Motors & Generators

Wyoming Master Electrician Exam — 2023 NEC (NFPA 70)

Learning Objectives

Upon completing 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 disconnecting means.

7.Calculate motor branch-circuit conductor ampacity and overcurrent protection ratings using the correct tables and multipliers.

8.Size motor feeders per 430.24 and apply the demand factors of 430.26 where permitted.

9.Understand the unique requirements for generators as separately derived systems under Article 445 and 250.30.

10.Coordinate overcurrent protection for motor and generator installations, recognizing the difference between branch-circuit, short-circuit, and ground-fault protection versus overload protection.

11.Identify common inspection failures and exam traps related to motor nameplate ratings, torque, and service conditions.

1.1 The Scope of Article 430 and Its Relationship to Other Articles

Article 430 is the primary reference for motors, motor circuits, and controllers. However, a master electrician must understand how this article interacts with others. For instance, the branch circuit supplying a motor is still a branch circuit, so Part II of Article 210 applies to general requirements, but 430.6(A) supersedes the general ampacity rules. Similarly, the feeder supplying multiple motors is governed by 430.24, not the standard 220.61 optional calculations.

Key Interaction: When a motor is part of an air-conditioning or refrigeration system, Article 440 takes precedence. Do not apply standard motor tables to hermetic refrigerant motor-compressors; they have their own nameplate ratings and multiplier rules (440.6(A)).

Critical Distinction: The code makes a clear separation between overload protection (designed to protect the motor, conductors, and equipment from excessive heating due to running overloads) and branch-circuit short-circuit and ground-fault protection (designed to protect the circuit from faults). A master must never confuse the two. Overload devices are typically sized at 125% to 130% of the motor nameplate full-load current (FLC), while short-circuit protection is sized based on the table values, often up to 250% or 300% of the FLC.

1.2 Motor Circuit Conductors: Sizing Beyond the Nameplate

The most common error on the exam and in the field is using the motor nameplate current rating to size conductors. The code requires you to use the tables in 430.248 (single-phase) and 430.250 (three-phase) to find the Full-Load Current (FLC). The nameplate is used only for overload relay selection and for the separate motor-running overload protection.

Branch-Circuit Conductor Sizing (430.22):

Single Motor: Conductors must have an ampacity of not less than 125% of the motor FLC (from the tables). For a motor with a service factor of 1.15 or greater, or a temperature rise of 40°C or less, you still use 125% of the table FLC. Do not adjust for the nameplate.

Torque Motors: Sizing is based on the nameplate current rating, not the table, because these motors draw current continuously in a stalled condition.

Wound-Rotor Motors: Secondary conductors must be sized to 125% of the secondary full-load current.

Feeder Conductor Sizing (430.24):

When a feeder supplies two or more motors, the ampacity must be at least 125% of the highest-rated motor FLC plus the sum of the FLCs of all other motors on that feeder. This is a "125% + 100% + 100%..." calculation. Do not multiply every motor by 125%.

Real-World Application: A feeder supplies three motors: 10 HP, 5 HP, and 2 HP (all 208V, three-phase). From Table 430.250, the FLCs are 30.8A, 16.7A, and 7.5A respectively. The feeder calculation is (30.8 × 1.25) + 16.7 + 7.5 = 38.5 + 16.7 + 7.5 = 62.7A. You would then select a conductor with an ampacity of at least 62.7A, typically a 4 AWG THHN at 75°C (85A) or a 6 AWG if the calculated value allowed, but here 6 AWG at 65A is insufficient.

Temperature Limitations: Remember the 75°C column for equipment terminations is the default for motors and controllers rated 100A or less, unless the equipment is specifically marked for 60°C. Do not use the 90°C column for ampacity adjustment unless you are doing derating for ambient temperature or conduit fill, and even then, the final ampacity cannot exceed the 75°C rating of the terminals.

1.3 Overcurrent Protection: The Art of Coordination

Overload Protection (430.32):

Motors with SF ≥ 1.15 or Temp Rise ≤ 40°C: Overload devices shall be sized at no more than 125% of the nameplate current rating.

All Other Motors: Sizing is limited to 115% of the nameplate current rating.

Next Standard Size Rule: If the calculated value does not correspond to a standard fuse, heater, or relay size, you may use the next higher standard size, but you are capped at a maximum of 130% (for the 125% rule) or 120% (for the 115% rule) of the nameplate.

