Motor Controller Overview

Motor controllers are defined in NFPA 70 (NEC) as “any switch or device that is normally used to start and stop a motor by making and breaking the motor circuit current”. The controller might utilize manual or automatic means to start and stop the motor and might also perform other supplementary functions including:

Motor starters are a form of motor controller that include the switching means necessary for starting and stopping the motor in combination with overload protection. Manual motor starters utilize contacts that are directly operated by mechanical means when a toggle switch or pushbutton is actuated by hand. Magnetic motor starters utilize a contactor with an electromagnetic coil, which may be electrically operated by control devices actuated by hand (either locally or remotely) or by automatic means. Combination motor starters combine a magnetic motor starter with an externally operable disconnecting means in a common enclosure. A contactor alone (without additional integral overload protection) might be utilized for motor control where overload protection is provided separately or where the motor contains integral thermal protectors.

Magnetic motor starters are classified by their starting methods, which are described in NEMA ICS 2 - Industrial Control and Systems Controllers, Contactors and Overload Relays Rated 600 Volts. Full-voltage starters are often called "across-the-line" starters because they apply full line voltage to start the motor. This results in a high starting current (typically 600 percent of full load current for standard motors, and higher for high-efficiency motors) and high starting torque. Reduced-voltage starters use various methods to limit starting current and/or torque. Reduced-voltage starting is commonly applied to larger motors, often to comply with utility company requirements for limiting line voltage dip. Multi-speed starters reconnect motor windings to achieve selected speeds. Reversing starters reconnect motor windings for rotation in either direction. Keep in mind that certain starting methods can only be applied to compatible motors with special winding arrangements. Common available starter configurations include:

Motor Protective Devices

Overcurrent protection for motors consists of protection against faults, such as short circuits and ground-faults, and protection against overloads. This protection is usually provided by a combination of fuses or circuit breakers for fault protection and overload relays for overload protection, though in some cases protection requirements may be satisfied by a single device. Overloads are defined in NFPA 70 (NEC) as “the operation of equipment in excess of rated ampacity that, when it persists for a sufficient length of time, would cause damage or dangerous overheating.” Causes of motor overloads include:

Overload relays are reusable and resettable and are of either the thermal or solid-state electronic type. Thermal overload relays utilize interchangeable current elements called “heaters”. Current elements/heaters use either a melting eutectic alloy (solder) or a bimetallic strip to open contacts and remove power from the motor upon sensing excessive heat from an overload condition. Solid-state electronic overload relays do not sense heat but rather use a specific measured current to determine overload conditions. Considerations for selection of overload relays include:

Disconnects for combination motor starters are of either the fusible switch or circuit breaker type. Circuit breaker disconnects use either motor circuit protectors or thermal magnetic circuit breakers. Motor circuit protectors are instantaneous-trip (magnetic-only) circuit breakers furnished with magnetic instantaneous tripping elements for short circuit protection, but not thermal inverse time tripping elements for overload protection (thermal magnetic circuit breakers are furnished with both types of tripping elements). Motor circuit protectors are only permitted for use as part of a listed combination motor controller with overload protection.

NEMA vs IEC Motor Controllers

Motor controllers are designed to meet standards published by either NEMA (National Electrical Manufacturers Association) or IEC (International Electrotechnical Commission).

NEMA/IEC motor controller comparison:

For a more in-depth evaluation of NEMA and IEC controllers, refer to NEMA ICS 2.4 – NEMA and IEC Devices for Motor Service-A Guide for Understanding the Differences.

NEMA motor controllers are most commonly specified in North America and IEC motor controllers are predominant in Europe, Asia, and most other global markets. IEC controller use in North America is primarily for installation in manufactured products and/or control panel assemblies by original equipment manufacturers (OEMs). Consequently the section text and this supporting document describe only NEMA controllers.

NEMA Motor Control Centers

Motor control centers are defined in NFPA 70 (NEC) as “an assembly of one or more enclosed sections having a common power bus and principally containing motor control units.” They provide an alternative to grouping individual enclosed motor controllers in a common location. Motor control centers may contain a combination of equipment including:

Wiring classes and types for NEMA motor control centers (MCCs) are defined in NEMA ICS 18 – Motor Control Centers.

NEMA Wiring Classes:

NEMA Wiring Types:

Arc-resistant motor control centers are available to reduce arc flash risk for personnel by containing and redirecting incident energy. IEEE C37.20.7 provides methods for testing switchgear for arc-resistant functionality. Although this standard does not specifically address motor control centers, it is commonly applied for that purpose. A revision to the standard incorporating an annex covering motor control centers has been proposed. IEEE C37.20.7 defines accessibility types, which may include suffix designations:

Motor Control Circuits

Most motor control circuits utilize either two-wire control or three-wire control. Two-wire control employs maintained contact control devices and three-wire control employs momentary contact devices. Two-wire control circuits provide low-voltage release, which is the automatic restarting of the motor after a power interruption and subsequent power restoration. Three-wire control circuits provide low-voltage protection, which requires the motor to be manually restarted after a power interruption and subsequent power restoration. Two-wire control is applied where automatic restarting of the motor after power interruption is desirable, and three-wire control is applied where automatic restarting of the motor after power interruption could create a hazardous condition for the operator.

