What Is a Motor Control Center (MCC)? Components, Types and How It Works

Key Takeaways

• Definition and Function – The motor control center (MCC) is an assembly of enclosed sections that share a common power bus and principally contain motor control units.

• The MCC Bucket Architecture – The typical motor unit, often called an MCC bucket, combines a disconnect, short-circuit protection, and a motor starter (a contactor plus an overload relay).

• Voltage and UL Standards Framework – In North America, low voltage MCCs are built to UL 845 (covering equipment rated up to 1,000 V AC or DC), whereas medium voltage MCCs (such as 7,200 V units) are built to UL 347.

• Distribution vs. Control Equipment – Switchgear and switchboards distribute power, while an MCC controls motors; a VFD is a discrete device rather than an assembly and functions as an individual unit inside an MCC bucket.

• Intelligent and Safety Trends – Modern intelligent MCCs integrate arc flash mitigation, cloud diagnostics, and Single Pair Ethernet or fieldbus networks to deliver real-time motor health telemetry and energy analytics.

• Mandated Maintenance Protocols – Standard maintenance compliance under NFPA 70B requires a documented electrical maintenance program with inspection intervals determined by ongoing equipment condition monitoring.

Introduction

MCC in electrical engineering stands for motor control center. In a plant with many electric motors, it is the lineup of steel cabinets where those motors are started, stopped and protected from one central place. The motor control center is an assembly of one or more enclosed sections having a common power bus and principally containing motor control units.

The idea behind it is simple. The most basic way to control an AC motor is a motor starter, made up of a contactor and an overload relay. When a site has only a few motors spread around, each starter can sit in a panel near its motor. Once you have many, it is often best to control them from a central location, and the MCC is designed for exactly that job.

This guide starts with the basics: what a motor control center is, what it does and how it is built. It then compares it with switchgear and with a variable frequency drive, and ends with the engineering details that matter when you specify, operate or maintain one.

What Is a Motor Control Center?

The Motor Control Center (MCC) is a floor-mounted assembly of one or more enclosed vertical sections having a horizontal common power bus and principally containing combination motor control units. ^[2]

Put simply, it is a row of standing steel cabinets. Heavy copper or aluminum bus bars run the length of the lineup, and each motor gets its own compartment that holds everything needed to switch and protect it. ^[2][3] You will also hear the assembly called an MCC panel, and its units called buckets or drawers. The names vary, but the equipment is the same.

What Does a Motor Control Center Do?

Each motor unit does five jobs:

1. Isolate: A disconnecting switch isolates the motor circuit. 

2. Protect the Wiring against Short Circuits: Fuses or a circuit breaker provide short-circuit protection. 

3. Protect the Motor against Overloads: The overload relay disconnects the motor when it draws too much current for too long, for instance, when a conveyor jams. 

4. Switch the Motor: The contactor closes to start the motor and opens to stop it. Some units use a solid-state controller instead. 

5. Give Operators Control and Feedback: Pushbuttons, selector switches, and indicator lights on the unit door let operators start and stop the motor and see its status. 

The overload relay protects the motor, but it does not provide short-circuit protection for the wiring, which is why a circuit breaker or fuses are also needed. ^[2][6]

Why Use an MCC Instead of Separate Starters?

Siemens highlights several key benefits of consolidating starters into a centralized motor control center:

• Faster and Easier Installation and Wiring

• Centralized Motor Control

• Generally Less Total Space

• Easier Future Changes, such as Larger Starters, Extra Starters or additional Vertical Sections

Intelligent MCCs add another advantage: built-in networking that captures motor data for predictive maintenance, process monitoring and diagnostics. ^[3]

Recommended Reading: Motor Control Circuit Guide: Contactors and Wiring  

How a Motor Control Center Is Built

Designs differ between manufacturers, but the anatomy is consistent. The figures below come from Rockwell Automation for the Allen-Bradley CENTERLINE 2100, a low-voltage motor control center built to UL 845. ^[3] These are typical values, not universal ones.