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

This is where the table values matter. The maximum rating of the protective device is a percentage of the FLC from the tables (430.248/250), not the nameplate.

Motor Type	Max % of FLC (Non-Time-Delay Fuse)	Max % of FLC (Time-Delay Fuse)	Max % of FLC (Instantaneous Trip CB)	Max % of FLC (Inverse Time CB)
Single-Phase, All Types	300%	175%	800%	250%
Squirrel Cage (Other than Design E)	300%	175%	800%	250%
Design E	300%	175%	1100%	250%
Wound Rotor	150%	150%	800%	250%
Direct Current	150%	150%	250%	200%

The "Next Size Up" Rule (430.52(C)(1) Ex. 1): If the calculated value does not correspond to a standard rating (e.g., 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600), you may go up to the next standard size. However, this allowance is strictly limited. You cannot exceed the maximum percentages listed in Table 430.52. For example, a 10 HP, 208V, three-phase motor has an FLC of 30.8A. With an inverse-time breaker, the max is 250% = 77A. The next standard size above 77A is 80A, which is permitted. But if the calculation yielded 80.5A, you could not go to 90A because 250% of 30.8A is 77A, and 90A exceeds that threshold.

Combination Controller: When a motor controller is marked "suitable for group installation," the branch-circuit protection can be higher, but this is a specialized engineering application rarely tested at the master level beyond recognition.

1.4 Motor Controllers and Disconnecting Means

Controller Requirements (430.83):

A controller must have a horsepower rating that is not less than the horsepower rating of the motor. Exception: For a stationary motor rated 2 HP or less and 300V or less, a general-use snap switch with an ampere rating of at least twice the motor FLC can be used.

Disconnecting Means (430.102 & 430.109):

The disconnecting means must be located in sight from the motor and the driven machinery. "In sight" means within 50 feet and visible.

The disconnect must be capable of being locked in the open position.

The disconnect must open all ungrounded conductors simultaneously.

For motors over 2 HP, the disconnect must be a horsepower-rated switch. For motors 2 HP or less, a general-use switch with an ampere rating of at least twice the motor FLC is acceptable.

Inspection Point: On a jobsite, a master must verify that the disconnect for a motor is not just a lighting switch. A standard toggle switch is not a motor-rated disconnect unless the motor is under 2 HP and the switch is rated for at least 200% of the motor current.

1.5 Generators: Article 445 and the Separately Derived System Concept

Generators are covered by Article 445, but their installation requirements are heavily influenced by Article 250 (grounding) and Article 700 (emergency systems) if they serve those loads.

Key Requirements (445.18):

Disconnecting Means: Generators must have a disconnect that is capable of being locked in the open position. This disconnect must open all ungrounded conductors.

Grounding: A generator is considered a separately derived system if there is no direct connection between the generator's neutral and the supply system's neutral (i.e., it is not a solidly interconnected neutral). In this case, you must install a system bonding jumper and a grounding electrode conductor per 250.30.

The Critical Exam Trap: If a generator is used as a backup for a service and the transfer switch is a "closed transition" or "make-before-break" type, the neutrals must be handled carefully. If the generator neutral is solidly bonded to the service neutral, it is not a separately derived system, and you do not install a new ground rod. If the transfer switch opens the neutral (a "4-pole" transfer switch), the generator becomes a separately derived system, requiring a new system bonding jumper and grounding electrode.

Sizing the Generator Feeder:

The feeder from the generator to the transfer switch must be sized based on the generator's rated output current. If the generator is rated 100 kW, 208V, three-phase, the output current is calculated as I = (kW × 1000) / (V × √3). For 100 kW, this is approximately 277.6A. The conductors must have an ampacity of not less than 115% of this value (per 445.13), which is 319A. You would typically select 400 kcmil copper or 600 kcmil aluminum.

Overcurrent Protection (445.12):

Generators must be protected against overcurrent. If the generator is provided with inherent overload protection that meets the requirements, no additional external overcurrent protection is needed. Otherwise, the ungrounded conductors must have overcurrent protection.

