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:

  • Providing motor overload protection and/or motor branch-circuit short-circuit and ground-fault protection
  • Allowing for selection of forward or reverse motor rotation
  • Allowing for selection of multiple motor speeds
  • Allowing for motor jogging or inching, which provide for momentary motor operation for the purposes of performing small movements
  • Allowing for motor plugging, which provides for momentary reversal of motor rotation for the purposes of braking

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. 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. Manual motor starters utilize contacts that are directly operated by mechanical means when a toggle switch or pushbutton is actuated by hand. Motor-starting switches are manually-operated horsepower rated switches that do not include overload protection.

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:

  • Full-voltage non-reversing
  • Full-voltage reversing
  • Two-speed, one-winding (constant horsepower or constant/variable torque)
  • Two-speed, two-winding (constant horsepower or constant/variable torque)
  • Reduced-voltage autotransformer
  • Reduced-voltage part-winding
  • Reduced-voltage wye delta (open or closed transition)

Manual motor starters are classified as either fractional-horsepower or integral-horsepower based on voltage and horsepower ratings. Fractional-horsepower starters are suitable for single phase motors through 1 Hp. Integral-horsepower starters are available for single phase motors up to 5 Hp and three phase motors up to 10 Hp. Manual motor starters are available in non-reversing, reversing, and two-speed configurations.


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:

  • Locked rotor condition
  • Load exceeds motor torque rating
  • Bad bearings
  • Low line voltage
  • Loss of phase on polyphase motor

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:

  • Melting alloy thermal overload relays must be manually reset, while bimetallic thermal and solid-state electronic overload relays often allow for selection of manual or automatic reset, or allow for remote reset.
  • Bimetallic thermal overload relays are available with ambient temperature compensation to ensure consistent operation where the motor is at a constant ambient temperature and the controller/overload relay is located elsewhere with varying ambient temperature conditions. Ambient temperature compensation is not required where the controller/overload relay is at a constant ambient temperature and the motor is located in a varying ambient temperature, nor if the controller/overload relay and motor are both located together in the same varying ambient temperature. Melting alloy thermal overload relays are not available with ambient temperature compensation. Solid-state electronic overload relays are not affected by ambient temperature.
  • Inverse-time overload relay trip class ratings (e.g. Class 10, 20, and 30) identify the maximum time in seconds for an overload relay to trip at 600 percent of its current rating (i.e. Class 20 relays will trip in 20 seconds or less at 600 percent of rated current). Melting alloy thermal overload relays have fixed trip class and current ratings according to selected heater elements. Bimetallic thermal overload relays have a fixed trip class rating but often allow for slight adjustment of current rating. Solid-state electronic overload relays have an adjustable current rating and often also allow for selection of trip class rating.
    • 1)    Class 20 (standard trip) is usually suitable for most general motor applications.
    • 2)    Class 30 (slow trip) may be suitable for motors driving high inertia loads requiring extended starting periods.
    • 3)    Class 10 (quick trip) may be suitable for hermetically sealed, submersible pump, and other motors which can only endure locked rotor current for a very short time, or motors which have a low ratio of locked rotor to full load current.
  • Solid-state electronic overload relays often include additional features such as phase loss protection, phase unbalance protection, and ground fault detection. Bimetallic thermal overload relays may be designed for a quicker trip response under a phase loss condition to provide limited protection.

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:

  • NEMA controllers are available in a relatively small number of standard controller sizes, designed conservatively for use in a broad range of applications. For example, one NEMA table lists 11 sizes (from size 00 through size 9) to cover motors from 2 Hp through 1600 Hp at 460V. IEC controllers do not come in standard sizes, but rather are selected based on assigned ratings and utilization categories for a specific and narrowly defined range of applications.
  • NEMA controllers are generally more robust devices than IEC controllers. NEMA contactor coils are commonly encapsulated for protection from contaminates and IEC contactor coils are commonly tape-wound.
  • NEMA controllers often have replaceable parts for improved maintainability over IEC controllers, which typically offer replaceable contacts only on larger horsepower-rated controllers.
  • NEMA controllers typically include factory assembly of contactors and overload relays, along with control and power circuit wiring. IEC controllers often involve field assembly of these components.
  • NEMA controllers are generally significantly more expensive than IEC controllers.
  • NEMA controllers are generally significantly larger than IEC controllers.

