Master Electrician Practice
Chapter III

Motors & Generators

Master Electrician Practice study guide with diagrams.

Motors & Generators — Master Exam Study Chapter

NH-MST | 2023 NEC (NFPA 70) | Open Book Reference

Learning Objectives

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

6.Apply the general requirements of Article 430 to motor branch circuits, feeders, and controllers, including the selection of conductor ampacity and overcurrent protection.
7.Calculate motor branch-circuit, feeder, and short-circuit/ground-fault protection sizes using the correct tables and percentages for various motor types and duty cycles.
8.Distinguish between the requirements for AC and DC motors, and between general motors and those used in specific applications like hermetic refrigerant compressors (Art. 440).
9.Identify the unique grounding, bonding, and overcurrent protection requirements for generators as separately derived systems (Art. 445, 250.30).
10.Supervise the installation of motor control circuits, disconnecting means, and controllers, ensuring compliance with clearance, location, and safety requirements.
11.Navigate the NEC efficiently to find motor and generator requirements, avoiding common code application errors.

1.1 The Scope of Article 430 — Beyond the Basics

Article 430 is the central hub for motor circuits and controllers. A master electrician must understand that this article does not exist in a vacuum. It interacts constantly with Article 240 (Overcurrent Protection) , Article 250 (Grounding and Bonding) , and Article 409 (Industrial Control Panels) . When you supervise a motor installation, you are not just checking the motor; you are checking the entire circuit from the last branch-circuit overcurrent device to the motor terminals.

Key Master-Level Concept: The NEC treats a motor circuit as a system with distinct components, each with its own sizing rules:

Branch-Circuit Conductors (430.22)
Branch-Circuit Short-Circuit and Ground-Fault Protection (430.52)
Motor Overload Protection (430.31 – 430.37)
Motor Controllers (430.81 – 430.91)
Disconnecting Means (430.101 – 430.113)

A common journeyman error is to use a single calculation for all of these. A master knows that the branch-circuit protection (fuse/breaker) is sized to handle starting current (inrush), while the overload relay is sized to protect the motor against running overcurrent. These are two entirely different calculations.

1.2 Motor Circuit Conductors — Sizing and Ampacity

1.2.1 Branch-Circuit Conductors (430.22)

Motor Branch Conductors: 125% Path Motor Branch Conductors: 125% Path NEC 430.6(A)(1) → 430.22(A) → 110.14(C) → Table 310.16 25 hp Motor 460V, 3-Phase Continuous Duty Nameplate: 31A Table 430.250 FLC = 34 A Never nameplate! 430.6(A)(1) Look up 430.22(A) Calc 34 A × 1.25 = 42.5 A 125% continuous duty Minimum ampacity Table 310.16 75°C Column 8 AWG THHN = 50 A 110.14(C) termination rule ✓ CORRECT SIZE 8 AWG THHN 50 A ≥ 42.5 A ✓ Terminations rated 75°C ⚠ TRAP LANE — Common Master Exam Errors ✗ Trap 1: Nameplate Using 31A nameplate instead of 34A Table FLC → undersized by 8% ✗ Trap 2: 90°C Column 14 AWG at 90°C = 25A looks OK but terminations rated 75°C → 110.14(C) ✗ Trap 3: Adjustments Ambient/bundling derating applies on top of 42.5A, not in place of 125% + ambient/bundling adjustments on top Master Electrician Practice — NEC 430.22(A) motor branch conductor sizing

The general rule is that conductors supplying a single motor must have an ampacity of not less than 125% of the motor's full-load current (FLC) as listed in the appropriate tables (430.247 – 430.250). This is not a suggestion; it is the minimum.

Master-Level Application:

Continuous Duty: The 125% factor is the starting point. You must then apply any correction factors for ambient temperature (310.15) and adjust for the number of current-carrying conductors in the raceway (Table 310.15(C)(1)). The final conductor size must be the one that satisfies all conditions.
Multiple Motors on One Branch Circuit: Per 430.22(C), the conductors must be sized at 125% of the largest motor FLC plus the sum of the FLCs of all other motors on that circuit.
Wound-Rotor Motors: The secondary conductors of a wound-rotor motor are sized at 125% of the full-load secondary current. This is a detail often overlooked.

