Motors and Generators

Chapter 1: Motors and Generators

Learning Objectives

Upon completing this chapter, the candidate will be able to:

1.1 General Installation Requirements (Article 430)

The Master electrician must view Article 430 not as a collection of isolated rules, but as a complete system. The Code requires that each motor have a branch circuit, a disconnecting means, a controller, and overload protection. The failure to coordinate these four elements is a primary source of exam traps.

Nameplate vs. Tables: A critical distinction exists between the motor nameplate current and the table values used for sizing conductors and protection. The full-load current (FLC) used for conductor sizing and branch-circuit protection is taken from NEC Tables 430.247 through 430.250, not the nameplate. The nameplate current is used exclusively for sizing the motor’s overload protection (per 430.32) and for the motor’s actual thermal limits. This is a classic exam trap: using nameplate amps to size a feeder conductor will result in an undersized conductor.

Terminal Housings: For motors larger than 100 hp or for motors with a maximum operating voltage over 600V, the terminal housing must be arranged so that the motor can be serviced without disturbing the connections to the controller (430.12). This is a field inspection point often overlooked on large industrial equipment.

1.2 Motor Circuit Conductors (Part II)

Branch-Circuit Conductors (430.22): The general rule is that conductors supplying a single motor must have an ampacity of not less than 125% of the motor’s FLC as listed in the tables. For a motor used in a continuous duty application, this is the baseline.

Feeder Conductors (430.24): When a feeder supplies several motors, the conductor ampacity must be sufficient for the sum of the FLCs of all motors on the circuit, plus 125% of the FLC of the highest-rated motor in the group. The calculation is:

Application Example: A feeder supplies a 25 hp, 208V, three-phase motor (FLC = 74.8A) and a 10 hp motor (FLC = 30.8A).

Motor + Loads (430.25): If a feeder supplies a motor and other loads (lighting, heating), the feeder must be sized for the sum of the other loads plus the motor load calculated per 430.24.

Voltage Drop: While the NEC does not mandate a specific percentage for voltage drop in all cases, 210.19(A) Informational Note suggests a maximum of 3% for branch circuits and 5% total. For motor circuits, the Master must account for the inrush current. Conductors must be sized to limit voltage drop to allow the motor to start; a stalled motor drawing locked-rotor current will trip overloads if the voltage collapses.

1.3 Overload Protection (Part III)

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

Sizing Rules (430.32):

The "Next Size Up" Rule (430.32(C)): If the standard sizes of overload relays (per 240.6) do not correspond to the calculated value, the next higher standard size is permitted, but only up to a maximum of 140% for motors with a 1.15 service factor, and 130% for all others. This is a specific allowance that does not apply to branch-circuit short-circuit protection.

Dual-Element Fuses vs. Inverse Time Breakers: The Master must understand that overload protection is distinct from short-circuit protection. Overload relays (heaters) are typically provided in the motor starter. If a motor is controlled by a variable frequency drive (VFD), the VFD’s internal electronic overload protection is often acceptable, provided it meets the requirements of 430.124.

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

This is the most calculation-heavy area for the Master exam. The device protects the conductors, the controller, and the motor against short circuits and ground faults, not overloads.

Sizing Rules (430.52): The maximum rating of the protective device is based on a percentage of the motor FLC (from tables), not the nameplate.

The "Next Size Up" Rule (430.52(C)(1)): If the calculated value does not correspond to a standard ampere rating (240.6), the next higher standard size is permitted. However, this allowance is strictly limited. The maximum permitted size is 800% of the FLC for motors other than those with a marked code letter, and specific caps apply based on the motor’s locked-rotor code letter (Table 430.52(B) Note).

Example: A 50 hp, 460V, three-phase motor has an FLC of 65A. Using an inverse time breaker: 65A × 250% = 162.5A. The next standard size up is 175A. This is permitted.