Vertical Sections

The structure is made of sections that contain wireways, doors, and plug-in or frame-mounted units. The standard vertical section measures 2,286 mm high, 508 mm wide, and 381 mm deep; 508 mm deep and 1,791 mm reduced-height versions are also available. ^[3] The training course by Siemens gives the same 20 by 90 inch (508 by 2,286 mm) width and height as the general rule.

Sections are joined side by side into a lineup. Back-to-back designs place units on both the front and rear, with each side having its own horizontal and vertical bus. 

Horizontal and Vertical Bus

The horizontal bus is the power spine of the lineup. In the CENTERLINE 2100, it is mounted near the vertical center of the structure and is available from 600 A to 3,200 A. ^[3]

Each section also has a vertical bus that carries power from the horizontal bus to the units. Rockwell's standard vertical bus is rated 300 A above and 300 A below the horizontal bus, giving an effective 600 A per section, with an optional 1,200 A effective rating.

The bus must also survive a fault. Bus bracing is rated in kiloamperes of symmetrical short-circuit current, and Rockwell offers 42 kA, 65 kA and 100 kA. ^[3] The rule is straightforward: the assembly must withstand the largest short-circuit current that can occur in its application. ^[2]

Units, Buckets and Space Factors

The compartments that hold motor controls are called units, or informally MCC buckets. Plug-in units have self-aligning stabs on the back that engage the vertical bus when the unit is pushed in. Rockwell rates its unit power stabs at 225 A. Units too large for stabs are frame-mounted, with connections made directly to the components. ^[2][3]

Unit heights come in fixed steps. Rockwell measures them in space factors: half a space factor is 165 mm high, a standard section holds six space factors, and up to twelve half-space-factor units fit in one section. Units can be interchanged and rearranged within a lineup.

Two details protect people when units go in and out. The ground stab makes contact first and breaks contact last, and optional automatic shutters cover the vertical bus as soon as a unit is removed. 

Recommended Reading: MCC Buckets: Engineering Marvels Revolutionizing Material Handling  

Wireways

Field cables reach the units through wireways that are kept separate from the power bus. In the CENTERLINE 2100, 152 mm high horizontal wireways run along the top and bottom of each section, and a vertical wireway on the right side runs 1,981 mm between them. ^[3]

Inside a Typical Starter Unit

The combination motor control unit includes an externally operable disconnecting means, branch-circuit overcurrent protection and a magnetic motor controller. ^[2] In practice, that gives you these components:

• Circuit Breaker or Fusible Disconnect Switch: Isolates the motor circuit and provides short-circuit protection.

• Contactor: Closes to start the motor and opens to stop it, driven by pushbuttons or other pilot devices.

• Overload Relay: Disconnects the motor on a sustained overload, such as a jammed conveyor.

• Pilot Devices: Pushbuttons, selector switches and indicator lights for local control and status.

• Terminal Blocks: Organized termination points for field wiring, depending on the NEMA wiring type.

The choice of breaker follows the job. Starter units commonly use instantaneous-trip (magnetic-only) breakers because the overload relay already handles overloads, while thermal-magnetic breakers are the norm where there is no starter, such as the main or a feeder unit. Current designs also offer inverse-time breakers in horsepower-rated starter units.  

Recommended Reading: Contactor vs Relay: Understanding the Differences and Applications  

Safety Interlocks

The unit door and the disconnect handle are mechanically interlocked. The disconnect cannot be switched on unless the door is closed, the door cannot be opened unless the disconnect is off, and the unit cannot be inserted or withdrawn while the disconnect is on. For tasks that need access without interrupting service, this safety interlock has an externally operated defeater. ^[3]

Types of MCC Units and Motor Starting Methods

The lineup supports more than basic starters. Rockwell's range includes contactor and starter units, metering units, main and feeder units, lighting and power panel units, transformer units, soft starter units, variable frequency drive units and programmable controller units. [3]