1.6 Three-Phase System Calculations for Motors

A master must be fluent in three-phase power calculations. The formula for three-phase current is:

I = (HP × 746) / (V × √3 × Efficiency × Power Factor)

However, the NEC tables in 430.250 already account for typical efficiencies and power factors. The exam will rarely ask you to calculate FLC from HP using the formula; instead, you will use the tables. But you must understand the relationship for generator sizing and feeder calculations.

Voltage Drop: While not a strict code requirement for motors (it is a recommendation in 210.19(A) Informational Note No. 4), voltage drop is a practical concern. A motor starting can draw 600% of its FLC. If the feeder is long, the voltage drop during starting can prevent the motor from starting. A master should calculate voltage drop for feeders over 100 feet. Use the formula: VD = (2 × L × I × K) / CM for single-phase, and VD = (√3 × L × I × K) / CM for three-phase, where K is the conductor constant (12.9 for copper, 21.2 for aluminum).

1.7 Code Navigation: Where to Find It

Topic	NEC Reference
Motor FLC Tables (Single-Phase)	Table 430.248
Motor FLC Tables (Three-Phase)	Table 430.250
Branch Circuit Conductor Sizing	430.22
Feeder Conductor Sizing (Multiple Motors)	430.24
Overload Protection (Running)	430.32
Short-Circuit & Ground-Fault Protection	430.52, Table 430.52
Motor Controllers	430.83
Disconnecting Means	430.102, 430.109
Motor Winding Protection (Thermal)	430.31
Adjustable Speed Drive Systems	430.120 – 430.130
Generators (General)	445.1 – 445.19
Generator Disconnect	445.18
Separately Derived System Grounding	250.30
Grounding Electrode Conductor Sizing	Table 250.66
System Bonding Jumper Sizing	250.102(C)
Emergency System Transfer Switches	700.5
Air-Conditioning (Hermetic Compressors)	440.6, 440.22
Voltage Drop (Informational)	210.19(A) Note 4

1.8 Inspection and Supervision Points for the Master

74.Nameplate vs. Table: Verify that the branch-circuit conductors and overcurrent devices are sized from the tables, not the nameplate. The nameplate is only for overload heaters.

75.Disconnect Location: Confirm the motor disconnect is within sight (50 feet) of the motor. If not, there must be a second disconnect at the motor location.

76.Lockout Capability: All disconnects must be capable of being locked in the open position. Check for a hasp or other locking mechanism.

77.Controller Rating: The controller must have a horsepower rating equal to or greater than the motor.

78.Grounding: For a separately derived generator, verify the system bonding jumper is installed at the generator (or at the first disconnecting means) and that a grounding electrode conductor is run to a permitted electrode. Do not bond the neutral at both the generator and the transfer switch if the transfer switch is a 3-pole switch.

79.Overload Heater Size: Check that the overload relay heaters are properly sized for the motor nameplate current. A common field error is leaving the factory-installed heaters in place when the motor is changed out.

1.9 Common Exam Traps

Trap 1: Using Nameplate Current for Conductors. Always use Table 430.250 for three-phase motors. The nameplate FLC is often lower than the table value.

Trap 2: Multiplying All Motors by 125% on Feeders. Only the largest motor gets the 125% multiplier. The others are added at 100%.

Trap 3: Ignoring the "Next Size Up" Cap. You can go to the next standard size, but you cannot exceed the maximum percentage in Table 430.52.

Trap 4: Treating a Generator as a Separately Derived System When It Is Not. If the neutral is solidly interconnected (e.g., a solid neutral transfer switch), it is not separately derived.

Trap 5: Applying Motor Rules to Hermetic Compressors. Article 440 uses the nameplate rating for branch-circuit selection, not the tables.

Trap 6: Forgetting the 115% vs. 125% Overload Rule. The service factor determines which multiplier you use for overload protection.

Trap 7: Sizing the Disconnect by Amperage Only. The disconnect must be horsepower-rated for motors over 2 HP.

1.10 Summary

Mastering motors and generators requires a disciplined approach to the code. Always start by identifying the type of motor and the specific article that applies. Use the tables for FLC, apply the correct multipliers for branch circuits versus feeders, and never confuse overload protection with short-circuit protection. For generators, the key is determining whether the system is separately derived, which dictates the grounding requirements. By understanding the logic behind the code—protecting conductors from heat, providing safe disconnection, and ensuring fault clearance—you will be prepared for both the exam and the responsibilities of a master electrician.

Motors & Generators