Motors & Generators — Kentucky Master Electrician 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.

5.Correctly size motor branch-circuit conductors, short-circuit and ground-fault protection, and overload protection using the tables in Article 430.

6.Understand the unique requirements for generators as separately derived systems, including grounding, bonding, and transfer switching.

7.Perform feeder calculations for multiple motors, including the application of demand factors.

8.Identify and apply the specific code rules for motor control circuits, disconnecting means, and adjustable-speed drive systems.

9.Navigate the NEC efficiently to locate motor and generator requirements during the open-book exam.

1.1 The Scope and Structure of Article 430

Article 430 is the cornerstone for all motor installations. It is a complex article that governs conductors, protection, controllers, and disconnects. A master electrician must understand how this article interrelates with others, particularly Article 240 (Overcurrent Protection), Article 250 (Grounding and Bonding), and Article 700 (Emergency Systems).

The key to mastering Article 430 is understanding that it provides a separate, distinct set of rules for motors that often override the general rules for other loads. For example, the branch-circuit conductor ampacity is based on a percentage of the motor's full-load current (FLC), not the rating of the overcurrent device. This is a fundamental shift from standard lighting and receptacle circuits.

Code Navigation:

Article 430: Motors, Motor Circuits, and Controllers

Part I: General (430.1 – 430.10)

Part II: Circuit Conductors (430.20 – 430.24)

Part III: Overload Protection (430.31 – 430.40)

Part IV: Branch-Circuit Short-Circuit and Ground-Fault Protection (430.51 – 430.58)

Part V: Controllers (430.81 – 430.91)

Part VI: Disconnecting Means (430.101 – 430.113)

Part VII: Motor Control Circuits (430.71 – 430.74)

Part IX: Motor and Motor Controller Trouble Shooting (Informational)

1.2 Motor Full-Load Currents: The Foundation

Every calculation for a motor circuit begins with the full-load current. The NEC provides tables for standard motors, and it is crucial to use these tables, not the nameplate rating, for conductor sizing and overcurrent protection.

Table 430.247: Full-Load Current, Direct-Current Motors.

Table 430.248: Full-Load Currents in Amperes, Single-Phase Alternating-Current Motors.

Table 430.250: Full-Load Current, Three-Phase Alternating-Current Motors.

Key Master-Level Distinction: For conductor sizing and short-circuit protection, you use the table FLC. For overload protection (the devices that protect the motor from mechanical overload and locked rotor), you use the nameplate full-load amperes (FLA). This is a critical distinction that is frequently tested.

Example: A 10 HP, 3-phase, 230V motor has a table FLC of 28A (per Table 430.250). Its nameplate might show an FLA of 26A. The branch-circuit conductors must be sized at 125% of the table value (28A × 1.25 = 35A). The overload relays, however, are selected based on the nameplate rating (26A), typically sized at 115% to 125% of that value.

1.3 Sizing Motor Branch-Circuit Conductors

Section 430.22 is the primary rule for branch-circuit conductors supplying a single motor. The conductor ampacity must be at least 125% of the motor's full-load current (from the tables).

Single Motor: 430.22(A) — Conductor ampacity ≥ 125% of Table FLC.

Multiple Motors: 430.24 — The feeder conductor must be sized to supply the largest motor at 125% of its FLC, plus the sum of the FLCs of all other motors on the feeder.

Formula for Multiple Motors (430.24):

Feeder Ampacity = (125% × FLC of Largest Motor) + (Sum of FLC of All Other Motors).

Critical Exception: 430.22(E) addresses motors used for short-time, intermittent, or periodic duty. In these cases, the conductor ampacity can be less than 125%, but not less than the motor's nameplate current. This is an advanced topic that requires careful reading of Table 430.22(E).

Inspection/Supervision Point: On site, verify that the conductors are sized for the table FLC, not the nameplate. A common error is undersizing conductors based on a motor's actual running current, which can lead to overheating under locked-rotor conditions.

1.4 Overload Protection (Part III)

Overload protection is designed to protect the motor, its branch-circuit conductors, and the controller from excessive heating due to motor overloads and failure to start. It does not protect against short circuits.

Sizing: 430.32 specifies the maximum size of the overload device.

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

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

The "Next Size Up" Rule: 430.32(C) permits the next higher standard size overload device if the values in (A) or (B) do not correspond to a standard rating. However, this next size up cannot exceed 140% for motors with a 1.15 SF or 40°C rise, or 130% for all others.

Dual-Element Fuses: If dual-element fuses are used for overload protection, they are sized differently. 430.32(A)(1) permits them to be sized at 150% of the nameplate FLA for motors with a 1.15 SF, and 130% for others (430.32(B)(1)).

Master-Level Insight: The overload relay is a separate device from the branch-circuit short-circuit and ground-fault protection device (fuse or breaker). The overload relay is typically located in the motor starter. The fuse or breaker protects the circuit from faults; the overload relay protects the motor from overload.

1.5 Branch-Circuit Short-Circuit and Ground-Fault Protection (Part IV)

This is the protection for the circuit against high-level faults. It is not the overload protection.

Sizing: 430.52 specifies the maximum rating of the protective device.

Non-time-Delay Fuses: 300% of the motor FLC (Table 430.52).

Dual-Element (Time-Delay) Fuses: 175% of the motor FLC.

Inverse-Time Circuit Breakers: 250% of the motor FLC.

The "Next Size Up" Rule: 430.52(C)(1) Exception No. 1 allows the next higher standard size device if the calculated value does not correspond to a standard rating. This is a critical allowance.

The "Must-Hold" Rule: 430.52(C)(1) Exception No. 2 is a master-level concept. If the next standard size is not sufficient to allow the motor to start (due to high inrush current), the protective device can be increased, but it is capped at a maximum of 400% for fuses and 800% for inverse-time breakers.

