A 3 pole 100 amp MCB is a three-pole miniature circuit breaker with a declared current rating of 100 A per pole under its specified conditions. It does not create a single 300 A circuit. Selection requires the line-to-line voltage, current in each phase, available fault current and the exact device configuration. A suitable current rating alone does not establish motor protection, neutral switching or compatibility with a distribution board.
Read 100 A as a per-phase rating
Three-phase currents flow in separate conductors. If each phase carries 80 A, it is not correct to describe the breaker as carrying 240 A against a 100 A limit. Conversely, lightly loaded phases do not compensate for an overloaded phase. Review the load schedule and the most heavily loaded conductor rather than averaging the three readings to justify the selection.
Power and current are also different quantities. Converting a three-phase power demand to current requires the supply voltage and relevant power-factor or efficiency assumptions. Use the equipment’s input-current information where available, especially for motors and electronic loads, rather than dividing watts by a single phase voltage.
Confirm the neutral and common-trip arrangement
A standard three-pole arrangement connects the three phases; it does not provide an extra neutral pole. A circuit may include a continuous neutral where permitted, but a requirement to disconnect or protect that conductor must be addressed explicitly in the design. Do not put a phase through an unprotected neutral pole as a shortcut.
Specify the manufacturer’s common-trip construction. Three separate single-pole devices placed next to one another are not automatically equivalent. The related three-phase miniature circuit breaker guide explains what the linked device does and which protection functions still require separate consideration.
Check the conductor, busbar and enclosure at 100 A
At this rating, terminations and assembly conditions deserve as much attention as the breaker. Verify the cable ampacity after correction factors, the terminal conductor range, approved busbar system and enclosure temperature limits. The IEC-based selection relationship in the Electrical Installation Guide remains IB ≤ In ≤ Iz; a 100 A label does not override a lower cable or assembly limit.
For an actual product review, compare the CQB2-125 miniature circuit breaker information with CHAC’s MCB product range. Confirm the exact 100 A three-pole variant and its technical documentation. A larger catalogue family is a starting point, not evidence that every configuration is available or approved.
Use the fault study to check breaking capacity
The three-phase fault current at the board must be considered independently of the normal operating current. Verify the breaker’s declared short-circuit capacity at the actual voltage and pole arrangement. If backup protection is proposed, obtain the specific tested combination and observe the applicable installation rules; do not infer protection from the upstream device’s larger ampere rating.
Selectivity is another separate question. Two breakers may both interrupt the fault safely yet trip together, shutting down more of the installation than intended. Use the manufacturer’s coordination data and the project’s continuity requirements to assess that consequence.
Add motor-specific protection where the load requires it
A three-pole distribution MCB does not automatically detect phase loss, phase reversal or damaging imbalance. A motor circuit may need an overload device, a phase-monitoring function and suitable coordination with the contactor. Schneider Electric’s Micrologic FAQ illustrates that even more elaborate breaker systems have model-specific limits on phase-loss detection.
Choose the MCB trip curve from the documented starting duty and required fault-clearing performance. A C-curve is not a universal motor setting. Keep startup behavior, overload protection and short-circuit interruption as distinct checks so that a device which tolerates starting current is not mistaken for a complete motor-protection scheme.
What must be confirmed for a 100 A three-pole feeder?
| Decision | Required check | Common mistake |
|---|---|---|
| Normal current | Each phase and corrected cable ampacity | Adding pole ratings to get 300 A |
| Fault duty | Available current at the declared voltage | Choosing by amperes only |
| Neutral | Required switching and protection arrangement | Assuming a fourth pole exists |
| Motor duty | Starting current, overload and phase monitoring | Treating common trip as phase-loss detection |
Frequently asked questions
Where is a three-pole 100 A MCB used?
Common applications include three-phase feeders, small distribution boards and equipment circuits within the device’s voltage and fault ratings. The application must meet local code and manufacturer limits.
Is a C-curve always right for a motor?
Not always. A C-curve may tolerate moderate inrush, but motor protection often needs a dedicated protector and overload coordination. Use measured or documented starting current.
Can a 3-pole MCB switch the neutral?
A standard three-pole device opens the three phases. If neutral switching is required, specify the correct four-pole or linked arrangement.
How is a DIN-rail MCB installed?
Qualified personnel should follow the exact mounting, terminal, line/load and tightening instructions. Perform the installation tests required by the adopted rules and commissioning plan; do not improvise a short-circuit test.
Is a 3-phase breaker rating per phase?
Yes, the declared current applies to each pole under the stated conditions. A 100 A three-pole breaker is not a 300 A single-path device, and the least-loaded phase does not offset an overload on another phase.
How do I calculate an MCB rating for a 3-phase circuit?
Determine the load current in each phase using the equipment input data and supply conditions. Then apply the adopted design rules, conductor correction factors, starting-duty review and fault-current check. Round-up arithmetic alone cannot establish the correct protective-device rating.
References
- Schneider Electric Electrical Installation Guide: protective-device ratings
- Schneider Electric: limitations and options for phase-loss protection
For a model-specific review, contact CHAC Electric with the circuit diagram, supply voltage, load details and destination-market requirements.