1.2.2 Feeder Conductors (430.24)

Feeder conductors supplying two or more motors must have an ampacity of 125% of the highest-rated motor FLC + the sum of the FLCs of all other motors on the feeder. This is a critical calculation for commercial and industrial work. Note the distinction: it is 125% of the largest motor, not 125% of the total load.

1.3 Overcurrent Protection — The Heart of the Matter

1.3.1 Motor Overload Protection (Part III, 430.31)

Overload protection is designed to protect the motor, the motor control apparatus, and the motor branch-circuit conductors against excessive heating due to motor running overloads and failure to start.

The Rule: Each continuous-duty motor rated more than 1 HP (or 1 kW for DC) must have an overload device sized at not more than 125% of the motor's nameplate full-load current rating (430.32(A)(1)).
The Exception: If the overload device sized at 125% is not sufficient to allow the motor to start, the next higher standard size is permitted, but never exceeding 140% of the nameplate rating for motors with a service factor of 1.15 or greater, or a temperature rise of 40°C or less. For all other motors, the absolute maximum is 130%.

Master's Inspection Point: Always verify the overload relay's heater element size against the motor nameplate, not the table FLC. The tables are for conductor sizing and branch-circuit protection; the nameplate is for overload protection.

1.3.2 Branch-Circuit Short-Circuit and Ground-Fault Protection (430.52)

Motor SCGF: Table 430.52 Max — NH Master Electrician (NEC 2023) Motor SCGF: Table 430.52 Max NEC 2023 · 430.52(C)(1) Exceptions 1 & 2 · 430.32 — NH Master MOTOR 34 A FLC OL RELAY 430.32 SCGF DEVICE Table 430.52 → to panel Table 430.52 Max Rating Inverse-time CB 250% × 34 A = 85 A max before next-size rule Time-delay fuse 175% × 34 A = 59.5 A next size = 60 A fuse Non-time-delay fuse 300% × 34 A = 102 A max before next-size rule Exception No. 1 — Next Size Up 430.52(C)(1) Exc. 1: if standard rating doesn't match, use next standard size. 85 A → 90 A breaker ✓ 102 A → 110 A fuse ✓ 59.5 A → 60 A fuse (std size) Exception No. 2 — 400% Max If starting current trips the lower device, rating may be increased but NOT above: 400% × 34 A = 136 A max Must prove starting current issue ⚠ TRAP — Master Depth SCGF device ≠ overload relay. Overload stays at 430.32: 125% × 34 A ≈ 45 A heater SCGF may exceed conductor ampacity. Motor conductors are jointly protected by OL relay (430.32) + branch device (430.52). Conductor ampacity per 430.22: 125% × 34 A = 42.5 A → 8 AWG @ 75°C (per Table 310.16) 90 A breaker protects this conductor because OL relay handles overloads. Key: 430.52(C)(1) table gives MAX before next-size; Exc. 1 allows next standard size; Exc. 2 allows up to 400% only for starting current. Overload relay (430.32) remains separate at ~125% FLC — this is the master-level distinction. Master Electrician Practice — NEC 430.52 Motor SCGF sizing

This is the device that protects the circuit from short circuits and ground faults. It is not the overload protection.

The Rule: The maximum rating of the protective device is determined by a percentage of the motor FLC from the tables (430.247 – 430.250), based on the type of motor and the type of protective device (inverse-time breaker, instantaneous trip breaker, or time-delay fuse).
The Table: Table 430.52 is your primary reference. For example, a NEMA Design B motor (the most common) with an inverse-time breaker is limited to 250% of FLC. With a time-delay fuse, it is 175%.
The "Next Size Up" Rule: If the calculated value does not correspond to a standard fuse or breaker size (240.6), you are permitted to go up to the next standard size. This is a critical allowance for starting current.
The "Why": This device must allow the high inrush current (typically 6–8 times FLC) during starting to pass without opening, but it must open quickly on a dead short.