Auxiliary contacts are often provided as a standard feature on magnetic motor starters or are available as an optional accessory. The auxiliary contacts operate in unison with the main power contacts and are used to perform various control and/or status indication functions, such as providing a holding circuit to keep the starter contactor coil energized after a momentary contact start pushbutton operator has been released.

Pilot devices provide local control and/or status indication for motor controllers and may include pushbuttons, selector switches, and indicating (pilot) lights. Pushbuttons and selector switches use manually controlled operators to open and close contacts. Pushbuttons are typically momentary-contact type (i.e. contacts change state upon button press and return to their normal state upon button release) and selector switches are typically maintained-contact type (i.e. contacts remain in their current state until a manual change in switch position). Pushbuttons may be furnished with or without illumination and are available with a variety of button operator styles (e.g. flush, extended). Selector switches may be furnished with knob, lever, or key operators and are available in multiple configurations (e.g. 2-position, 3-position, etc.). Indicating lights may be furnished with LED, incandescent, or neon lamp sources and are available with lenses in a variety of colors or clear. Devices may be standard or heavy duty (as defined in NEMA ICS 5) and are generally available in two nominal sizes, 22 mm and 30 mm.

Control and timing relays are used in motor control circuits to perform various functions. Timing relays may be pneumatic or solid-state, with timed contacts applied for either on or off delay. Pneumatic timing relay timing period is controlled by the rate at which air moves through an adjustable or fixed orifice.

Control power transformers are used to deliver control power to the motor controller at the appropriate voltage. They are sized to accommodate the burden of all connected contactor coil(s) and auxiliary devices, and often include designated spare capacity. Control power may also be derived at line voltage or from a separate control power source.

Sequencing control involves the interconnection of magnetic starter control circuits to start and stop a number of separate motors in a definite sequence. Sequencing control is often applied to prevent large inrush currents resulting from multiple motors starting at the same time.

NFPA 70 (NEC) Requirements

Article 430 contains requirements for motors, motor circuits, and motor controllers, including parts specifically covering the following subjects:

Article 440 contains requirements for air-conditioning and refrigeration equipment and associated branch circuits and controllers, including special considerations necessary for hermetic refrigerant motor-compressors.

Key Requirements:

Other Applicable Standards

IEEE C37.20.7 – IEEE Guide for Testing Metal-Enclosed Switchgear Rated up to 38 kV for Internal Arcing Faults; provides basis for testing and rating of arc-resistant motor control centers.

NECA 402 - Standard for Installing and Maintaining Motor Control Centers.

NEMA 250 - Enclosures for Electrical Equipment (1000 Volts Maximum); classifies enclosure types according to degree of protection provided; common NEMA types available for motor control center enclosures include:

NEMA ICS 1 - Industrial Control and Systems General Requirements; describes usual service conditions for industrial control and systems equipment, including altitude and ambient temperature limits.

NEMA ICS 2 - Industrial Control and Systems Controllers, Contactors and Overload Relays Rated 600 Volts; includes parts specifically covering the following subjects:

NEMA ICS 2.3 - Instructions for the Handling, Installation, Operation, and Maintenance of Motor Control Centers.

NEMA ICS 5 - Industrial Control and Systems:  Control Circuit and Pilot Devices; includes parts specifically covering the following motor control components:

NEMA ICS 6 - Industrial Control and Systems:  Enclosures (references NEMA 250 for enclosure types, described above).

NEMA ICS 18 – Motor Control Centers; includes definitions of wiring classes and types (see NEMA Motor Control Centers above for detailed descriptions).

NEMA KS 1 - Heavy Duty Enclosed and Dead-Front Switches (600 Volts Maximum).

NETA ATS – Acceptance Testing Specifications for Electrical Power Equipment and Systems; includes the following applicable sections:

UL 98 - Enclosed and Dead-Front Switches.

UL 489 - Molded-Case Circuit Breakers, Molded-Case Switches and Circuit Breaker Enclosures.

UL 845 - Motor Control Centers.

UL 869A - Reference Standard for Service Equipment.

UL 1053 - Ground-Fault Sensing and Relaying Equipment.

How to Specify

Start under PART 2, progress to PART 3, and finish up with PART 1.

MANUFACTURERS Article:  Specify acceptable manufacturers. A basis of design manufacturer or product may be specified if applicable.


“Description” Paragraph:  Indicate whether or not motor control centers are arc-resistant type. Before specifying arc-resistant motor control centers, consider the additional cost of equipment and design issues related to discharge of arc flash gases which often involves a plenum and associated ductwork to a designated area, usually outdoors.

