A motor control center (MCC) is a factory-built assembly that groups the starters, drives and protection devices for many motors in one engineered enclosure, fed from a common busbar. Instead of scattering motor control equipment around a plant, an MCC puts it in one place where it can be protected, monitored and maintained. This practical guide from Paklink LLC walks facility engineers through what an MCC contains, how to specify one, and what to check for Dubai’s conditions. If you are planning a motor-heavy facility, our electrical contractor services for industrial facilities cover the full scope from design to commissioning.
What is a motor control center (MCC)?
An MCC is a low voltage assembly made up of vertical sections, each holding individual units, usually called buckets or drawers, that control a single motor or a small group of loads. All units share a horizontal busbar that carries power across the lineup, and each unit takes its supply from a vertical busbar inside its section.
MCCs appear wherever a plant has many pumps, fans, compressors, conveyors or mixers: water treatment works, factories, oil and gas sites, and large HVAC plant rooms. They are built to the IEC 61439 family of standards for low voltage switchgear and controlgear assemblies, and the starter devices inside follow IEC 60947-4-1.
How is an MCC different from a distribution board?
An MCC controls and protects motors, while a distribution board splits a supply into protected circuits for general loads. The two often sit in the same electrical room, which is why they get confused.
| Feature | Motor control center | Distribution board |
|---|---|---|
| Main purpose | Start, stop, control and protect motors | Distribute power to lighting, sockets and small equipment |
| Typical contents | Starters, drives, overload and motor protection, control gear | Circuit breakers and busbars |
| Control functions | Yes, including interlocks and remote control | Usually none |
| Typical load | Motors from small pumps to large process drives | General circuits and small power |
| Link to automation | Often connected to PLC or SCADA | Rarely |
What does an MCC contain?
A standard MCC unit combines a way to isolate the motor, a way to switch it, and a way to protect it. Looking at a single motor feeder, you will usually find:
- Isolation and short-circuit protection: a circuit breaker or fuse switch that disconnects the unit and clears short circuits.
- Switching: a contactor, soft starter or variable frequency drive (VFD) that starts and stops the motor.
- Overload protection: a thermal overload relay or an electronic motor protection relay that guards against sustained overcurrent.
- Control circuit: pushbuttons, selector switches, indicator lamps and auxiliary relays for local and remote operation.
- Terminals and wiring: for incoming power, the motor cable and control connections.
At the lineup level, the MCC also has the main incoming device, the horizontal and vertical busbars, earthing bars and cable entry space.
Which starter type should you use?
The right starter depends on the motor size, the load and what the supply can tolerate. The four common choices differ mainly in how they handle the starting current, which can be several times the motor’s full-load current.
| Starter type | How it works | Best for | Watch out for |
|---|---|---|---|
| Direct-on-line (DOL) | Connects the motor straight to full voltage | Small motors and simple loads | High starting current and mechanical shock |
| Star-delta | Starts with the windings in star, then switches to delta | Medium motors that start with low load | Reduces starting current to roughly one third, but also reduces starting torque, and the changeover causes a brief transient |
| Soft starter | Ramps up the voltage electronically | Pumps, fans and conveyors needing a smooth start | Does not control speed during running |
| Variable frequency drive (VFD) | Varies voltage and frequency to control speed | Loads where speed control saves energy or improves the process | Generates harmonics and needs proper cooling and cabling |
A good specification states which type each motor gets, rather than leaving it to the panel builder to decide.
Should you choose a fixed or withdrawable MCC?
Choose withdrawable if downtime matters, and fixed if budget and simplicity matter more. The difference is how each unit is mounted and how easily it can be isolated.
| Construction | How units are mounted | Advantage | Trade-off |
|---|---|---|---|
| Fixed | Bolted in place and wired directly | Lowest cost, compact | Maintenance usually needs a local shutdown |
| Plug-in | Units connect to the busbar through plug-in contacts but are removed with the power off | Faster replacement than fixed | The section may need to be isolated first |
| Withdrawable | Units slide out on rails and disconnect from the busbar and control circuits | A failed starter can be swapped quickly, often with the rest of the MCC running | Higher cost and more floor space |
In a continuous process plant, a failed starter on a withdrawable MCC is often a short, controlled job. On a fixed MCC it can mean a planned outage of the whole section. That difference is why many oil, gas and water facilities specify withdrawable designs.
What standards and separation forms apply to an MCC?
An MCC should be designed and verified to the IEC 61439 series, which sets requirements for temperature rise, dielectric strength, short-circuit withstand and other performance limits. Three specification points matter most:
- Form of internal separation: Forms 1 to 4 describe how busbars, functional units and terminals are kept apart inside the assembly. Higher forms, such as Form 4, give better protection during live maintenance, with higher cost and space needs.
- Short-circuit rating: the MCC must withstand the fault current available at its location. IEC 61439 expresses this through ratings such as the rated short-time withstand current. Always specify it against the actual fault level.
- Coordination type: IEC 60947-4-1 defines Type 1 and Type 2 coordination for motor starters. With Type 2, the starter is expected to remain suitable for use after a short circuit, apart from light contact welding that is easily separated. Type 1 allows damage to the starter, as long as it does not endanger people or the installation.
If internal arc protection matters for your operators, ask whether the assembly has been tested for arc fault containment, and to which standard. You can browse the standards on the IEC website.
