Circuit Breaker Size Calculator
About Circuit Breaker Size Calculation
This calculator estimates the recommended circuit breaker size (A) from electrical load power (kW), network voltage (230 V single-phase or 400 V three-phase), demand factor, power factor (cosφ), and a chosen current margin. It also provides indicative copper and aluminum cable cross-sections (mm2) based on the selected breaker and correction factors for ambient temperature and installation method. It is useful for quick sizing of protective devices and feeder cables in typical low-voltage 230/400 V systems.
Tips and Tricks
Step 1 - Apply demand factor to power. For a single dedicated load, the calculator uses k ≈ 1. For a group line, it reduces the total installed power by a simultaneity (demand) factor k (0…1). The effective active power is computed as P_eff = P_total × k, where P_total is in kW and P_eff is in kW.
Step 2 - Convert power to operating current. The calculator converts P_eff (kW) to watts and divides by voltage and power factor. For single-phase 230 V: I_oper = P / (U × cosφ). For three-phase 400 V: I_oper = P / (√3 × U × cosφ). Here P is in W, U in V, and the result I_oper is in A. The use of 230/400 V corresponds to the European low-voltage supply convention (EN 50160).
Step 3 - Add sizing margin. A margin is applied to cover expected growth, long-term loading, and uncertainty: I_calc = I_oper × (1 + margin/100), where margin is in %. The breaker is selected from the first standard rating that is not less than I_calc.
Step 4 - Pick the breaker from a standard series. The calculator rounds up to the nearest standard rating in a typical set (e.g., 6, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100 A). This reflects common MCB nominal currents used under IEC 60898-1 in many LV distribution boards. If your local market uses a different series (for example, 12 A instead of 13 A), the “next higher standard size” principle stays the same.
Step 5 - Compute the “actual margin”. After choosing I_n, the calculator reports how much the selected breaker exceeds the calculated current: margin_actual = (I_n / I_calc − 1) × 100%. A very large actual margin can hint that you are between standard sizes or that the assumed cosφ / demand factor is conservative.
Step 6 - Correct cable current capacity for conditions. Cable sizing is based on the chosen breaker current I_n, but adjusted by correction coefficients for conditions: k_cond = k_temp × k_inst. The calculator then computes a “table current” requirement: I_table = I_n / k_cond. Lower k_cond (hotter ambient or worse cooling) increases I_table, which forces a larger cross-section. This approach follows the correction-factor logic used in IEC 60364-5-52 / HD 60364 for current-carrying capacity.
Step 7 - Choose minimum cross-section from lookup tables. The calculator finds the smallest conductor section S (mm2) whose allowable current I_allow (A) is at least I_table. Copper and aluminum use separate lookups because aluminum typically needs a larger section for the same current. Treat these as indicative: final selection should also consider voltage drop, short-circuit withstand, protective device characteristics (B/C/D curves), grouping of multiple circuits, and national annexes to IEC/HD 60364.
Common practical inputs. Many household mixed loads are often approximated with cosφ around 0.95, while motor-heavy circuits are often closer to 0.8. For groups of socket outlets or mixed appliances, a demand factor k below 1 is commonly used because not everything runs at full power at the same time. A margin of 10-25% is frequently used in preliminary sizing, but the “best” margin is the one justified by the design context and applicable rules.
FAQs
Why does the breaker rating round up instead of matching the calculated current exactly?
MCBs and other breakers are manufactured in standard nominal currents, so selection follows the “next higher standard size” rule. The calculator picks the smallest standard rating In that is not less than Icalc, which reduces nuisance tripping during sustained operation near the design load.
What is the difference between operating current and calculated current with margin?
Operating current is computed directly from power, voltage, and cosφ using I_oper. Calculated current adds your selected percentage margin: I_calc = I_oper × (1 + margin/100). The circuit breaker size is chosen from I_calc, not from I_oper.
Do ambient temperature and installation method change the breaker size?
In this calculator, temperature and installation method affect the cable cross-section recommendation via correction factors, but they do not change the breaker selection step. In real designs, overall coordination still matters (IEC 60364-5-52 / HD 60364): the protective device rating and the conductor capacity must be compatible under the actual installation conditions.
How should I choose cosφ for this circuit breaker calculator?
Use cosφ ≈ 1 for purely resistive heating and similar loads, around 0.95 for typical mixed household circuits, and around 0.8 for motor-driven equipment. If you are unsure, picking a slightly lower cosφ is a conservative way to avoid underestimating current.
Are the copper/aluminum cable sections final values?
No—treat them as a fast, indicative starting point based on current-carrying capacity and simple correction factors. Final cable selection should also check voltage drop limits, protective device time/current characteristics, short-circuit conditions, and local wiring rules that implement IEC/HD 60364.