Starter units themselves come in several configurations. Siemens lists full voltage non-reversing (FVNR), full voltage reversing (FVR), two-speed, part winding, reduced voltage autotransformer and wye-delta starters. ^[2]

DOL, Star-Delta and Soft Starters

Full voltage starting, also called direct-on-line (DOL) or across-the-line starting, connects the motor straight to the supply. ^[2][9] It is simple, but it draws a high starting current. Reduced-voltage methods lower that current, and Rockwell's application guide compares two of them with full voltage starting:

These figures come from Rockwell's worked example, which assumes a full-voltage starting current of 600% of full-load current. ^[9]

Star-Delta starting connects the motor windings in star during start, lowering the voltage across each winding to 57.7% of full voltage. It requires a motor with all winding leads brought out. 80% of standard IEC-style 50 Hz motors are built for wye-delta starting, but fewer than 1% of standard 60 Hz NEMA-style motors are, except in large frames: from about 200 hp, many manufacturers make the connections standard. ^[9]

Soft starters reduce voltage electronically instead of by reconnection. The wye-delta method gives more torque per ampere, but its starting torque is fixed at 33%, while a soft starter lets you adjust the current limit to match the load. The common open-transition version also briefly disconnects the motor at changeover, and in some cases the following current peak briefly exceeds even the locked-rotor current.

NEMA Starter Sizes and IEC Ratings

NEMA rates starters by maximum horsepower, in sizes 00 to 9, covering 2 hp to 1,600 hp at 460 V. ^[2] The kilowatt values below are converted at 0.746 kW per hp. ^[7]

IEC works differently. IEC ratings are the maximum operational current, and IEC does not specify sizes. The product standard for contactors and motor starters is IEC 60947-4-1, which applies to circuits up to 1,000 V AC or 1,500 V DC. IEC 60947-4-1 is currently Edition 5.0, published in November 2023, with a corrected version issued in March 2026. ^[10]

NEMA Classes and Wiring Types

When you order a motor control center, you also choose how much wiring the factory does. NEMA defines two classes and three wiring types: ^[2][3]

Low Voltage and Medium Voltage MCCs

In North America, low voltage MCCs are built to UL 845, which covers motor control centers for use under the US National Electrical Code (NFPA 70), the Canadian Electrical Code and Mexico's NOM-001-SEDE. Its current Edition 6, published in June 2021 and last revised in February 2025, applies to MCCs rated not more than 1000 V AC or DC and circuits with up to 200,000 A of available short-circuit current. ^[4] Individual product lines can be narrower: the CENTERLINE 2100, for example, is designed for power systems rated 600 V or less. ^[3]

Medium Voltage MCCs follow different standards. CENTERLINE 1500 is built to UL 347 and NEMA ICS 3-2, with a maximum rated voltage of 5,000 V or 7,200 V and nominal ratings from 2,400 V to 6,900 V. Its power cell holds non-load-break isolation switches, power fuses, contactors, and current and control power transformers. ^[5]

Outside North America, the IEC 61439 series covers low voltage switchgear and controlgear assemblies, and in IEC terminology an MCC is generally a controlgear assembly. Part 2 of the series, IEC 61439-2:2020, sets requirements for power switchgear and controlgear assemblies rated up to 1,000 V AC or 1,500 V DC.

MCC vs Switchgear vs Switchboard

The short answer is purpose. The main difference is purpose. Switchgear and switchboards primarily distribute and protect electrical power, while motor control centers (MCCs) primarily start, stop and protect motors. ^[2]

• Switchgear provides power distribution and protection, typically at higher levels of the electrical distribution system. Low-voltage power circuit breaker switchgear uses individually mounted, compartmentalized circuit breakers, commonly with drawout construction. Typical US standards include UL 1558 and IEEE C37.20.1. ^[6]

• Switchboards distribute power to loads, panelboards and control equipment. They can be installed downstream of switchgear and use group-mounted or individually compartmentalized circuit breakers. The typical US standard is UL 891. ^[6]

• Motor Control Centers combine motor control units within a common assembly. These units typically include a disconnecting means, fuses or a circuit breaker, an overload relay, and a contactor or solid-state controller. The typical US standard is UL 845. ^[2][4][6]

MCC cannot be considered a switchboard because switchboards are supposed to contain circuit breakers only, not motor controllers. Then, a switchboard is intended for power distribution, not motor control. 

