Enter your load power, voltage system, and power factor to find the right cable cross-section
This calculator solves the three key decisions in every electrical circuit: what cable size to use, which circuit breaker to install, and whether the cable run is short enough to avoid excessive voltage drop. Enter your load power, voltage system, power factor, and load type — the calculator returns a complete, coordinated result where the cable safely handles the current and the breaker correctly protects the cable.
The most important principle in circuit protection is that the breaker rating must sit between the load current and the cable ampacity. If the breaker is smaller than the load current, it trips during normal operation. If the breaker is larger than the cable can handle, the cable overheats before the breaker trips — a fire hazard. This calculator enforces this rule automatically: if the smallest suitable breaker exceeds the cable capacity, the calculator upsizes the cable until the chain is correct.
The formula depends on whether your supply is single-phase or three-phase:
| System | Formula | Example |
|---|---|---|
| Single-phase (1×230V) | I = P ÷ (V × cosφ) | 2300W ÷ (230V × 1.0) = 10.0A |
| Three-phase (3×400V) | I = P ÷ (√3 × V × cosφ) | 10000W ÷ (1.732 × 400 × 0.85) = 17.0A |
| Three-phase (3×230V) | I = P ÷ (√3 × V × cosφ) | 5000W ÷ (1.732 × 230 × 0.9) = 13.9A |
Circuit breakers are rated not only by their nominal current (10A, 16A, 32A) but also by their tripping characteristic — how they respond to short-duration overcurrents like motor startup surges. The letter before the rating describes this behavior:
| Type | Magnetic trip range | Typical use |
|---|---|---|
| B | 3–5× nominal | Lighting, sockets, heaters, resistive loads, EC motors |
| C | 5–10× nominal | AC induction motors, compressors, air conditioning |
| D | 10–20× nominal | Heavy motors, welding equipment, transformers |
Choosing the wrong characteristic causes problems in both directions. A type B breaker on a large AC motor trips at every startup because the inrush current exceeds 5× nominal. A type D breaker on a lighting circuit provides slower short-circuit protection than necessary. Match the characteristic to the load type, and the calculator does this automatically based on your selection.
Traditional AC induction motors draw a large inrush current at startup — typically 6–8 times their nominal current for a fraction of a second. This is why they need type C breakers that tolerate higher overcurrent before tripping. EC motors (electronically commutated motors) work differently. They have built-in inverter electronics that soft-start the motor, limiting inrush current to near-nominal levels. Their power factor is also much higher — typically 0.95 to 0.99 compared to 0.7–0.85 for AC motors.
For EC motors, use type B breakers and a power factor of 0.95–1.0. This applies to modern ventilation fans, recuperator units, VRF systems with inverter-driven compressors, and energy-efficient circulation pumps. Using type C for an EC motor is not dangerous, but it provides slower short-circuit protection than necessary — type B is the correct choice.
The way a cable is installed directly affects how much heat it can dissipate, which determines its maximum current rating. A cable clipped to a wall in open air (method C) cools much more efficiently than one buried inside an insulated wall (method A1). The same 6 mm² cable might carry 41A on a wall surface but only 34A inside a wall conduit — a difference that can push you to the next cable size up.
| Method | Description | Typical use |
|---|---|---|
| A1 | Insulated cable in conduit in thermally insulated wall | Residential walls with insulation |
| A2 | Multi-core cable in conduit in thermally insulated wall | Same, multi-core variant |
| B1 | Insulated cable in conduit on wall | Surface-mounted conduit, commercial |
| B2 | Multi-core cable in conduit on wall | Same, multi-core variant |
| C | Cable clipped directly to wall surface | Visible runs, workshops, basements |
| E | Cable on tray or ladder in free air | Industrial, data centers, plant rooms |
| D2 | Cable directly buried in ground | Outdoor supply, garden lighting, EV charging |
Choose the method that matches the actual installation. If the route is unknown, do not assume B1: identify the wall, conduit, and insulation arrangement or ask a qualified electrician. Cables through insulated walls often need A1 or A2, which can require a larger cable.
