Breaker Size Calculator determines the appropriate circuit breaker size for electrical loads based on current or power input. It helps electrical engineers and other MEP designers select protective devices that comply with IEC/NEC standard series and continuous-load rules.
About breaker sizing. Breaker sizing ensures that protective devices trip reliably under overload or short-circuit conditions while allowing normal current to flow. Selecting the correct breaker is crucial to prevent overheating, nuisance tripping, or damage to equipment and conductors.
About current and power inputs. Loads can be expressed either in amperes (A) or in kilowatts (kW) with known voltage, power factor (PF), and phase. This calculator lets you choose between entering load current directly or calculating it from power and supply conditions. Three-phase systems distribute current across three conductors, reducing per-conductor current compared to single-phase systems.
About continuous load. Electrical codes such as NEC and IEC require that continuous loads (running for 3 hours or more) be protected by breakers rated at least 125% of the load current. This prevents overheating during prolonged operation. The calculator automatically applies this multiplier when the “Continuous” option is selected.
About standard series. Breakers are manufactured in standardized current ratings. Common IEC/EN sizes include 6, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, and 200 A. Common NEC/UL sizes include 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, and 200 A. This calculator automatically rounds up to the nearest standard size.
Calculation. This calculator determines breaker size from either direct current input or power-based current calculation. It applies code-required multipliers for continuous loads and safety factors, then rounds up to the nearest standard breaker size.
For the “By Current” workflow, the breaker size is based on load current:
If continuous: Ibase = 1.25 × Iload
Else: Ibase = Iload
Ireq = Ibase × Safety Factor
For the “By Power” workflow, the current is calculated first:
Single-phase: I = (P × 1000) / (V × PF)
Three-phase: I = (P × 1000) / (√3 × V × PF)
Then the same steps as above apply.
- Example 1: 18 A non-continuous → 18 A × 1.00 ≈ 18 A → next IEC size = 20 A
- Example 2: 18 A continuous → 18 × 1.25 = 22.5 A → next IEC size = 25 A
- Example 3: 4.5 kW @ 230 V 1φ, PF 0.9 → ≈21.7 A → continuous → 21.7 × 1.25 = 27.1 A → next IEC size = 32 A
Corresponding tools. To evaluate conductor sizes, use the Voltage Drop Calculator. To estimate upstream loads, use the Heating Load Calculator or Cooling Load Calculator.
