2 Motor Nameplate vs. Table Values — The Master’s Judgment Call

The single most common error in motor work is confusing nameplate full-load current (FLC) with table full-load current. The NEC requires you to use Table 430.247 through 430.250 for conductor sizing, overcurrent protection, and controller sizing — NOT the nameplate. The nameplate is used only for motor overload protection (430.32) and for certain locked-rotor current determinations.

Key tables you must memorize:

For three-phase motors, the table values are based on standard voltages: 115, 200, 230, 460, and 575 volts. If you have a 240-volt motor, use the 230-volt column. If you have a 480-volt motor, use the 460-volt column. This is a classic exam trap — rounding up to the next standard voltage is NOT permitted for table selection.

1.3 Branch-Circuit Conductors — Sizing Per 430.22

Branch-circuit conductors supplying a single motor must have an ampacity of at least 125% of the motor’s FLC (from the tables, not nameplate). This is 430.22(A). For a motor with a service factor of 1.15 or greater, or a temperature rise of 40°C or less, the 125% factor is still applied — the service factor does not increase conductor sizing.

Special cases you must know:

Voltage drop is not a code requirement for motor conductors, but the master must consider it for long runs — the NEC only recommends it in 210.19(A) Informational Note. However, if voltage drop is excessive, the motor may not start, and the inspector may flag it under 110.3(B) if the manufacturer specifies a minimum voltage.

1.4 Motor Overload Protection — The 430.32 Maze

Overload protection is distinct from short-circuit protection. Overloads are thermal — they protect the motor from running too hot. Short-circuit protection protects the conductors and equipment from fault currents.

430.32(A) — Motors with service factor ≥ 1.15 OR temperature rise ≤ 40°C: overload device must be sized at no more than 125% of nameplate current.

430.32(B) — All other motors: overload device must be sized at no more than 115% of nameplate current.

430.32(C) — If the required setting is not sufficient for starting: you may increase the overload setting up to a maximum of 140% for motors in (A) and 130% for motors in (B). This is a permissive rule — you must document that the motor cannot start otherwise.

Exam trap: Many candidates use table FLC for overloads. Wrong. Overloads are always based on nameplate current. The nameplate is the motor’s thermal identity; the table is the circuit’s electrical identity.

Thermal protection devices (heaters, solid-state relays) must be selected based on the motor nameplate and the actual ambient temperature. If the motor is in a hot environment, the overload must be adjusted — this is an engineering judgment call that the master must supervise.

1.5 Short-Circuit and Ground-Fault Protection — 430.51 Through 430.58

The branch-circuit OCPD (fuse or breaker) protects the conductors, the controller, and the motor against short circuits and ground faults — NOT overloads (the overload relay does that).

Maximum permitted settings per 430.52(C)(1):

430.52(C)(1) Exception 1: If the maximum permitted value does not allow the motor to start, you may go higher, but the absolute ceiling is 400% for non-time-delay fuses, 225% for time-delay fuses, and 400% for inverse-time breakers. This exception is frequently tested.

430.52(C)(1) Exception 2: For motors over 100 hp, you may use the next higher standard rating per 240.6(A) if the calculated value falls between standard sizes.

Critical coordination point: The branch-circuit OCPD must also protect the motor control circuit. If the control transformer is tapped from the motor branch circuit, its primary protection must comply with 430.72(C). A common field issue is a control transformer protected at 250% when the code requires no more than 167% for a 600-volt class transformer.

1.6 Motor Controllers — 430.81 Through 430.91

A controller is any device that starts and stops a motor. It can be a manual switch, a magnetic contactor, a solid-state starter, or a variable frequency drive (VFD).

Controller rating (430.83): The controller must have a horsepower rating at the motor’s voltage that is not less than the motor’s horsepower. For motors over 100 hp, the controller can be a listed combination controller if it has been tested and marked.

Exception for stationary motors 1/8 hp or less: a general-use snap switch or a branch-circuit breaker can serve as the controller if the motor is within sight of the switch.

VFDs and solid-state controllers (430.124 through 430.126): These are covered by Part X. The VFD must be listed for the motor type and application. The master must verify that the VFD’s input current rating is adequate for the motor FLC, and that the VFD is protected against overcurrent per its listing. Important: When a VFD is used, the motor overload protection is often built into the drive, and the drive’s internal electronic overload must be set per the motor nameplate. The branch-circuit OCPD must be sized per the VFD manufacturer’s instructions, which may differ from 430.52.

Disconnecting means for the controller (430.102): A disconnecting means must be provided in sight of the controller location. The controller disconnecting means must disconnect the motor and the controller from all ungrounded supply conductors.