Motor Overload vs. Short Circuit: If the branch-circuit short-circuit protection is sized too low, it will trip on motor starting inrush (locked-rotor current). If sized too high, it may not protect the conductors. The Master must ensure the conductors have an ampacity sufficient for the overload relay setting, but the short-circuit device protects the conductor from fault current.

Coordination (430.52(C)(3)): For "coordinated" systems in critical operations (hospitals, data centers), the branch-circuit protective device may be increased beyond the 800% cap if a short-circuit study demonstrates that the device will clear a fault before the motor starter contacts weld or the conductors are damaged. This requires engineering documentation.

1.5 Disconnecting Means and Controllers (Parts V & VI)

Disconnecting Means (430.101): A disconnecting means must be provided for each motor and controller. The disconnect must be located in sight from the motor and the driven machinery. "In sight" is defined as visible and not more than 15.2 m (50 ft) from the equipment.

Controller Disconnect: The controller disconnect must open all ungrounded supply conductors. For a motor with a rating over 600V, the disconnect must be a circuit breaker or a motor-circuit switch.

Controller Rating (430.83): The controller must have a horsepower rating not lower than the horsepower rating of the motor. A controller rated for a larger motor can control a smaller one. However, a controller with a lower horsepower rating cannot control a larger motor, even if the current rating is sufficient.

Motor Disconnect (430.109): The disconnecting means must be a listed motor-circuit switch, a molded-case circuit breaker, or a listed instantaneous-trip circuit breaker. A general-use switch is only permitted for stationary motors of 2 hp or less.

1.6 Generators and Separately Derived Systems (Article 445 & 250.30)

Generators are the heart of many commercial and industrial installations. The Master must determine if the generator is a separately derived system or a non-separately derived system.

Separately Derived System (SDS): A generator is considered an SDS if there is no direct electrical connection (metallic path) between the generator’s output conductors and the service conductors, other than through the grounding and bonding path. This typically occurs when a generator feeds a transfer switch that opens the neutral conductor (a 4-pole transfer switch).

Grounding an SDS (250.30): For a separately derived system, the Master must:

Non-Separately Derived System: If the generator is connected to a transfer switch that does not switch the neutral (a 3-pole transfer switch), the generator is not an SDS. In this case, the generator’s neutral is solidly connected to the utility neutral, and the ground-fault current path is back to the utility source. The generator frame must be bonded to the equipment grounding conductor, but a separate grounding electrode is not required (though it is often recommended for lightning protection).

Generator Conductors (445.13): The ampacity of the conductors from the generator terminals must be not less than 115% of the generator’s nameplate current rating. This is a distinct rule from motor conductors.

Generator Protection (445.12): Generators must be protected from overloads. If the generator is driven by a prime mover (engine), the protective device must be rated to carry the rated current of the generator continuously, but must trip on short circuit.

1.7 Commercial and Industrial Applications

Feeder Taps for Motor Circuits (430.28): The NEC permits taps for motor branch circuits under specific conditions. A tap conductor must have an ampacity of at least one-third of the rating of the overcurrent device protecting the feeder, and must terminate in a single circuit breaker or set of fuses. The tap must be enclosed and not exceed 7.6 m (25 ft) in length.

VFD Installations (430.120-430.132): Variable frequency drives are treated as "power conversion equipment." The input conductors are sized per 430.122, which requires the branch-circuit conductor ampacity to be not less than 125% of the rated input current of the VFD. The output conductors (motor side) are sized per 430.22. The VFD’s internal overload protection is acceptable if it meets the requirements of 430.124.

Inspection Point: When inspecting a VFD installation, verify that the motor thermal protection is set to the motor nameplate FLC, not the VFD’s rated output current. A common mistake is leaving the VFD at its factory default of 100% of its own rating, which may not protect a smaller motor.

1.8 Code Navigation

1.9 Inspection and Supervision Points

As a Master, you are responsible for the final sign-off. Verify the following on every motor installation:

1.10 Common Exam Traps