Quick Answer: ACB or MCCB?
Use an ACB (Air Circuit Breaker) for the main incoming position, bus-ties, and any circuit above 1000A where you need draw-out capability and advanced protection. Use an MCCB (Molded Case Circuit Breaker) for branch circuits, feeder protection, and motor circuits from 16A to 1600A where compact size and cost matter most. In most switchboards, you will use both: one or two ACBs at the incoming/bus-tie positions and dozens of MCCBs for outgoing feeders.
What Is an Air Circuit Breaker (ACB)?
An ACB is a large-frame, low-voltage circuit breaker designed for high currents (typically 800A to 6300A). It uses atmospheric air and an arc chute — a stack of insulated metal plates — to extinguish the arc when the contacts separate. ACBs are built as open-frame devices that mount on a draw-out chassis, allowing the breaker to be physically withdrawn from the switchboard for inspection or replacement without de-energizing the bus.
Key characteristics:
- Current range: 800A to 6300A (some models start at 400A)
- Breaking capacity: 42 kA to 150+ kA at 400V
- Mounting: Draw-out (withdrawable) on chassis rails
- Trip unit: Electronic with LCD display, communication ports, metering
- Size: Large — typically 400-600 mm wide
- Typical position: Main incoming, bus-tie, generator, large feeder
NAIJI Electric's EZMX1 air circuit breaker delivers up to 6300A rated current and 135 kA breaking capacity in one of the world's most compact form factors. It features TH (tropical/humid) rating, integrated communication networking, and multiple controller options.
What Is a Molded Case Circuit Breaker (MCCB)?
An MCCB is a compact circuit breaker enclosed in a molded insulating housing, designed for branch circuit and feeder protection from 16A to 1600A. The molded case provides the structural frame, insulation between phases, and arc-extinguishing chambers. MCCBs are fixed-mount devices — once installed, they cannot be withdrawn without disconnecting cables.
Key characteristics:
- Current range: 16A to 1600A (frame sizes 100A, 250A, 400A, 630A, 800A, 1600A)
- Breaking capacity: 10 kA to 70 kA at 400V (up to 100 kA for premium models)
- Mounting: Fixed — bolted to bus bars or DIN rail (smaller frames)
- Trip unit: Thermal-magnetic (standard) or electronic (adjustable)
- Size: Compact — 75 to 200 mm wide depending on frame
- Typical position: Branch feeders, motor circuits, distribution panels
NAIJI Electric's EZM1 MCCB series covers 16A to 630A with rated insulation voltage of 800V, supporting working voltages up to 690V. CCC certified with overload, short-circuit, and undervoltage protection built in.
ACB vs MCCB: Side-by-Side Comparison
| Parameter | ACB (Air Circuit Breaker) | MCCB (Molded Case) |
|---|---|---|
| Current Range | 800A - 6300A | 16A - 1600A |
| Breaking Capacity (400V) | 42 - 150+ kA | 10 - 70 kA |
| Mounting | Draw-out (withdrawable) | Fixed (bolted) |
| Trip Unit | Electronic (LCD, comms, metering) | Thermal-magnetic or electronic |
| Communication | Modbus, Profibus, Ethernet (standard) | Optional (higher-end models only) |
| Metering | Built-in V, I, P, E, PF, THD | None or basic (electronic trip only) |
| Maintenance | Withdraw breaker, bus stays live | Must de-energize to service |
| Size (per pole) | Large (400-600 mm) | Compact (75-200 mm) |
| Cost | High (5-20x MCCB cost) | Low to moderate |
| Electrical Life | 8,000 - 20,000 operations | 1,000 - 5,000 operations |
| Mechanical Life | 20,000 - 50,000 operations | 10,000 - 20,000 operations |
| Standards | IEC 60947-2 | IEC 60947-2 |
| Typical Application | Main incoming, bus-tie, generator | Branch feeders, motors, distribution |
When to Choose an ACB
- Main incoming breaker: The primary protection point where utility power enters your switchboard — highest current, highest fault level, needs draw-out for maintenance
- Bus-tie (bus coupler): Connects two bus sections and must handle full bus current with draw-out capability
- Generator incoming: Protects and switches generator power, often requiring special protection schemes (reverse power, under-frequency)
- Large feeders above 1000A: Any circuit exceeding MCCB range
- Critical circuits requiring zero downtime: Draw-out design allows breaker swap without de-energizing the bus
- Energy management: Built-in metering provides real-time power monitoring without separate meters
When to Choose an MCCB
- Branch circuit protection: Individual feeders to motors, lighting panels, HVAC, UPS systems
- Motor protection: Thermal-magnetic trip provides overload and short-circuit protection for motors
- Distribution panels: Multiple outgoing circuits in a compact space
- Budget-constrained projects: MCCBs cost a fraction of ACBs for the same current rating in the overlap range
- Space-limited installations: Compact form factor fits more circuits per panel
- Circuits below 800A: ACBs are over-specified for small circuits
Typical Switchboard Layout: ACB + MCCB Together
In a standard industrial or commercial LV switchboard, ACBs and MCCBs work together in a hierarchy:
- Main incoming ACB (e.g., EZMX1 at 2000A) — receives power from the transformer and provides main protection
- Bus-tie ACB (optional) — connects two transformer sections for redundancy
- Large feeder MCCBs (e.g., EZM1 at 400-630A) — feeds sub-distribution boards and large motors
- Small feeder MCCBs (EZM1 at 16-250A) — individual circuit protection
This hierarchy ensures selective coordination: if a fault occurs on a branch circuit, only the local MCCB trips, not the main ACB. The upstream breaker remains closed, keeping all other circuits energized.
Selective Coordination: Why It Matters
Selective coordination (also called discrimination) means that only the breaker closest to the fault trips, while all upstream breakers remain closed. This minimizes the impact of a fault to the smallest possible section of the system.
ACB + MCCB combinations achieve selectivity through:
- Current selectivity: The ACB trip threshold is set higher than the MCCB rating
- Time selectivity: The ACB has a deliberate time delay (short-time pickup) that lets the downstream MCCB trip first
- Zone-selective interlocking (ZSI): Advanced ACBs can communicate with downstream breakers to accelerate tripping when the fault is on the ACB's own bus
Quick Selection Guide
| Circuit Type | Current | Recommended |
|---|---|---|
| Main incoming from transformer | 1000-6300A | ACB (EZMX1) |
| Bus-tie / bus coupler | 1000-6300A | ACB |
| Generator incoming | 800-3200A | ACB with reverse power protection |
| Large motor (>200 kW) | 400-1000A | MCCB electronic trip (EZM1) |
| Sub-distribution feeder | 100-630A | MCCB |
| Small motor (<50 kW) | 16-100A | MCCB thermal-magnetic |
| Lighting panel feeder | 63-250A | MCCB |
| UPS / IT load | 100-800A | MCCB or ACB depending on criticality |
NAIJI Electric Low Voltage Solutions
NAIJI Electric provides a complete low-voltage protection lineup:
- EZMX1 ACB — up to 6300A / 135 kA, TH-rated, integrated communication, one of the world's most compact ACBs
- EZM1 MCCB — 16-630A, 800V insulation, AC 400/500/690V, CCC certified, overload + short-circuit + undervoltage protection
- GCS Switchgear — Draw-out LV switchgear platform (4000A bus) for housing ACBs and MCCBs in MCC and distribution applications
- GGD Switchgear — Fixed-type LV switchgear (3150A bus) for cost-effective distribution
Need help designing a complete LV switchboard with proper ACB/MCCB coordination? Contact the NAIJI Electric engineering team.