Motors and Generators — Master Electrician Study Chapter

TX-MST-KNOW | 2026 NEC (NFPA 70)

Learning Objectives

Upon completing this chapter, you will be able to:

6.Apply the general requirements for motor installations, including nameplate data interpretation and terminal housing requirements.

7.Size motor branch-circuit conductors, short-circuit and ground-fault protection, and overload protection using the correct NEC tables and percentages.

8.Calculate motor feeder conductor and protection sizes for multiple motors, applying demand factors correctly.

9.Understand the unique requirements for generators, including grounding, transfer switches, and separately derived system rules.

10.Navigate the NEC efficiently to locate motor and generator requirements for open-book exam scenarios.

11.Identify common inspection failures and exam traps related to motor and generator installations.

1.1 Motor Circuit Components and the Code Structure

A complete motor circuit consists of several distinct components, each with its own NEC requirements. As a master electrician, you must be able to identify these components and apply the correct code sections. The primary components are:

Branch-Circuit Conductors: Supply the motor and must be sized at 125% of the motor's full-load current (FLC), not the nameplate current rating.

Branch-Circuit Short-Circuit and Ground-Fault Protection (BCSCGFP): Protects the circuit against short circuits and ground faults, not overloads. This is typically a fuse or circuit breaker.

Motor Overload Protection: Protects the motor, branch-circuit conductors, and control apparatus against excessive heating due to motor overloads or failure to start. This is typically a heater element, solid-state relay, or electronic overload relay.

Motor Controllers: A device that governs the electric power delivered to the motor.

Disconnecting Means: A device that disconnects all conductors from the circuit supply.

Key Distinction: The nameplate current rating is used for sizing overload protection, conductor ampacity for the motor's terminal housing, and disconnecting means. The NEC Table FLC values (Table 430.247 through 430.250) are used for sizing branch-circuit conductors, BCSCGFP devices, and feeder conductors.

1.2 Sizing Branch-Circuit Conductors

The branch-circuit conductors supplying a single motor must have an ampacity of not less than 125% of the motor's full-load current as determined by the appropriate NEC table (430.22). This is a fundamental rule.

Single Motor: Conductor ampacity ≥ 1.25 × Table FLC.

Multiple Motors on One Branch Circuit: This is a specific case where a branch circuit supplies two or more motors. The conductors must be sized at 125% of the highest-rated motor's FLC plus the sum of the FLCs of all other motors on that circuit (430.22(C)). This is a common exam calculation.

Important: When applying conductor ampacity, you must consider the temperature rating of the terminals and the conductors, as well as any derating factors for ambient temperature or conductor bundling. The final ampacity after derating must still be sufficient.

1.3 Overload Protection

Overload protection is designed to protect the motor from drawing excessive current for a prolonged period. The requirements are found in Article 430, Part III.

Sizing: The overload device must be sized at no more than 115% of the motor's nameplate current rating for motors with a marked service factor of 1.15 or greater, or a marked temperature rise of 40°C or less. For all other motors, the maximum is 125% of the nameplate current rating (430.32(A)(1)).

Exception: If the motor is not capable of starting successfully with the maximum permitted overload device, the next higher standard size or rating is permitted, but it cannot exceed 130% for the 115% rule or 140% for the 125% rule (430.32(C)). This is a critical exception for motors with high starting torque requirements.

Dual-Element Fuses: When using dual-element (time-delay) fuses for overload protection, they must be sized at no more than 150% of the motor nameplate current rating (430.32(A)(1) Exception No. 1). This is a special allowance because these fuses have a time-delay characteristic that permits motor starting current to flow without opening.

Exam Trap: Do not confuse the percentages for overload protection with those for branch-circuit short-circuit and ground-fault protection. Overloads are based on nameplate current; short-circuit protection is based on Table FLC.

1.4 Branch-Circuit Short-Circuit and Ground-Fault Protection

The BCSCGFP device must protect the circuit conductors, the motor controller, and the motor itself against overcurrent due to short circuits and ground faults. The maximum permitted ratings are found in Table 430.52.

Selection: The rating of the BCSCGFP device must not exceed the values listed in Table 430.52 for the specific type of motor and protection device used. For example, a squirrel-cage motor (not a Design B energy-efficient type) can have a non-time-delay fuse rated at 300% of FLC, an inverse-time circuit breaker at 250% of FLC, or a time-delay fuse at 175% of FLC.

Design B Energy-Efficient Motors: For Design B motors, the maximum rating for an inverse-time circuit breaker is 250% of FLC, but this can be increased to 400% if the motor will not start with the 250% breaker (430.52(C)(1) Exception No. 2). This is a common point of confusion and a frequent exam question.

Next Higher Standard Size: If the calculated value does not correspond to a standard rating of a fuse or circuit breaker, the next higher standard size is permitted (430.52(C)(1) Exception No. 1). This is a key difference from conductor sizing, where you must always round up to the next available ampacity.

1.5 Feeder Sizing and Protection

A feeder supplying two or more motors must have an ampacity sufficient for the largest motor's FLC × 1.25 plus the sum of the FLCs of all other motors on the feeder (430.24). This is a core calculation for commercial and industrial installations.

Feeder Protection: The feeder's short-circuit and ground-fault protection device must be sized to protect the feeder conductors. The maximum rating is the sum of the ratings of the BCSCGFP devices for all motors supplied by the feeder, plus the rating of any other loads (430.62(A)). This is a more complex calculation and often results in a larger device than what is needed for the conductor ampacity alone.

Feeder Conductor Tap Rules: Taps from a feeder to a motor are permitted if they meet specific requirements for length and ampacity (430.28). A tap of 25 feet or less must have an ampacity of at least one-third of the rating of the feeder's overcurrent device. A tap of more than 25 feet must have an ampacity of at least the motor's FLC.