The motor control center receives its power through the power distribution system of the plant, which can include transformers, substations and switchboards. The overcurrent device protecting its bus can sit upstream, for example in a switchboard, or be installed as a main inside the lineup.

MCC vs VFD

This comparison mixes two categories. MCC is an assembly of sections, busbars and motor control units that controls multiple motors, typically through a separate unit for each motor. VFD is a solid-state motor controller that adjusts the frequency and voltage supplied to an AC motor to control its speed. ^[1]

The two can work together. MCC can contain VFD units alongside conventional motor starters, combining power distribution, protection and motor control within one assembly. ^[2][3]

The design question is which control unit each motor needs. The direct-on-line (DOL) starter or soft starter operates a motor at its normal full operating speed after startup, while a VFD allows adjustable-speed operation. ^[2]

Speed control offers substantial energy-saving potential for centrifugal fans and pumps. Under the applicable affinity-law assumptions, power varies approximately with the cube of speed, so a 20% speed reduction can reduce power demand by about 50%. In a US Department of Energy example, a fan motor drawing 16.4 kW at full speed requires only 2.8 kW at half speed, including drive and motor losses. ^[7]

For constant-torque loads, such as conveyors and hoists, power demand falls approximately in proportion to speed, so the energy-saving potential from reducing speed is generally smaller.

Recommended Reading: What Is a VFD (Variable Frequency Drive)? Complete Engineering Guide to VFD Drives  

Intelligent MCCs and PLC Integration

Intelligent MCC connects its motor control devices to a network. IntelliCENTER version by Rockwell uses EtherNet/IP or DeviceNet to capture data for predictive maintenance, process monitoring and diagnostics. ^[3] Square D Model 6 iMCC by Schneider Electric is another example of the category.

The practical gain is remote access. Rockwell's system allows users to adjust overload relay parameters, monitor phase currents, and reset tripped relays remotely across the network while maintaining closed unit doors. Its integration software can also add the MCC's devices to the PLC programming environment automatically, which reduces PLC development time for industrial automation projects.

Arc Flash Safety, PPE and Working Space

MCC buses are braced for fault currents of up to 100 kA in Rockwell's range, so arc flash safety belongs in every design and maintenance decision. Maintenance matters here too: improper maintenance can lead to arcing faults, and arcing faults can lead to dangerous arc-flash incidents. ^[8]

Standards That Apply

• NFPA 70E is the standard for electrical safety in workplaces with electrical hazards. Its hierarchy of risk controls runs from elimination at the top to PPE at the bottom, as the least effective control.

• IEEE 1584-2018, still the active edition, provides the mathematical models used to determine the arc-flash hazard distance and the incident energy workers could be exposed to. IEEE is revising it under project P1584, whose scope covers three-phase AC systems from 208 V to 15 kV and excludes recommendations for PPE.

• IEEE 1683-2025, published in January 2026, is IEEE's guide to reducing shock and arc-flash hazards in low-voltage MCCs. It replaced the 2014 edition.

Design Features That Reduce Exposure

• Arc-Resistant Construction: ArcShield MCCs are tested to IEEE C37.20.7 for Type 2 accessibility, which protects personnel on all sides of the enclosure during an internal arcing fault. ^[3]

• Closed-Door Disconnection: Some units let you disconnect a unit from the vertical bus without opening the enclosure door.

• Remote Diagnostics: Networked units let you monitor and troubleshoot without opening doors, which keeps you out of the arc flash boundary.