For A1, A2, B1, B2, C, and E, cable ampacity ratings assume 30°C air temperature. For D2 (cable directly buried in ground), the reference is 20°C ground temperature and the calculator uses the applicable ground-temperature factor. In hotter conditions, a cable can dissipate less heat and its safe current rating drops. For PVC cables in air, the correction factor follows √((70 − T) ÷ 40), where T is air temperature and 70°C is the maximum conductor temperature.
| Air temp | Correction factor | Effect on 36A cable |
|---|---|---|
| 20°C | 1.12 | 40.3A |
| 25°C | 1.06 | 38.2A |
| 30°C | 1.00 | 36.0A (reference) |
| 35°C | 0.94 | 33.8A |
| 40°C | 0.87 | 31.3A |
| 45°C | 0.79 | 28.4A |
| 50°C | 0.71 | 25.6A |
At 45°C the cable loses over 20% of its capacity — enough to change the required cable size by one or two steps. Always account for the worst-case temperature your cables will experience, not average conditions.
Power factor determines how much current a device draws for a given power output. A lower power factor means higher current and potentially larger cables. Here are typical values:
| Load type | Typical cosφ |
|---|---|
| Resistive (heaters, kettles, incandescent) | 1.0 |
| LED lighting with driver | 0.90–0.95 |
| EC motors, VRF, recuperators | 0.95–0.99 |
| Fluorescent lighting (compensated) | 0.85–0.95 |
| AC induction motors (small) | 0.75–0.85 |
| AC induction motors (large) | 0.80–0.90 |
| Welding equipment | 0.50–0.70 |
Cables are manufactured in standard sizes measured in square millimeters (mm²). The standard series is: 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, and 240 mm². Each size has a maximum current rating that depends on insulation type, installation method, and ambient temperature. This calculator uses reference values for PVC-insulated copper cables based on IEC 60364-5-52, covering all standard installation methods from conduit in insulated wall (A1) to free air on cable tray (E) and direct burial (D2).
Even when a cable can handle the current without overheating, long runs introduce voltage drop — the voltage loss caused by current flowing through cable resistance. If the drop is too large, motors run slower, lights dim, and electronics may malfunction. Most standards limit drop to 3–5% of nominal voltage. For 230V, that means no more than about 7–11.5V. Enable the voltage drop check to verify whether your cable meets these thresholds over the actual run length, and the calculator will suggest a larger cable if needed.
On a single-phase 230V circuit with cosφ = 1.0, a 3kW load draws 13.0A. With this calculator's default B1 installation method at 30°C, the reference result is 1.5 mm² with a B16 breaker. Select the actual installation method and check voltage drop; local rules or installation conditions can require a larger cable.
Use type B for resistive loads (heaters, lighting, sockets) and for EC motors with soft-start electronics. Use type C for AC induction motors and compressors that draw high startup current. Use type D only for heavy industrial loads like welding machines or large transformers. When in doubt, check whether your device has an inverter or soft-start — if yes, type B is sufficient.
EC motors have built-in inverter electronics that limit startup current, so type B is correct. Set the power factor to 0.95–0.99. A typical 150W recuperator fan on 230V draws only about 0.65A — a B6 breaker with 1.5 mm² cable is more than sufficient.
Three-phase distributes the same total power across three conductors, so each carries less current. A 10kW load on single-phase 230V draws 43.5A (needs 10 mm²), but on three-phase 400V it draws only about 14.4A (needs 2.5 mm²). This is why industrial and large residential installations use three-phase supply.
Choose the method that matches the actual installation: B1 for conduit on a wall surface, A1 or A2 for insulated walls, C for cables clipped directly to a wall, E for cable trays in free air, and D2 for cable directly buried in ground. If you cannot identify the route and insulation, do not default to B1; confirm it with the installer or a qualified electrician.
For A1, A2, B1, B2, C, and E, ratings assume 30°C air temperature. D2 buried cable uses 20°C ground temperature as its reference and a separate ground-temperature factor. At 45°C in air, a PVC cable carries about 21% less current than at 30°C, which may require a larger cable size.
The calculator may select a larger cable than the current alone requires for two reasons: the matching circuit breaker nominal exceeds the smaller cable's capacity (protection coordination), or the voltage drop over the cable length exceeds 3%. Both are safety requirements, not conservative estimates.
No. This calculator provides a reference estimate based on standard conditions. Real installations involve additional factors — derating for grouped cables, ambient temperature corrections, protection coordination, and compliance with local regulations. Always have a qualified electrician verify cable sizing for permanent installations.