Master's Trap: The "next size up" rule applies to the branch-circuit short-circuit protection. It does not apply to conductor sizing (430.22) or overload protection (430.32). You cannot use it to justify a larger conductor.

1.4 Motor Controllers and Disconnecting Means

1.4.1 Controllers (Part VII)

A controller is any switch or device that is normally used to start and stop a motor. The controller must have an ampere rating not less than 115% of the motor's FLC (430.83). For a motor over 100 HP, the controller must be rated for the motor's horsepower.

Master-Level Supervision: You must ensure the controller is capable of interrupting the stalled-rotor current of the motor. A simple light switch is not a compliant motor controller for a large motor.

1.4.2 Disconnecting Means (Part IX)

Location: The disconnecting means must be in sight from the motor and the driven machinery location (430.102). "In sight" means visible and not more than 15 m (50 ft) from the equipment.
Type: It must be a motor-circuit switch rated in horsepower, or a circuit breaker.
The Exception: If the disconnecting means is not in sight, it must be capable of being locked in the open position (lockable) and must be provided with a warning sign at the motor location.
Controller Disconnect: The controller must also have a disconnecting means (430.102(A)). This can be the same device if it is in sight.

Master's Trap: The disconnecting means for the motor must disconnect the motor and the controller from all ungrounded supply conductors. It is a common error to only disconnect the controller, leaving the motor leads energized from a separate source.

1.5 Generators — Article 445 and Separately Derived Systems

Generators are a major part of the master's scope, especially in commercial and industrial settings. The NEC treats them as power sources, and the rules are found in Article 445 and Article 250 (Part II) .

1.5.1 Generator Sizing and Rating (445.10)

The generator must have a nameplate showing the rated voltage, current, power factor, and frequency. The ampacity of the generator's conductors must be not less than 115% of the generator's nameplate current rating (445.13). This is a different factor than the 125% used for motors.

1.5.2 Overcurrent Protection (445.12)

Generators are required to have overcurrent protection. The protective device must be rated to carry the generator's rated current. If the generator is a separately derived system, the overcurrent device is typically located at the point where the generator output conductors connect to the distribution system.

1.5.3 Separately Derived Systems (250.30)

This is the most critical concept for a master. A generator is a separately derived system when there is no direct electrical connection between the generator's output and the supply system's grounded conductor (neutral). This is the case for most portable and standby generators with a transfer switch that opens the neutral.

Master-Level Requirements for a Separately Derived System (250.30(A)):

69.Grounding Electrode: The system must have a grounding electrode conductor connected to a grounding electrode (e.g., a ground rod). The size of this conductor is based on the largest ungrounded supply conductor (Table 250.66).
70.System Bonding Jumper: The grounded conductor (neutral) must be bonded to the equipment grounding conductor and the grounding electrode conductor at the source (the generator) or at the first disconnecting means. This is the single point of bonding for that system.
71.Equipment Grounding: All non-current-carrying metal parts of the generator and the downstream equipment must be connected to the equipment grounding conductor.

Master's Trap: If the generator is a non-separately derived system (i.e., the neutral is solidly connected to the utility neutral, as in a permanently installed generator with a solidly connected neutral), the rules are different. In that case, you do not install a new grounding electrode, and the neutral is not re-bonded to the frame. This is a common point of failure on inspections.

1.6 Special Applications — Hermetic Refrigerant Compressors (Article 440)

Hermetic Compressor Circuits (440) - Master Depth Hermetic Compressor Circuits — Art. 440 NH Master Electrician — NEC 2023 / NFPA 70 • Open-Book Master Depth M Hermetic Compressor Nameplate Rated-Load Current 24 A (not Table 430.250) Conductors NEC 440.32 Conductor ampacity 125% × rated-load 24 × 1.25 = 30 A ✓ 30 A min per Table 310.16 (10 AWG @ 60°C) NEC 440.22(A) SCGFD protection 175% of rated-load 24 × 1.75 = 42 A ✓ 45 A breaker (next size up) If needed for starting: 225% = 54 A max NEC 440.12 Disconnecting means 115% of rated-load 24 × 1.15 = 27.6 A ⚠ Misapplication Trap ✗ Using Table 430.250 (motor FLC tables) gives wrong answer — Art. 440 uses nameplate RLC ✗ Applying Table 430.52 percentages to compressor often undersized! Master Electrician Practice — NEC 440.32 / 440.22(A) / 440.12 • Hermetic compressor branch-circuit sizing Art. 440.12