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.


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. Auxiliary contacts are often available as an optional accessory for integral-horsepower manual motor starters, but not for fractional-horsepower manual motor starters or motor-starting switches.

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:

  • Motor Circuit Conductors (Part II)
  • Motor and Branch-Circuit Overload Protection (Part III)
  • Motor Branch-Circuit Short-Circuit and Ground-Fault Protection (Part IV)
  • Motor Feeder Short-Circuit and Ground-Fault Protection (Part V)
  • Motor Control Circuits (Part VI)
  • Motor Controllers (Part VII)
  • Motor Control Centers (Part VIII)
  • Disconnecting Means (Part IX)
  • Adjustable-Speed Drive Systems (Part X)

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:

  • NEC 430.6 requires conductor ampacity and branch-circuit short-circuit and ground-fault overcurrent devices for general motor applications to be selected from specific tables (not from motor nameplate values). It also requires motor overload protection to be selected based on motor nameplate current rating (not from specific tables).
  • NEC 430.75 requires a disconnecting means for motor control circuits that, where separate from the motor controller disconnecting means, must be located adjacent to the motor controller disconnect.
  • NEC 430.87 requires each motor to have its own individual controller (with certain exceptions).
  • NEC 430.102 requires a disconnecting means for each motor controller located within sight of the controller (with certain exceptions). It also requires a disconnecting means for each motor located within sight of the motor and the driven machinery (with certain exceptions). It permits the controller disconnect to also serve as the motor disconnect, if it is also located within sight of the motor.
  • NEC 430.107 requires at least one of either the controller disconnecting means or the motor disconnecting means to be readily accessible.


Other Applicable Standards

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:

  • Type 1: Indoor use; provides a degree of protection against ingress of falling dirt.
  • Type 2: Indoor use; provides a degree of protection against ingress of falling dirt and dripping and light splashing of water.
  • Type 3R: Indoor or outdoor use; provides a degree of protection against ingress of falling dirt, rain, sleet, and snow; undamaged by external formation of ice on the enclosure.
  • Type 12: Indoor use (without knockouts); provides degree of protection against ingress of falling dirt, circulating dust, lint, fibers, and flyings, dripping and light splashing of water, and light splashing and seepage of oil and non-corrosive coolants.

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:

  • General Standards for Manual and Magnetic Controllers (Part 1)
  • AC Noncombination Magnetic Motor Controllers Rated 600 Volts (Part 2)
  • Nonmagnetic Motor Controllers (Part 3)
  • Overload Relays (Part 4)
  • AC Combination Motor Controllers (Part 6)
  • Disconnect Devices for Use in Industrial Control Equipment (Part 8)

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

  • Industrial Control Relays (Part 2)
  • Pushbuttons, Selector Switches, Indicating Lights, and Pushbutton Stations (Part 5)

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

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:

  • Section – Switches, Air, Low-Voltage
  • Section – Circuit Breakers, Insulated-Case/Molded-Case
  • Section 7.14 – Ground-Fault Protection Systems, Low-Voltage
  • Section – Motor Control, Motor Starters, Low-Voltage

UL 98 - Enclosed and Dead-Front Switches.

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

UL 1053 - Ground-Fault Sensing and Relaying Equipment.

UL 60947-1 – Low-Voltage Switchgear and Controlgear – Part 1: General Rules and UL 60947-4-1 – Low-Voltage Switchgear and Controlgear – Part 4-1: Contactors and Motor-Starters – Electromechanical Contactors and Motor-Starters; together replace UL 508 for investigation of motor controllers for new products; UL 508 listings for existing products are withdrawn as part of the final of three transitional phases, effective January 27, 2017.


How to Specify

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

MANUFACTURERS Article:  Specify acceptable manufacturers.


“Service Conditions” Paragraph:  NEMA ICS 1 (referenced by NEMA ICS 2) 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:

  • Indicate whether the available fault current or short circuit current rating are indicated on the drawings or are to be determined by a short circuit study performed in accordance with Section 26 0573 – Power System Studies.
  • Indicate whether or not series ratings are permitted. 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.

