MCCB (Molded Case CB) Symbol
Definition: The MCCB (Moulded Case Circuit Breaker) symbol represents a self-contained, factory-assembled overcurrent protective device housed in a moulded insulating case, rated from 15 A to 2500 A, used in circuit diagrams to indicate branch-feeder or main-circuit protection in industrial, commercial, and residential distribution systems per IEC 60947-2 and ANSI/UL 489.
Also known as: moulded case circuit breaker, molded case circuit breaker, MCCB, industrial circuit breaker, frame breaker, thermal-magnetic breaker.
What the MCCB (Molded Case CB) symbol means
The MCCB symbol denotes a circuit breaker that combines a thermal-magnetic trip mechanism (or electronic trip unit on larger frames) within a moulded insulating case, providing overcurrent protection (overload and short-circuit), switching capability, and fault isolation for power circuits. The Line pin receives power from the supply side and the Load pin delivers protected power to the downstream circuit.
In industrial and commercial electrical diagrams the MCCB symbol marks major protection points in the distribution hierarchy — switchboard incomers, motor branch circuits, feeder protection, and main distribution board breakers. An MCCB is distinguished from an MCB (miniature circuit breaker, typically ≤ 125 A) by its higher current and interrupting capacity and by its adjustable trip settings in larger frame sizes.
How to identify the MCCB (Molded Case CB) symbol
The MCCB symbol is drawn as a rectangle or standardised circuit-breaker symbol with two connection points: Line (supply side) and Load (downstream side). The IEC representation (per IEC 60617-07) shows a contact symbol — two lines representing open contacts — within or adjacent to a rectangle labelled 'MCCB'. The ANSI/IEEE representation uses a diagonal line crossing a circuit path with a box or the letters 'CB' indicating the overcurrent trip function. Many EDA tools use a simplified rectangular block labelled 'MCCB' with the current rating shown.
Function in a circuit
An MCCB provides three protective functions in one device: (1) thermal overload protection — a bimetallic strip heats under sustained overcurrent and trips the mechanism after a time-current delay; (2) magnetic short-circuit protection — a magnetic coil trips the breaker instantaneously on high fault currents (typically 5–10× rated current); (3) manual switching — the operating handle manually opens or closes the contacts for isolation and maintenance. Adjustable electronic trip units on larger frames provide separate settings for long-time, short-time, instantaneous, and earth-fault protection.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60947-2 is the primary standard for moulded case circuit breakers, defining rated currents, interrupting capacities, trip characteristics, and test requirements. IEC 60617-07 defines the circuit-breaker symbol as a normally closed contact with an operating element indicator. |
|---|---|
| ANSI/IEEE 315 | ANSI/UL 489 (UL Standard for Safety — Molded-Case Circuit Breakers, Molded-Case Switches, and Circuit Breaker Enclosures) governs MCCB construction and ratings in North America. ANSI C37.13 covers low-voltage switchgear breakers. ANSI Y32.2 / IEEE 315-1975 defines the circuit-breaker schematic symbol. |
| Key difference | IEC 60617 uses a stylised contact-and-bar symbol for circuit breakers; ANSI Y32.2 / IEEE 315 typically shows a diagonal slash through the circuit path with a box representing the trip mechanism. Functionally identical; the glyph style differs between European and North American drawings. |
Terminals / pins
| Pin | Name |
|---|---|
| line | Line |
| load | Load |
Typical values
Current ratings (frames): 15–2500 A (standard frames: 100 A, 160 A, 250 A, 400 A, 630 A, 800 A, 1600 A, 2500 A). Voltage ratings: 240–690 V AC (IEC), 240–600 V AC (ANSI). Interrupting capacity: 10–150 kA (kAIC) depending on frame size and class. Poles: 1, 2, 3, or 4. Trip classes: Class B (5–10×), Class C (10–20×) for instantaneous trip ratio.
Where the MCCB (Molded Case CB) symbol is used
- Main and sub-main incomer protection in industrial switchboards and motor control centres (MCCs)
- Feeder circuit protection in commercial building distribution boards
- Individual motor-branch protection for motors up to several hundred kilowatts
- Generator output breaker in standby and prime power generator sets
- Solar PV DC-side protection in utility-scale inverter systems (DC MCCB variants)
- Data centre PDU (power distribution unit) incomer and branch protection
Example
In an industrial motor control centre (MCC) diagram, the MCCB symbol shows its Line pin connected to the 480 V three-phase bus bar and its Load pin feeding a contactor and overload relay assembly. The MCCB is rated 100 A, 65 kAIC, with the thermal-magnetic trip set for the motor's full-load current. Upstream, a 400 A MCCB protects the entire MCC feeder from the main switchboard.