Example: A 3-phase, 230V, 10 HP motor has a table FLC of 28A. An inverse-time breaker is to be used.

250% × 28A = 70A. A 70A breaker is a standard size, so it is permitted.

Coordination with Overloads: The branch-circuit protective device must be coordinated with the overload relay so that the overload relay operates first on a sustained overload, and the branch-circuit device operates on a short circuit. This is a key design principle.

1.6 Motor Controllers and Disconnecting Means

Controller (Part V): The controller is any switch or device that governs the starting and stopping of the motor. It must have an ampere rating not less than 115% of the motor's FLC (430.83). A horsepower rating is also required for general-purpose controllers.

Disconnecting Means (Part VI): A disconnecting means must be provided to disconnect the motor and controller from the circuit.

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

Type: Must be a motor-circuit switch (rated in horsepower) or a circuit breaker.

Rating: Must have an ampere rating of at least 115% of the motor's FLC (430.110).

Inspection/Supervision Point: A common violation is installing a standard light switch as a motor disconnect. The disconnect must be horsepower-rated to safely interrupt the locked-rotor current of the motor.

1.7 Generators: Separately Derived Systems

Generators are treated as separately derived systems (SDS) when they have no direct electrical connection to the supply system, other than through bonding and grounding connections. This is a critical concept for grounding and bonding.

Key Rules:

Grounding (Article 250, Part I & II): The generator's frame and the system neutral must be grounded. The grounding electrode conductor must be sized from Table 250.66 based on the largest ungrounded conductor.

Bonding: The generator's neutral (grounded conductor) must be bonded to the generator frame and the equipment grounding conductor at the generator location (250.30(A)).

Transfer Switches (Article 701, 702): A transfer switch is required to prevent backfeeding. It must be listed and approved for the purpose.

Feeder Sizing: The generator feeder must be sized based on the calculated load, not the generator's nameplate kVA rating. The generator's rated output is the maximum it can supply, but the feeder must be sized for the calculated demand.

Master-Level Insight: The neutral of a generator used as an SDS must be switched in the transfer switch. This is a critical detail to ensure that the separately derived system is properly grounded and bonded in either mode of operation.

1.8 Adjustable-Speed Drive Systems (VFDs)

Article 430, Part X, covers adjustable-speed drive systems. These are complex systems that require special consideration.

Harmonics: VFDs create harmonics that can cause overheating in conductors and transformers.

Conductor Sizing: Conductors must be sized based on the drive's input current, not the motor's FLC (430.122).

Grounding: VFDs require a dedicated equipment grounding conductor run with the phase conductors. The grounding conductor must be sized to handle the high-frequency currents.

Motor Protection: The VFD itself often provides overload protection for the motor. If so, separate overload relays may not be required (430.124).

Inspection/Supervision Point: Verify that the conductors between the VFD and the motor are sized correctly and that the shielding and grounding are properly installed to prevent electromagnetic interference.

1.9 Code Navigation: Quick Reference Table

Concept	NEC Article/Section	Key Table
Motor FLCs (DC)	430.6(A)(1)	Table 430.247
Motor FLCs (1-Phase)	430.6(A)(1)	Table 430.248
Motor FLCs (3-Phase)	430.6(A)(1)	Table 430.250
Branch-Circuit Conductors (Single Motor)	430.22(A)	—
Feeder Conductors (Multiple Motors)	430.24	—
Overload Protection Sizing	430.32(A), (B), (C)	—
Branch-Circuit Short-Circuit Protection	430.52	Table 430.52
Controller Rating	430.83	—
Disconnecting Means Rating	430.110	—
Disconnecting Means Location	430.102	—
Motor Control Circuits	430.71 – 430.74	—
Adjustable-Speed Drive Systems	430.120 – 430.126	—
Grounding – Separately Derived Systems	250.30	Table 250.66
Transfer Switches	701.5, 702.5	—

1.10 Common Exam Traps and Supervision Points

Exam Traps:

96.Table vs. Nameplate: Using the motor nameplate FLA instead of the Table FLC for conductor sizing or short-circuit protection.

97.The 125% Factor: Forgetting to apply the 125% factor to the largest motor only when sizing a feeder for multiple motors.

98.The "Next Size Up" Rule: Applying the "next size up" rule for branch-circuit protection when the calculated value does match a standard size.

99.Overload vs. Short-Circuit: Confusing the purpose and sizing rules for overload relays versus branch-circuit short-circuit and ground-fault protection devices.

100.Generator Grounding: Treating a generator as a non-separately derived system when it is, in fact, an SDS, leading to improper grounding and bonding.

Inspection/Supervision Points:

Verify Nameplate vs. Table: Check that the conductors and overcurrent devices are sized based on the Table FLC, not the motor's nameplate.

Check the Disconnect: Ensure the disconnect is in sight of the motor and is horsepower-rated.

Confirm Overload Sizing: Verify the overload relay heaters are sized based on the motor nameplate FLA and are properly installed.

Inspect the Grounding: For generators, confirm that the neutral is bonded to the frame and that a proper grounding electrode conductor is installed.

Review the Feeder Calculation: For a panelboard supplying multiple motors, re-calculate the feeder size to ensure it meets the requirements of 430.24.

1.11 Conclusion

Mastering motors and generators requires a systematic approach. Always start by identifying the motor's full-load current from the correct table. Then, apply the specific rules for conductors, overload protection, and short-circuit protection in the correct order. For generators, the primary focus is on the correct application of the rules for separately derived systems. By understanding the why behind the code rules and navigating the article structure with precision, you will be well-prepared for the master exam and for supervising safe, code-compliant installations in the field.

Motors & Generators