Enclosures” Paragraph:  Specify default enclosure environment types per NEMA 250 (see description under Other Applicable Standards above), where not indicated on drawings.

Include “Magnetic Motor Starters” paragraph where applicable.

  • Configuration” Paragraph:  Select the first choice option to specify non-reversing as the default starter type. Select the second choice option if starter types are indicated on drawings.
  • Minimum starter size may be specified.
  • Indicate whether or not substitution of suitable non-standard starter sizes for specified NEMA sizes is permitted. Some manufacturers offer non-standard “half-sizes” that fall between standard NEMA sizes.
  • Disconnects” Paragraph:  Select disconnect type. Use option “circuit breaker or disconnect switch type as indicated” if disconnect types are indicated on drawings. Indicate circuit breaker type where applicable. See Motor Protective Devices above for a description of differences between motor circuit protectors and thermal magnetic circuit breakers.
  • Overload Relays” Paragraph:  Select overload relay type. See Motor Protective Devices above for descriptions of bimetallic thermal, melting alloy thermal, and solid-state overload relays.
  • Pilot Devices Required” Paragraph:  Include this optional paragraph to specify pilot devices required for motor starters, if requirements are not clearly indicated on drawings.

Include “Manual Motor Starters” paragraph where applicable.

  • Configuration” Paragraph:  Select the first choice option to specify non-reversing as the default starter type. Select the second choice option if starter types are indicated on drawings.
  • Include “Fractional Horsepower Manual Motor Starters” and “Integral Horsepower Manual Motor Starters” paragraphs where applicable and edit features according to project requirements.

Include “Motor-Starting Switches” paragraph where applicable.


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

Overload Relays” Paragraph:

  • Inverse-Time Trip Class Rating” Paragraph:  Select the first choice option to specify Class 20 as the default trip class rating. Select the second choice option if trip class ratings are indicated on drawings. See Motor Protective Devices above for description of overload relay inverse-time trip class ratings.
  • Indicate if/when automatic and/or remote reset is to be employed. Automatic or remote overload relay reset may be desirable where access to controller/overload relay is not convenient. Automatic reset should normally be limited to three-wire control arranged to prevent automatic restarting of motors, which can result in hazardous conditions for the operator or damage to motor from cycling of overload relay.
  • Edit features for applicable overload relay types (bimetallic thermal, melting alloy thermal, solid-state) according to project requirements. A basis of design product may be specified for solid-state overload relays if applicable.

Circuit Breakers” Paragraph:

  • Motor Circuit Protectors” paragraph is turned on automatically via linking if specified for combination magnetic motor starters. Edit optional features and accessories according to project requirements.
  • Molded Case Circuit Breakers” Paragraph is turned on automatically via linking if specified for combination magnetic motor starters.
    • 1)    Indicate application of thermal magnetic and electronic trip circuit breakers, which may be specified according to frame sizes.
  • Thermal Magnetic Circuit Breakers” Paragraph:
    • 1)    Include field-adjustable magnetic instantaneous trip setting, specified according to frame size, as applicable. Adjustable trip response settings can provide increased flexibility by allowing the time-current curve to be shaped to improve system selectivity.
    • 2)    Include interchangeable trip units as applicable.
  • Electronic Trip Circuit Breakers” Paragraph:
    • 1)    Edit available field-adjustable trip response settings according to project requirements. Circuit breaker trip functions are often described by a combination of the abbreviations L, S, I, and G (L=Long time, S=Short time, I=Instantaneous, G=Ground fault).
    • 2)    Edit optional features and accessories according to project requirements.

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.

  • 1)    Include optional services of a manufacturer's authorized representative according to project requirements. Edit choice to indicate extent of responsibilities.
  • 2)    Applicable NETA ATS testing requirements for components are turned on automatically via linking according to selections made under PART 2.
  • 3)    Motor Starters:  Indicate if NETA ATS tests listed as optional are required.
  • 4)    Circuit Breakers:  Indicate which circuit breakers require testing and if NETA ATS tests listed as optional are required.

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.