Motors & Controls

Motors & Controls — Vermont Master Electrician Exam Study Chapter

Learning Objectives

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

1.1 The Scope and Structure of Article 430

Article 430 is the single most important article for motor installations. It is a self-contained system of rules that governs every aspect of a motor circuit, from the branch-circuit conductors to the final overload protection. As a master electrician, you must understand that a motor circuit is not a simple "wire and breaker" scenario. It requires a layered approach:

A common journeyman-level error is treating the motor circuit like a lighting circuit. A master understands that the SCGFP is not sized to protect the motor from overload; that is the job of the overload relay. The SCGFP is sized to protect the conductors and the motor from short circuits and ground faults, and it is deliberately sized larger than the running current to permit the high inrush current during starting.

1.2 Motor Full-Load Currents: Tables 430.247–430.250

The foundation of all motor calculations is the full-load current (FLC), not the nameplate current rating. The NEC mandates that you use the values from Tables 430.247 through 430.250 for all calculations involving conductor ampacity, disconnecting means, and SCGFP sizing.

Critical Master Point: The motor nameplate full-load amperes (FLA) is used only for sizing the overload protection (430.32). The table FLC is used for everything else. This is a classic exam trap. For example, a 10-hp, 3-phase, 460-volt motor may have a nameplate FLA of 13.2 A, but Table 430.250 gives an FLC of 14 A. You must use 14 A for conductor sizing and SCGFP sizing.

1.3 Sizing Branch-Circuit Conductors (430.22)

The general rule for a single motor is that the branch-circuit conductors must have an ampacity of not less than 125% of the motor's FLC from the tables.

Formula: Conductor Ampacity ≥ FLC (from Table 430.250) × 1.25

Example: A 25-hp, 3-phase, 208-volt motor has a table FLC of 74.8 A. The minimum conductor ampacity is 74.8 A × 1.25 = 93.5 A. You would select a conductor with a 75°C termination rating, such as a #3 AWG THHN (rated 100 A at 75°C), not a #4 AWG (rated 85 A).

Special Cases (430.22):

1.4 Sizing Motor Feeders (430.24)

A feeder supplying two or more motors must have an ampacity sufficient for the sum of all the loads. The rule is:

Feeder Ampacity = (125% × FLC of the largest motor) + (Sum of FLCs of all other motors) + (Other loads on the feeder)

This is a critical calculation for commercial and industrial work. The 125% factor applies only to the single largest motor on the feeder, not to all of them.

Example: A feeder supplies three motors: a 10-hp (14 A), a 15-hp (21 A), and a 25-hp (34 A) at 460V, 3-phase. The feeder ampacity must be at least (34 A × 1.25) + 21 A + 14 A = 42.5 A + 35 A = 77.5 A. You would size the conductors for at least 77.5 A.

Important: This calculation is for the conductors. The feeder overcurrent protection is sized differently (see 430.62).

1.5 Overload Protection (430.31–430.40)

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

Sizing Rules (430.32):

Example: A motor has a nameplate FLA of 10 A and a service factor of 1.15. The maximum overload relay size is 10 A × 1.25 = 12.5 A. You would select the next standard size up if the relay is not adjustable, but you cannot exceed the 125% limit unless the relay is set to trip at or below 12.5 A.

The "Next Size Up" Rule (430.32(C)): If the motor is not overloaded when it is running, and the motor is not part of a group installation, you may use the next higher standard size of overload relay if the calculated value does not correspond to a standard size. However, the relay must be set to trip at no more than the calculated value. This is a common point of confusion.

Dual-Element Time-Delay Fuses as Overloads: In some cases, dual-element time-delay fuses can be used for overload protection, but they must be sized per 430.36 and are limited to 125% of the motor nameplate current for motors with a service factor of 1.15 or more.

Master Supervision Point: On site, always verify the motor nameplate service factor and temperature rise. A motor with a 1.0 service factor is common in modern, high-efficiency designs. These motors are much less forgiving, and the overloads must be set at 115% of nameplate, not 125%.

1.6 Branch-Circuit Short-Circuit and Ground-Fault Protection (430.51–430.58)

This is the device that protects the branch-circuit conductors, the controller, and the motor against overcurrents due to short circuits and ground faults. It is not an overload device.

Sizing Rules (430.52): The maximum rating or setting of the SCGFP is based on a percentage of the motor FLC from the tables. Table 430.52 provides these percentages:

Example: A 15-hp, 3-phase, 460-volt Design B motor has an FLC of 21 A. Using an inverse-time circuit breaker, the maximum SCGFP rating is 21 A × 250% = 52.5 A. You would select a 50 A breaker. If you use a dual-element time-delay fuse, the maximum is 21 A × 175% = 36.75 A, so you would select a 35 A fuse.

The "Next Size Up" Rule (430.52(C)(1) Ex. 1): If the calculated value does not correspond to a standard rating, you may go up to the next standard size. However, this is a maximum limit. You can always use a smaller device, provided it allows the motor to start.

Master Supervision Point: The most common field violation is oversizing the SCGFP. A 60 A breaker on the 15-hp motor above would be a code violation. The breaker is there to protect the conductors from a short circuit; a larger breaker may not open fast enough to prevent catastrophic damage to the motor and conductors.

1.7 Sizing Feeder Overcurrent Protection (430.62)

The feeder SCGFP is sized to protect the feeder conductors. The rule is more complex than for a single motor.

Rule (430.62(A)): The feeder protection device must be rated to protect the feeder conductors, but it must also be sized to allow the largest motor on the feeder to start. The maximum rating is:

Feeder SCGFP Max = (Largest Motor SCGFP rating per 430.52) + (Sum of FLCs of all other motors)

Example: Using the feeder from Section 1.4 (10-hp, 15-hp, 25-hp motors), and assuming all are Design B with inverse-time breakers:

You would select a 100 A or 110 A breaker, whichever is standard, but you cannot exceed 120 A. Note that you do not add 125% of the largest motor's FLC here; you use the maximum SCGFP rating for that motor.

1.8 Controllers and Disconnecting Means (430.81–430.110)

Controllers (430.81): A controller is any switch or device that is normally used to start and stop a motor. It must be rated for the horsepower of the motor, or be a listed motor controller. A simple snap switch (light switch) can only be used for motors rated at 2 hp or less and 300 V or less (430.83(C)).

Disconnecting Means (430.102):

The "In Sight" Rule (Article 100): "In sight" means the equipment is visible and not more than 15 m (50 ft) apart. This is a critical definition for a master to enforce on site.

Disconnect Rating (430.109): The disconnecting means must have a horsepower rating not less than the motor's horsepower. A general-use switch with a horsepower rating can be used. For motors over 100 hp, the disconnecting means must be a motor-circuit switch, a molded-case circuit breaker, or a listed industrial control panel.

1.9 Adjustable-Speed Drive Systems (430.120–430.132)

Variable Frequency Drives (VFDs) are ubiquitous in modern commercial and industrial installations. Article 430, Part X, has specific requirements.

Master Supervision Point: VFDs generate harmonic currents. The neutral conductor in a 4-wire system supplying nonlinear loads must be counted as a current-carrying conductor for derating purposes (310.15(E)). This is a subtle but critical detail for feeder sizing to VFD panels.

1.10 Generators (Article 445)

Generators are treated as separately derived systems when they have no direct connection to the utility source. Key points for a master:

Code Navigation: Where to Find It

Inspection and Supervision Points

As a master, you are responsible for the final sign-off. Here is your checklist:

Common Exam Traps