How do you size and specify an MCC?
Start from the motor list, then work outward to the busbar and the room. A clear specification reduces redesign and avoids arguments later. The steps are:
- Build the motor schedule. List every motor with its power, voltage, duty, starting method and whether it needs speed control.
- Define the control philosophy. Decide which motors are controlled locally, from the MCC, or remotely from a control system, and which need interlocks.
- Set the electrical ratings. Specify the busbar current rating, the short-circuit rating and the incoming supply arrangement.
- Choose the construction. Decide fixed, plug-in or withdrawable, and the form of internal separation.
- Plan for growth. Include spare units and spare space for future motors, because plant expansion is common and adding sections later is harder.
- Define the environment. State the ambient temperature, ingress protection (IP) rating, altitude and corrosion risk.
- Specify documentation and testing. List the drawings, test reports and certificates required at handover.
How does Dubai’s climate affect MCC design?
Heat is the main climate factor, followed by dust and humidity. Standard assumptions for indoor equipment typically limit the ambient temperature to 40°C, with a lower 24-hour average. In a hot electrical room, or an unconditioned one, the real temperature can exceed what the components were rated for.
| Condition | Effect on the MCC | Design response |
|---|---|---|
| High ambient temperature | Components run hotter, which reduces their capacity and shortens life | Air-conditioned electrical rooms, derating where needed, adequate ventilation |
| Dust and sand | Build-up inside panels, blocked filters, tracking risk | Suitable IP rating, filtered ventilation, regular cleaning |
| Humidity and condensation | Corrosion and insulation problems | Anti-condensation heaters and sealed cable entries |
| Coastal or chemical air | Corrosion of metalwork and terminals | Protective finishes and corrosion-resistant materials |
A panel specified for mild-weather assumptions can run into trouble in summer, so state the real site conditions in the specification and ask the supplier to confirm how the design copes with them.
How do VFDs affect an MCC?
VFDs save energy and give process control, but they bring heat and harmonics that the MCC design must handle. A drive converts the supply in a way that draws distorted current from the network. This harmonic distortion can overheat transformers, cables and capacitor banks, and it can disturb sensitive equipment.
The usual measures are:
- Line reactors or DC chokes to reduce harmonic current
- Harmonic filters where limits are strict
- Enough ventilation and cooling for the heat the drives release
- Screened motor cables and proper earthing to limit electrical noise
Harmonics are best measured rather than guessed. A Class A power quality analyzer such as the Janitza UMG 512-PRO records the data you need to size filters and to check compliance after commissioning.
How does an MCC connect to PLC and SCADA systems?
An MCC connects to automation through hardwired signals or through a digital communication network. In a conventional MCC, each unit is wired to the control system with individual cables for start, stop and status. In an intelligent MCC, motor protection relays and drives communicate over an industrial network such as Modbus or PROFINET, which reduces wiring and adds data such as motor current, trip history and run hours.
The benefits are easier troubleshooting and better maintenance planning. The cost is more engineering up front, plus the need to agree the communication protocol and cybersecurity rules with the controls team early.
What happens during MCC testing and commissioning?
A sound process tests the MCC at the workshop first, then again on site. The usual stages are:
- Workshop checks: visual inspection, mechanical operation, wiring checks, insulation and dielectric tests, and functional tests of the control circuits.
- Delivery and installation: inspection for transport damage, correct positioning, cable termination and earthing.
- Pre-commissioning tests: insulation resistance, continuity and protection relay settings.
- Functional tests: each motor is tested for correct rotation, starting, stopping, interlocks and alarms, first uncoupled where the process allows.
- Handover: as-built drawings, test reports, relay settings records, manuals and training for operators.
Ask the contractor which of these they carry out themselves, and which documents you will receive.
What mistakes do MCC projects commonly make?
Most MCC problems start at specification, not in the equipment. Five mistakes come up repeatedly:
- No spare capacity, which forces expensive modifications when the plant grows.
- Underestimating heat, so the room and the panel run hotter than designed.
- Specifying a drive without checking harmonics, which then affects other equipment.
- Poor access and clearance, which makes withdrawable units hard to remove and maintenance unsafe.
- Incomplete documentation, which slows every future fault investigation.
Planning a motor-heavy facility in the UAE?
If you are specifying or upgrading an MCC and want a second opinion on starters, separation, ratings or climate conditions, talk to the team at Paklink LLC. You can read more about the scope we cover on our electrical contracting services in Dubai page.
Frequently asked questions
What does MCC stand for?
MCC stands for motor control center. It is a factory-built assembly that houses the starters, drives and protection for multiple motors.
What is the difference between an MCC and a distribution board?
An MCC starts, controls and protects motors. A distribution board splits a supply into protected circuits for general loads such as lighting and small power.
What is a withdrawable MCC?
It is an MCC whose units slide out on rails and disconnect from the busbar and control circuits, so a failed starter can be replaced quickly without a long shutdown.
When should I use a VFD instead of a soft starter?
Use a VFD when you need speed control during running, such as for pumps or fans where energy savings matter. Use a soft starter when you only need a smooth start and stop.
What standards apply to an MCC?
MCC assemblies follow the IEC 61439 series, and the motor starters inside follow IEC 60947-4-1. Local utility and client specifications may add further requirements.