Key facts
- The MCCB (Moulded Case Circuit Breaker) symbol represents a factory-assembled overcurrent protective device rated 15–2500 A, providing thermal-magnetic or electronic trip protection within a moulded insulating case.
- Two pins define the symbol: Line (supply/input side) and Load (downstream/protected side); current flows from Line through the breaker contacts to Load.
- IEC 60947-2 is the primary international standard governing MCCB ratings, trip characteristics, and interrupting capacities; ANSI/UL 489 covers North American MCCB requirements.
- MCCBs differ from MCBs (miniature circuit breakers) in their higher current range (up to 2500 A vs typically ≤ 125 A for MCBs), higher interrupting capacity (up to 150 kA), and larger physical size with adjustable trip settings on larger frames.
- A dual-element MCCB trip combines a bimetallic thermal element (time-delayed response for overloads) with a magnetic coil (instantaneous response for short circuits), providing selective coordination with upstream and downstream protection devices.
- Interrupting capacity (kAIC) is the maximum prospective fault current the MCCB can safely interrupt without damage; this value must exceed the available fault current at the installation point.
- IEC 60617-07 defines the circuit-breaker schematic symbol; ANSI Y32.2 / IEEE 315-1975 defines the North American symbol. Both represent the MCCB as a switching element with a trip indicator.
Frequently asked questions
What does the MCCB symbol mean in a circuit diagram?
The MCCB (Moulded Case Circuit Breaker) symbol represents a factory-assembled circuit breaker rated 15–2500 A that provides overcurrent protection (overload and short-circuit), switching capability, and fault isolation for power circuits. In a diagram it marks a protection point in the distribution system, showing where circuit protection and isolation are provided.
What do the Line and Load pins mean on the MCCB symbol?
The Line pin is the supply side of the MCCB, connected to the power source (utility incomer, bus bar, or upstream feeder). The Load pin is the downstream side, connected to the protected circuit, motor, or sub-distribution board. Current flows from Line through the MCCB contacts to Load; the MCCB opens this path when an overcurrent is detected.
What is the difference between an MCCB and an MCB?
An MCB (Miniature Circuit Breaker) is rated up to 125 A, has a fixed trip setting, and is used for residential branch circuits (lighting, sockets). An MCCB (Moulded Case Circuit Breaker) is rated 15–2500 A, often has adjustable trip settings, provides higher interrupting capacity (up to 150 kA), and is used for industrial and commercial feeder and main protection. MCCBs are physically larger and more expensive than MCBs.
What does interrupting capacity (kAIC) mean for an MCCB?
Interrupting capacity (kAIC — kiloamperes interrupting capacity) is the maximum prospective short-circuit current the MCCB can safely interrupt without being destroyed or causing a dangerous arc. For example, a 65 kAIC MCCB can safely clear a fault of up to 65,000 A. The selected MCCB's kAIC rating must equal or exceed the available fault current at the point of installation, calculated from the supply transformer impedance.
What standard defines the MCCB circuit breaker symbol?
The circuit-breaker schematic symbol is defined by IEC 60617-07 for IEC-format drawings and by ANSI Y32.2 / IEEE 315-1975 for North American drawings. MCCB construction and performance requirements are governed by IEC 60947-2 internationally and ANSI/UL 489 in North America.
What are typical MCCB frame sizes?
Standard MCCB frame sizes (defined by the maximum continuous current rating of the frame) are typically 100 A, 160 A, 250 A, 400 A, 630 A, 800 A, 1600 A, and 2500 A. Smaller current-rated breakers can be fitted in a larger frame with a lower trip setting, but the kAIC rating is determined by the frame, not the trip setting.
Can an MCCB be used as a motor disconnect switch?
Yes. An MCCB can serve as both the motor branch-circuit short-circuit protection device and the motor disconnecting means, provided it is rated for the motor's locked-rotor current (NEC 430.52 or IEC 60947-4-1). For motors, the MCCB is often supplemented by a separate overload relay or uses a motor-rated MCCB with a dedicated motor-protection trip characteristic.
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