“Configuration” Paragraph:
•    Indicate arrangement. Use option “front-mounted units, front- and rear-mounted units, or back-to-back configuration as indicated” if arrangement is indicated on drawings.
•    Select NEMA classification and wiring type. See NEMA Motor Control Centers above for detailed descriptions. Class I, Type B wiring is generally the most common. Class II wiring may be appropriate where more complex interconnection and remote control is involved, keeping in mind that the additional manufacturer engineering and factory wiring generally adds to both cost and lead time.

“Arc-Resistance Rating” Paragraph:

Include “Service Entrance Motor Control Centers” paragraph where applicable.

Include “Motor Control Centers with Busway Transitions” paragraph where applicable.

“Service Conditions” Paragraph:  NEMA ICS 1 (referenced by NEMA ICS 18) describes usual service conditions for industrial control and systems equipment, including altitude and ambient temperature limits. Consult manufacturer for unusual service conditions and edit paragraphs accordingly.

“Short Circuit Current Rating” Paragraph:  Motor control center sections and units are each assigned short circuit current ratings independently. Combination motor control units may be assigned a short circuit current rating that is higher than the interrupting capacity of the overcurrent protective device in the unit. Series ratings are permitted (as described in NEMA ICS 18), but are not commonly applied. NFPA 70 (NEC) 240.86 includes requirements for application of series ratings, including limitations on motor contribution.

Include “Selectivity” paragraph where applicable. Among other applications, selective coordination may be required to comply with NFPA 70 (NEC) requirements for emergency systems (see Article 700), legally required standby systems (see Article 701), or critical operations power systems (see Article 708). NEMA ABP 1 – Selective Coordination contains useful information on selective coordination.

Include “Main Devices” paragraph where applicable.

“Bussing” Paragraph:

“Conductor Terminations” Paragraph:  Specify main and neutral lug material and type. Aluminum lugs are typically standard with copper lugs offered as an optional feature for an additional cost. Since copper lugs are not suitable for use with aluminum conductors, do not specify them unless only copper conductors will be used. Mechanical lugs are typically standard with compression lugs offered as an optional feature for an additional cost.

“Enclosures” Paragraph:

Include “Surge Protective Devices” paragraph if motor control center may contain factory-installed, internally mounted surge protective devices. Coordinate with Section 26 4300 - Surge Protective Devices.

Include “Ground Fault Protection” paragraph where applicable. Among other applications, ground fault protection may be required to comply with NFPA 70 (NEC) 215.10, 230.95, and 240.13. NEMA PB 2.2 - Application Guide for Ground Fault Protective Devices for Equipment includes useful information on ground fault protection.

Include “Arc Flash Energy-Reducing Maintenance Switching” paragraph where applicable. Energy-reducing maintenance switching might be one solution for complying with arc energy reduction requirements of NFPA 70 (NEC) 240.87, revised for 2014 edition to apply to any circuit breaker 1200A or higher (previously applied only to circuit breakers with non-instantaneous trip). This requires a circuit breaker with an electronic trip unit.

Include appropriate “Owner Metering” paragraph if motor control center contains metering for Owner’s use.


Include “Feeder Units” paragraph where applicable. Select feeder unit type. Use option “circuit breaker or fusible switch type as indicated” if feeder unit types are indicated on drawings.

Include “Combination Magnetic Motor Starter Units” paragraph where applicable.

Include cross references to other sections specifying motor control center unit components. Coordinate with applicable sections.


“Overload Relays”, “Fusible Devices” and “Circuit Breakers” paragraphs are turned on automatically via linking.

“Overload Relays” Paragraph:

“Fusible Devices” Paragraph:

“Circuit Breakers” Paragraph:

MOTOR CONTROL ACCESSORIES Article:  Edit features for accessories according to project requirements. See Motor Control Circuits above for description of auxiliary contacts, pilot devices, control and timing relays, and control power transformers.

EXAMINATION article is optional.

INSTALLATION is turned on automatically via linking. Applicable installation requirements for components are turned on automatically via linking according to selections made under PART 2.

FIELD QUALITY CONTROL article is optional.

ADJUSTING, CLEANING, CLOSEOUT ACTIVITIES and PROTECTION articles are optional. Under CLOSEOUT ACTIVITIES indicate requirements for demonstration and training where applicable.

SECTION INCLUDES:  Corresponding components will be activated via linking according to selections made under PART 2.

RELATED REQUIREMENTS: Article will automatically include other sections cited within the specification text (except for standard Division 1 cross references). Other sections may be listed because they include items that might be expected to be found within this Section or include items important for the completion of the work that are not specified in an obvious location (e.g. isn’t obvious from the section title).

REFERENCE STANDARDS: Article will automatically include standards cited within the specification text. If the Consolidated List of Citations option is active, cross sectional links (not visible in the links window) will activate the reference standard in Section 01 4219 – Reference Standards as well.


SUBMITTALS: Edit according to project requirements.

QUALITY ASSURANCE: Qualifications for manufacturer and product listing organization may be included.