1.6 Motor Controllers and Disconnecting Means

Controller Rating: The controller must have a horsepower rating that is not less than the horsepower rating of the motor it controls (430.83). For a motor with a higher current rating than the equivalent horsepower rating, the controller must be rated for the motor's current.

Disconnecting Means: A disconnecting means must be provided for each motor and controller. It must be capable of disconnecting the motor and controller from all ungrounded supply conductors (430.102). The disconnecting means must be located in sight from the motor and the driven machinery (430.102(B)).

In Sight: The NEC defines "in sight" as being visible and not more than 50 feet (15.2 m) apart from the other equipment. If the disconnecting means is not in sight from the motor, it must be capable of being locked in the open position.

Rating: The disconnecting means must have an ampere rating of at least 115% of the motor's full-load current (430.110(A)). This is a common calculation that is often overlooked.

1.7 Generators and Separately Derived Systems

Generators are covered in Article 445, but their installation must also comply with other relevant articles, particularly those covering grounding and bonding (Article 250).

Generator Nameplate: All generators must have a nameplate providing the manufacturer's name, rated voltage, current, power factor, frequency, and other required data (445.11).

Overcurrent Protection: Generators must be protected from overcurrent by a circuit breaker or fuses. The rating of this protection is based on the generator's rated current and the type of protection used (445.12). For a generator that is a separately derived system, the overcurrent device must be located at the generator or at the first disconnecting means.

Separately Derived System: A generator is considered a separately derived system if there is no direct electrical connection (including a solidly grounded circuit conductor) between the generator's output and the supply system. This is a critical distinction.

Grounding: For a separately derived system, the generator's neutral (grounded conductor) must be grounded at the generator or at the first disconnecting means by a system bonding jumper and a grounding electrode conductor (250.30).

Bonding: The generator frame must be bonded to the grounded conductor and the grounding electrode system.

Transfer Switches: When a generator is used as a backup power source, a transfer switch is required to prevent the generator from back-feeding the utility system (702.5). This is a critical safety requirement to protect utility workers.

Types: Transfer switches can be automatic or manual. They must be rated for the load they will carry and must be listed for the purpose.

Grounding: The transfer switch must be designed to switch the grounded (neutral) conductor if the generator is a separately derived system. If the generator is not a separately derived system (i.e., it is a "non-separately derived" system), the neutral is not switched.

1.8 Code Navigation: Where to Find It

Concept	NEC Article/Section
Motor Branch-Circuit Conductors	430.22
Motor Feeder Conductors	430.24
Motor Overload Protection	430.32, 430.36
Motor Short-Circuit Protection	430.52, Table 430.52
Motor Controllers	430.83, 430.84
Motor Disconnecting Means	430.102, 430.109, 430.110
Motor Tables (FLC)	Table 430.247, 430.248, 430.249, 430.250
Generators	Article 445
Separately Derived Systems	250.30
Transfer Switches	702.5
Grounding and Bonding	Article 250
Services	Article 230

1.9 Inspection and Supervision Points

As a master electrician, you are responsible for the final installation. Here are key points to verify on-site:

71.Nameplate vs. Table: Verify that the correct current value (nameplate vs. table) was used for each component. A common error is using the nameplate current for conductor sizing.

72.Overload Protection: Confirm that the overload protection is sized correctly based on the motor's nameplate and service factor. Check that the correct exception was applied if a larger device was needed for starting.

73.Disconnecting Means Location: Ensure the disconnecting means is in sight of the motor and is capable of being locked in the open position if not.

74.Controller Rating: Verify that the controller's horsepower rating is equal to or greater than the motor's horsepower rating.

75.Generator Grounding: For a separately derived generator, confirm that the system bonding jumper is installed at the correct location (generator or first disconnecting means) and that a grounding electrode conductor is connected to an acceptable electrode.

76.Transfer Switch Operation: Verify that the transfer switch is correctly wired to prevent back-feeding and that the neutral conductor is handled correctly (switched for SDS, not switched for non-SDS).

77.Feeder Taps: If taps are present, measure the length and verify the ampacity of the tap conductors meets the requirements of 430.28.

1.10 Common Exam Traps

Confusing Nameplate and Table Currents: This is the most common error. Overloads use nameplate; conductors and short-circuit protection use table FLC.

Rounding Down: Always round up to the next standard size for conductors and overcurrent devices (when permitted). Never round down.

Forgetting the 125% Factor: The 125% factor for the largest motor is frequently forgotten in feeder calculations.

Design B Motor Exception: The 400% allowance for an inverse-time breaker for Design B motors is a specific exception. It is not a general rule.

In Sight Definition: Remember the 50-foot rule for "in sight." A disconnect that is 60 feet away is not "in sight."

Separately Derived vs. Non-Separately Derived: Understanding the difference is crucial for grounding and transfer switch wiring. A generator with a bonded neutral is not automatically an SDS if it is connected to a system with a grounded neutral.

Overload Protection for Starting: The exception to allow a larger overload device for starting is only permitted if the motor will not start with the standard device. It is not a design choice.

Feeder Protection Calculation: The feeder protection is based on the sum of the branch-circuit protection devices, not the sum of the motor FLCs. This is a critical distinction that is often missed.

Summary

Mastering motor and generator calculations requires a systematic approach. Always start by correctly identifying the type of motor and the applicable NEC table. Then, carefully apply the correct percentages for each component, keeping in mind the distinction between nameplate and table currents. For generators, the key is understanding the concept of a separately derived system and its implications for grounding and bonding. By understanding the logic behind the code and practicing these calculations, you will be well-prepared for the master's exam and for supervising safe, code-compliant installations in the field.

Motors and Generators