• Infrared Windows: Viewing windows let you inspect a unit without opening its door.

These features work higher up the hierarchy of risk controls than PPE. PPE still matters, but the level of PPE selected from an arc-flash study may not be enough if protective devices do not operate as expected. ^[8] For more on arc flash mitigation inside MCC units, read more about MCC buckets.

Working Space in Front of an MCC

OSHA sets minimum clear working space for equipment likely to need examination, adjustment, servicing or maintenance while energized. For 151 V to 600 V to ground, the minimum depth is 0.9 m with exposed live parts on one side and nothing live or grounded opposite, 1.0 m with grounded parts opposite, and 1.2 m with exposed live parts on both sides of the worker. Installations built before 16 April 1981 may have 0.7 m in the first case. The 480 V system is 277 V line to neutral, so it falls in this range. ^[9] The working space must also be at least as wide as the equipment or 762 mm, whichever is greater. For installations built on or after 13 August 2007, headroom must be at least 1.98 m, or the equipment's height if that is greater. No working space is required behind an MCC with no renewable or adjustable parts on the back and connections accessible from elsewhere, and MCCs must be located in dedicated spaces and protected from damage. 

How to Maintain a Motor Control Center?

Maintenance is now written as a requirement, not a recommendation. NFPA 70B changed from a recommended practice into the Standard for Electrical Equipment Maintenance, with mandatory language that authorities having jurisdiction can enforce. It sets out what must be maintained, what maintenance must be done and at what intervals, and it has a dedicated chapter on motor control equipment. ^[8]

Three requirements shape an MCC maintenance plan:

1. Documented Program: The equipment owner must implement and document an electrical maintenance program (EMP) appropriate to the safety and operational risks. ^[8]

2. Condition-based Intervals: Maintenance intervals come from an equipment condition assessment that weighs physical condition, criticality and operating environment. ^[8]

3. Regular Review: The EMP must be audited at least once every five years, with a named EMP coordinator. ^[8]

On the equipment itself, the task lists in NFPA 70B include infrared thermography, for example, for circuit breakers, and the standard requires surfaces, enclosures, and insulating materials to be kept clean and free of contaminants. ^[8] MCC features make this easier. Plug-in units let you work on a unit away from energized equipment, and networked devices share diagnostic data for predictive maintenance.

How to Specify a Motor Control Center?

Key decisions outlined in selection guides from both Rockwell and Siemens include: ^[2][3]

1. Power System: Voltage, frequency, number of phases and available fault current.

2. Incoming Power: Main circuit breaker, main fusible switch, main lugs only, or a splice to an existing MCC.

3. Bus Ratings: Horizontal bus ampere rating and plating, vertical bus rating, and bus bracing at least equal to the available fault current.

4. Enclosure: NEMA/UL Type 1 (IP20 to IP40), Type 12 (IP54 with bottom plates), Type 3R (IP44) or Type 4 (IP65) for the environment.

5. Units: Motor horsepower, starting method (FVNR, FVR, reduced voltage, soft starter or VFD) and disconnect type for each unit.

6. Wiring: NEMA class and wiring type.

7. Extras: Voltage and source of control power (for example, control transformers), additional equipment such as transformers, panelboards and PLCs, plus networking and arc-resistant construction. 

Common Mistakes 

• Undersized Bus Bracing: Bracing below the available fault current breaks the basic rule that the assembly must withstand the largest possible short circuit in its application.

• Assuming Star-Delta Suits any Motor: The method needs a motor with all winding leads brought out (six leads, or 12 on dual-voltage motors), and most standard 60 Hz NEMA-style motors below about 200 hp are not built that way.

• Quoting an Old Voltage Limit: The current UL 845 edition covers MCCs rated up to 1000 V AC or DC, so check the edition before citing a 600 V limit.

• Calling any Motor Panel an MCC: The custom panel of starters and disconnects is normally an industrial control panel, covered by UL 508A in the US, rather than an MCC.