1.7 Disconnecting Means — 430.101 Through 430.113

Every motor must have a disconnecting means that:

The “in sight” rule (Article 100): In sight means visible and not more than 15 m (50 ft) apart. If the motor is not in sight of its disconnecting means, you must install a second disconnecting means at the motor — this is a common field inspection point.

For cord-and-plug-connected motors (430.109(C)): The plug and receptacle can serve as the disconnecting means if they are in sight of the motor and the motor is not over 1/3 hp, or if the plug is a horsepower-rated attachment plug.

Grouped motor disconnects (430.112): A single disconnecting means can serve a group of motors if the group is under one controller and the motors are in sight of the disconnect.

1.8 Feeder Sizing for Multiple Motors — 430.24 and 430.62

This is the heart of the master-level calculation. When a feeder supplies two or more motors, the feeder conductor ampacity must be:

430.24: The sum of the FLC of all motors on the feeder (table values) plus 125% of the highest-rated motor in the group.

Formula: Feeder ampacity = (125% × largest motor FLC) + (sum of all other motor FLCs)

Example: Three motors: 10 hp @ 460V (14 A), 5 hp @ 460V (7.6 A), 2 hp @ 460V (3.4 A). Feeder = (14 × 1.25) + 7.6 + 3.4 = 17.5 + 11.0 = 28.5 A. Use a 30 A conductor per 240.6(A) after applying 430.62 for the OCPD.

Feeder OCPD (430.62(A)): The feeder protection must be sized at the sum of the largest branch-circuit OCPD (not the motor FLC) plus the FLC of all other motors. This is a subtle but critical difference from conductor sizing.

Formula: Feeder OCPD = (largest branch OCPD) + (sum of FLC of all other motors)

Exam trap: Do not use 125% of the largest branch OCPD. The 125% factor is already built into the branch OCPD calculation. If the calculated feeder OCPD does not correspond to a standard rating, you may round up to the next standard size per 430.62(A).

430.62(B) — Combination loads: If the feeder also supplies lighting or other loads, add the largest motor OCPD plus the FLC of all other motors plus the other loads. The feeder OCPD must not exceed the ampacity of the feeder conductors.

1.9 Motor Control Circuits — 430.71 Through 430.74

Control circuits (the low-voltage wiring that operates the contactor coil) are often overlooked. Key rules:

Field inspection point: Many installers use a 15 A fuse for a control transformer when the code requires a 10 A fuse for a 120 VA transformer at 120V. The master must check the transformer VA rating and calculate the primary current.

1.10 HVAC Equipment — Article 440 Interaction

Article 440 covers hermetic refrigerant motor-compressors, which are common in commercial rooftop units and heat pumps. These are NOT treated as standard motors.

Key differences:

440.12: The disconnecting means for HVAC equipment must have an ampere rating of at least 115% of the sum of the RLC plus all other loads (fans, heaters). For a hermetic compressor, the disconnect must also be capable of interrupting the locked-rotor current.

440.22: The branch-circuit OCPD must be sized per the equipment nameplate or per 440.22(A) which permits up to 175% of the RLC for time-delay fuses and 225% for inverse-time breakers.

Exam trap: Do not apply 430.52 to hermetic compressors. The percentages are different, and the basis is RLC, not table FLC.

1.11 Adjustable-Speed Drives and Part X — 430.120 Through 430.126

Part X of Article 430 was significantly revised in the 2023 NEC. Key requirements:

Master’s check: When a VFD is used, the motor’s thermal protection is often provided by the drive’s electronic overload. The drive must be programmed with the motor nameplate FLC. If the drive is bypassed (for maintenance), a separate overload relay must be provided for the bypass circuit.

1.12 Generators and Separately Derived Systems — Article 445 and 705

Motors and generators are closely linked. A generator supplying motor loads must be sized to handle the starting current (locked-rotor) of the largest motor plus the running current of all other loads. Article 445 covers generators, and Article 705 covers interconnected electric power production sources.

Key points for the master:

705.20: If the generator is interconnected with the utility, the output conductors must be sized per the generator’s rated output, and the overcurrent protection must be coordinated with the utility’s requirements.

1.13 Code Navigation — Where to Find It

1.14 Inspection and Supervision Points

When you are the master on the job, verify these items on every motor installation:

1.15 Common Exam Traps

1.16 Summary

The master electrician’s role in motor installations is to verify, calculate, and supervise. The NEC’s Article 430 is a carefully structured set of rules that separates thermal protection (overloads) from fault protection (short-circuit), and that treats conductors, controllers, and disconnects as distinct elements. The most important skill is knowing which current value to use — table FLC for circuit elements, nameplate for thermal elements, and RLC/BCSC for hermetic compressors. Master these distinctions, practice the feeder calculations, and you will be prepared for the motor questions on the Maine Master exam.