Motor control center MCC basics
Definition and Purpose
Section titled “Definition and Purpose”A motor control center (MCC) concentrates the starting, protection, and control of multiple electric motors in a centralized location, typically between 10 and 50 motors in a single installation. According to the National Electrical Code (NEC), an MCC is an assembly of one or more enclosed sections sharing a common power bus and containing mainly motor control units. These devices are used in large commercial and industrial buildings where numerous low-voltage three-phase motors (220 V to 600 V) must be operated from a single point, such as machine rooms or electrical rooms. There are also medium-voltage MCCs for higher-power motors, which operate in ranges from 2300 V to 15 000 V and use vacuum contactors.
Components and Structure
Section titled “Components and Structure”An MCC consists of one or more vertical metal cabinets housing common power buses and removable compartments for motor controllers. The typical structure includes a main three-phase copper or aluminum busbar, distribution busbars, and trays for control and power cables. Each motor control unit (bucket or cubicle) is a removable module containing the following elements:
- Circuit breaker or short-circuit protection fuses.
- Electromagnetic contactor or solid-state starter.
- Overload relay (thermal or electronic).
- Disconnect switch for circuit isolation.
- Connection terminals for the motor and control circuit.
- Optionally, control transformer, pilot lights, start/stop pushbuttons, variable frequency drive (VFD), or integrated PLC.
Each module connects to the power bus via plug-in connectors, facilitating maintenance or replacement without de‑energizing the entire MCC. Larger units may be bolted in place. Field wiring enters from the top or bottom of the cabinets.
Types of Starters in MCC
Section titled “Types of Starters in MCC”MCCs integrate various starters depending on the motor control and protection requirements:
- Direct‑on‑line (DOL) starter: Connects the motor directly to the line, applying full voltage. This is the simplest and most economical method, but it produces a starting current of 5 to 8 times the rated current. It is used in small motors where the allowable voltage drop permits.
- Star‑delta starter: Reduces the starting current to approximately one‑third by connecting the motor first in star and then in delta.
- Soft starter: Electronically controls the voltage applied to the motor to limit starting current and torque.
- Variable frequency drive (VFD): Allows starting, stopping, and speed regulation of the motor by varying the supply frequency and voltage; it is the most versatile and efficient option.
In particular, the DOL starter consists of a contactor and an overload relay, controlled by a circuit with start and stop pushbuttons. The high starting current follows the electrical relationship:
I_a = (V – E) / R_a
| Variable | Description |
|---|---|
| I_a | Armature current (A) |
| V | Supply voltage (V) |
| E | Back electromotive force (V) |
| R_a | Armature winding resistance (Ω) |
At standstill, speed is zero and thus E = 0, producing a very high initial current that decreases as the motor accelerates.
Heat Dissipation
Section titled “Heat Dissipation”The heat generated by the electrical equipment is a critical factor in designing the ventilation or air‑conditioning system for the room where the MCC is installed. The following table lists typical heat losses for MCC sections and starters of various sizes at low and medium voltage.
| Component | Heat loss |
|---|---|
| MCC section (per module) | 500 W |
| LV starter size 00 | 50 W |
| LV starter size 0 | 50 W |
| LV starter size 1 | 50 W |
| LV starter size 2 | 100 W |
| LV starter size 3 | 130 W |
| LV starter size 4 | 200 W |
| LV starter size 5 | 300 W |
| LV starter size 6 | 650 W |
| MV starter 200 A | 400 W |
| MV starter 400 A | 1300 W |
| MV starter 700 A | 1700 W |
LV = low voltage, MV = medium voltage.
These values serve to calculate the total thermal load that the MCC contributes to the environment.
Installation Considerations
Section titled “Installation Considerations”The MCC location must ensure adequate space for operation, ventilation, and maintenance. A minimum front clearance of 1,5 m / 5 ft is recommended for module extraction, and overhead free space for crane access if heavy units are installed. In dusty or corrosive environments, it is preferable to place the MCC in a separate air‑conditioned room, keeping ambient temperature below 40 °C / 104 °F and relative humidity controlled. Floors must have adequate fire resistance, and all cable penetrations must be sealed with firestop barriers.
Applicable Standards
Section titled “Applicable Standards”Motor control centers are designed and tested in accordance with international standards:
- IEC 61439‑1 and IEC 61439‑2: Low‑voltage switchgear and controlgear assemblies.
- UL 845: US standard for motor control centers.
- NEMA ICS 18: National Electrical Manufacturers Association standards for MCCs.
- NFPA 70 (NEC): Article 430 on motors and controllers, and Article 409 on industrial control centers.
- IEEE C37.20.7: Guide for internal arc testing in medium‑voltage equipment.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”How many starter modules can an MCC contain?
Section titled “How many starter modules can an MCC contain?”A typical vertical MCC can accommodate 10 to 50 removable modules, depending on starter size and bus capacity (from 600 A to 3000 A).
What is the heat loss of a size 2 starter?
Section titled “What is the heat loss of a size 2 starter?”A low‑voltage size 2 starter dissipates approximately 100 W of heat to the environment.
What starting current does a direct‑on‑line starter produce?
Section titled “What starting current does a direct‑on‑line starter produce?”A DOL starter produces a starting current between 5 and 8 times the motor rated current, which can exceed 500 A in medium‑power motors.
Up to what voltage do low‑voltage MCCs reach?
Section titled “Up to what voltage do low‑voltage MCCs reach?”Standard low‑voltage MCCs cover voltages from 220 V to 600 V; above that, medium‑voltage MCCs are used, operating up to 15 000 V.
What is the maximum recommended ambient temperature for an MCC?
Section titled “What is the maximum recommended ambient temperature for an MCC?”Ambient temperature should not exceed 40 °C / 104 °F to ensure the service life of electronic components and avoid thermal trips.
How much thermal load does a complete MCC section contribute?
Section titled “How much thermal load does a complete MCC section contribute?”Each MCC section (without starters) dissipates about 500 W, a value that must be added to the installed starters to calculate the total heat gain of the room.
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/heat-gain-equipment-d_1668.html
- electrical4u.com: https://www.electrical4u.com/direct-online-starter-or-dol-starter/