How to Size a Molded Case Circuit Breaker for a Power Supply?
Selecting the right molded case circuit breaker (MCCB) is essential to keep electrical systems safe and reliable. Whether you are designing a distribution panel or upgrading an industrial machine, the correct breaker rating prevents overloads, short-circuit failures, and unnecessary downtime. So, how do you correctly size an MCCB for a power supply? Here is a clear and practical guide used by electrical engineers worldwide.
1. Determine the Load Current (Full Load Current)
Calculate the current based on the load power:
Current (A) = Power (W) ÷ Voltage (V)
For three-phase systems:
I = P ÷ (√3 × V × Power Factor)
This value helps select the breaker rated current (In).
2. Set the Breaker Trip Rating Based on Application
Once current is known, choose a breaker rating slightly above the load current to avoid nuisance trips:
| Load Type | Suggested Breaker Size |
|---|---|
| Resistive loads (heater, lighting) | 100% to 125% of load current |
| Inductive loads (motor, pumps) | 125% to 150% of load current |
For motors, use manufacturer recommendations and local standards such as IEC 60947-2.
3. Check Short-Circuit Capacity (Icu/Ics)
Different installations face different fault levels. Ensure the chosen MCCB can withstand the short-circuit current available at the installation point:
Industrial substations: 25–65 kA or higher
Commercial buildings: 10–36 kA
Small facilities: 6–10 kA
Breakers must meet or exceed the fault level to avoid catastrophic damage.
4. Consider Voltage, Poles, and Installation Environment
Voltage: Match the system (AC 230/415V, 690V, etc.)
Poles:
1P for single-phase
3P/4P for three-phase systems
Ambient temperature: High temperatures may require derating
5. Choose Protection Features
Different trip units offer different protections:
| Trip Unit Type | Protection |
|---|---|
| Thermal-Magnetic | Overload + Short circuit |
| Electronic Trip Unit | Adjustable protection, energy measurement, selectivity |
Quick Example
A 3-phase motor:
Power: 45 kW
Voltage: 400V
Power Factor: 0.85
Current = 45,000 ÷ (1.732 × 400 × 0.85) ≈ 76.6 A
Choose an MCCB:
100A rating
36 kA breaking capacity
3-pole
Adjustable trip (electronic or thermal-magnetic)
Why Choose SSPD MCCB?
✔ Wide range: 16A–1600A, 10–80 kA
✔ Strong short-circuit performance
✔ Fully compliant with IEC 60947-2
✔ Compatible with Schneider-type accessories (shunt trip, UVR, auxiliaries)
✔ OEM/ODM support for global distributors
We supply reliable MCCB solutions for switchgear builders, wholesalers, and industrial projects in 80+ countries.
📩 Contact us for technical datasheets and quotations:
[email protected] | www.shendianele.com