Article 440 is an overlay on Article 430 for a specific type of motor. A hermetic compressor has the motor and compressor sealed inside a common housing. The motor is cooled by the refrigerant, so its characteristics are different.

Key Differences:

Rated-Load Current (RLC): Instead of using the motor FLC tables, you use the nameplate RLC of the compressor.
Branch-Circuit Conductors: Sizing is based on 125% of the RLC (440.32).
Overload Protection: The overload device must be selected to trip at not more than 140% of the RLC (440.52(A)(1)).
Short-Circuit Protection: The maximum fuse/breaker size is based on a percentage of the RLC, but the percentages in Table 440.22(A) are different from Table 430.52. For example, the maximum for a time-delay fuse is 175% of RLC, and for an inverse-time breaker, it is 225%.

Master's Trap: Do not use the motor FLC tables for a hermetic compressor. The RLC on the nameplate is the governing factor. This is a frequent source of exam errors.

1.7 Code Navigation — Quick Reference

ConceptNEC 2023 Location
**Motor Article Scope**Article 430
**Motor FLC Tables**Tables 430.247, 430.248, 430.249, 430.250
**Branch-Circuit Conductors**430.22
**Feeder Conductors (Multiple Motors)**430.24
**Motor Overload Protection**430.32, 430.33
**Branch-Circuit Short-Circuit Protection**430.52, Table 430.52
**Standard Fuse/Breaker Sizes**240.6
**Motor Controllers**430.81 – 430.91
**Disconnecting Means**430.101 – 430.113
**Hermetic Compressors**Article 440
**Generators**Article 445
**Separately Derived Systems**250.30
**Grounding Electrode Conductor Sizing**Table 250.66
**Conductor Ampacity Correction**310.15, Table 310.15(C)(1)

1.8 Inspection and Supervision Points

As a master, you are responsible for the final sign-off. On a motor installation, check these items in the field:

89.Conductor Sizing: Verify the conductors are sized for 125% of FLC plus any derating factors. Check the temperature rating of the terminals.
90.Overload Heater Sizing: Match the heater element to the motor nameplate FLC, not the table value.
91.Disconnect Location: Confirm the disconnect is within sight (≤50 ft) and is a horsepower-rated switch or breaker.
92.Controller Rating: Ensure the controller's ampere rating is at least 115% of the motor FLC.
93.Generator Bonding: For a separately derived generator, verify there is exactly one system bonding jumper and a proper grounding electrode conductor. Check that the neutral is not bonded at the transfer switch if it is bonded at the generator.
94.Lockout/Tagout: Ensure the disconnecting means is capable of being locked in the open position for maintenance.

1.9 Common Exam Traps

Trap 1: Using the wrong FLC. For a standard motor, use the tables. For a hermetic compressor, use the nameplate RLC.
Trap 2: Applying the "next size up" rule to conductors. This rule is only for the short-circuit protection device, not for conductor ampacity.
Trap 3: Confusing overload protection with short-circuit protection. They serve different purposes and have different sizing rules.
Trap 4: Forgetting the 115% factor for generator conductors. It is not 125% like a motor.
Trap 5: Misapplying the separately derived system rules. If the generator neutral is solidly connected to the utility, it is not a separately derived system, and you must not install a new grounding electrode or bond the neutral again.
Trap 6: Ignoring the "in sight" rule. The motor disconnect must be visible and within 50 feet. If not, it must be lockable and have a warning sign.

This chapter provides the foundational theory and code references necessary for the motors and generators portion of the NH Master Electrician exam. Focus on the relationships between the articles and the why behind the rules. A master does not just memorize numbers; they understand the system.

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