Circuit Breaker Size Calculator
About Circuit Breaker Size Calculation
This Circuit Breaker Size Calculator calculates the required breaker size from load power for a 230 V single-phase or 400 V three-phase supply. It determines the operating current, selects a standard circuit breaker rating and provides a recommended copper or aluminum cable cross-section while accounting for power factor, demand factor, optional current margin, ambient temperature and installation method.
Guidelines and recommendations
Design load power
Design load power is determined from the total active power P in kW and the demand factor k. For a single load, k = 1 is used. For a group of loads, the available values are 1, 0.9, 0.8, 0.7, 0.6 and 0.5. The factor is selected by the user and is not reduced automatically according to the number of connected devices.
Pdesign = P × k
A value of k = 1 means that 100% of the installed power is included in the calculation. For example, k = 0.8 means that the simultaneous design load is taken as 80% of the total installed power. If the simultaneous load cannot be estimated reliably, use k = 1.
Converting power to operating current
Operating current is calculated from the design power, supply voltage and power factor cosφ. Power entered in kW is first converted to watts by multiplying by 1000.
For a 230 V single-phase supply:
Ioperating = Pdesign × 1000 / (230 × cosφ)
For a 400 V three-phase supply:
Ioperating = Pdesign × 1000 / (√3 × 400 × cosφ)
Power factor accounts for the increase in current caused by the reactive component of the load. The calculator uses cosφ = 1 for resistive loads, 0.95 for mixed loads and 0.8 for motors, pumps and compressors. The lower the cosφ, the higher the operating current for the same active power.
Three-phase calculation assumes that the load is evenly distributed across the three phases. The formula containing the √3 factor is valid under this assumption.
Optional current margin and breaker selection
Additional current margin is entered as a percentage and is 0% by default. If a margin r is specified, the operating current is increased by that percentage.
Idesign = Ioperating × (1 + r / 100)
With a 0% margin, Idesign = Ioperating. This margin is an optional user-defined value and not a mandatory fixed factor required by the standard.
Circuit breaker rating is selected as the smallest standard value that is not lower than the design current. The calculator uses the standard series 6, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80 and 100 A. For example, a design current of 18.3 A results in a 20 A breaker, while 20.1 A results in a 25 A breaker.
Actual breaker margin shows the percentage difference between the selected standard breaker rating and the design current:
Margin = (In / Idesign - 1) × 100%
If the design current exceeds 100 A, the calculator displays >100 because higher breaker ratings are not included in the series used by this calculator.
Cable cross-section selection
Cable cross-section is selected according to the chosen breaker rating so that the permissible continuous current of the cable is not lower than the breaker rated current. For a single-phase circuit, tabulated values for two loaded conductors are used. For a three-phase circuit, values for three loaded conductors are used.
The calculation is based on cable with PVC insulation and a maximum conductor operating temperature of 70 °C. For copper conductors, the calculator checks standard cross-sections of 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185 and 240 mm2. For aluminum conductors, it checks 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185 and 240 mm2.
Installation method determines the base current-carrying capacity of the cable. The IEC 60364-5-52 reference methods used are A2 for a multicore cable in conduit in a thermally insulated wall, B2 for a multicore cable in conduit mounted on a wall, and C for a cable mounted directly on a wall or on an unperforated cable tray. If no installation method is selected, method B2 is used.
Ambient temperature correction
Temperature correction factor adjusts the cable current-carrying capacity relative to the reference ambient air temperature of 30 °C. If temperature correction is disabled, kT = 1 is used. When enabled, the following factors for PVC insulation are applied:
- 10 °C and below -
1.22 - 15 °C -
1.17 - 20 °C -
1.12 - 25 °C -
1.06 - 30 °C -
1.00 - 35 °C -
0.94 - 40 °C -
0.87 - 45 °C -
0.79 - 50 °C -
0.71 - 55 °C -
0.61 - 60 °C -
0.50
For temperatures between the listed points, the factor for the next higher temperature is used. For example, at 12 °C the calculator applies the 15 °C factor 1.17, while at 31 °C it applies the 35 °C factor 0.94. For temperatures below 10 °C, the factor 1.22 is used. Above 60 °C, no cable cross-section recommendation is produced.
Required tabulated current-carrying capacity is calculated by dividing the breaker rated current by the temperature correction factor:
Itable = In / kT
The calculator then selects the smallest cable cross-section whose tabulated permissible current is not lower than Itable, taking into account conductor material, number of loaded conductors and installation method. As a result, a higher ambient temperature or a less favorable installation method may require a larger cable cross-section even when the circuit breaker rating remains unchanged.
European standards used as reference
IEC 60364-5-52 and HD 60364-5-52 - Low-voltage electrical installations. Part 5-52: Selection and erection of electrical equipment. Wiring systems. These documents provide the principles used for conductor current-carrying capacity, reference installation methods A2, B2 and C, and temperature correction factors for PVC insulation.
IEC 60364-4-43 and HD 60364-4-43 - Low-voltage electrical installations. Part 4-43: Protection for safety. Protection against overcurrent. The selection of the protective device follows the general principle of coordinating the design current, circuit breaker rating and permissible cable current.
IB ≤ In ≤ Iz
EN 60898-1 - Electrical accessories. Circuit-breakers for overcurrent protection for household and similar installations. Circuit-breakers for a.c. operation. This standard applies to circuit breakers used for overcurrent protection of wiring in household and similar electrical installations.
FAQs
Why does the calculator select a 20 A breaker when the calculated current is 18 A?
Circuit breaker ratings are selected from a standard series, so there is no 18 A breaker in the series used by the calculator. The next standard rating that is not lower than the design current is selected, so an 18 A result leads to a 20 A circuit breaker.
How should I choose the demand factor for a group of loads?
If all connected loads may operate at the same time, use k = 1. Values of 0.9, 0.8, 0.7, 0.6 or 0.5 should only be used when the simultaneous load can reasonably be expected to be lower than the total installed power.
Why is the three-phase current lower for the same load power?
In a balanced three-phase system, the total power is distributed across three phases and the formula includes the √3 factor. Therefore, for the same total load power, the current in a 400 V three-phase system is lower than in a 230 V single-phase system.
Why does the recommended cable cross-section change with temperature or installation method?
The permissible continuous current of a cable depends on how effectively it can dissipate heat. Higher ambient temperature and installation conditions with poorer heat dissipation reduce the cable current-carrying capacity, so a larger conductor cross-section may be required for the same circuit breaker rating.
What does the actual circuit breaker margin show?
It is the percentage difference between the selected standard circuit breaker rating and the calculated design current. This helps show how much higher the selected standard rating is than the actual calculated load, for example the difference between 18.3 A and a 20 A breaker is relatively small, while the difference between 20.1 A and a 25 A breaker is noticeably larger.