Conclusion

A motor control center is the central home for the motors in a plant: a lineup of sections sharing a common bus, with one plug-in unit per motor that isolates it, protects it and switches it. Its construction is standardized enough to specify by a short list of ratings, from horizontal bus current and bracing to NEMA class and wiring type.

It is also different from the equipment it is often confused with. Switchgear and switchboards distribute power. A VFD controls motor speed and can sit inside an MCC unit. Build safety in from the start with arc-resistant options and remote operation, and keep the equipment in its designed condition with a documented NFPA 70B maintenance program.

Frequently Asked Questions

Q. What Does MCC Mean in Electrical?

A. MCC stands for motor control center: an assembly of enclosed sections with a common power bus that principally contains motor control units. It groups the starters and protection for many motors in one central location.

Q. What Is an MCC Panel?

A. "MCC panel" is a common name for a motor control center. It is a factory-built assembly listed to UL 845 in North America. The custom panel of motor controllers, overload relays and disconnects is normally an industrial control panel, covered by UL 508A in the US.

Q. What Is the Difference Between an MCC and Switchgear?

A. Switchgear distributes and protects power, with every protective device drawout and individually compartmented. MCC controls motors, with each unit containing a disconnect, short-circuit protection, an overload relay and a contactor or solid-state controller.

Q. What Is the Difference Between a VFD and an MCC?

A. VFD is a solid-state controller that varies a motor's speed by changing the frequency and voltage it receives. On the other hand, an MCC is the assembly that can house VFDs alongside starters, soft starters and other units.

Q. How Much Does a Motor Control Center Cost?

A. There is no list price for a complete lineup, because each one is configured to order. Cost follows the selections described above: the number of sections and units, bus rating and bracing, enclosure type, networking and arc-resistant construction. It is better to request quotes against a complete specification.

Q. Can You Build Your Own Motor Control Panel?

A. Yes, but a shop-built assembly of starters, overload relays and disconnects is normally an industrial control panel, not an MCC. In the US, UL 508A covers industrial control panels. MCC, by contrast, is a factory-built assembly listed to UL 845. 

Q. How Often Should a Motor Control Center Be Maintained?

A. NFPA 70B sets maintenance intervals from an equipment condition assessment of physical condition, criticality and operating environment, rather than one fixed interval for all equipment. Your documented maintenance program defines the schedule, and you must audit it at least every five years.

References

[1] ECFR. 29 CFR 1910.399 - Definitions Applicable to this Subpart [Cited 2026 October 20] Available at: Link

[2] Siemens Energy and Automation. Motor Control Centers, STEP 2000 Course [Cited 2026 October 20] Available at: Link

[3] Rockwell Automation. CENTERLINE 2100 Motor Control Centers Selection Guide [Cited 2026 October 20] Available at: Link

[4] ANSI. UL 845, Motor Control Centers, Edition 6 [Cited 2026 October 20] Available at: Link

[5] Rockwell Automation. CENTERLINE 1500 Medium Voltage Motor Control Centers Selection Guide [Cited 2026 October 20] Available at: Link

[6] GT Engineering. Switchgears, Switchboards and Motor Control Centers: What are the Differences? [Cited 2026 October 20] Available at: Link

[7] US Department of Energy. Adjustable Speed Drive Part-Load Efficiency, Motor Systems Tip Sheet 11 [Cited 2026 October 20] Available at: Link

[8] IEEE. The New NFPA 70B-2023 Standard for Electrical Maintenance, paper ESW2023-18 [Cited 2026 October 20] Available at: Link

[9] Rockwell Automation. Wye-Delta and Solid-State Starters, Publication 150-AT005A-EN-P [Cited 2026 October 20] Available at: Link

[10] IEC. IEC 60947-4-1:2023, Low-Voltage Switchgear and Controlgear, Part 4-1: Contactors and Motor-Starters [Cited 2026 October 20] Available at: Link