Free Online Calculators
Find the right breaker & cable cross-section
Convert watts to amps with I = P / (U × cos φ), or I = P / (√3 × U × cos φ) on three-phase. The calculator then gives the next standard breaker and the minimum cable size per IEC 60364-5-52 — or, the other way round, the largest breaker for an existing cable.
A 1.5 mm² cable takes a breaker of at most 16 A, 2.5 mm² up to 20 A and 4 mm² up to 25 A (multicore cable in conduit, single-phase, 30 °C). At 230 V a 16 A breaker supplies up to 3,680 W, at 400 V three-phase 11,085 W. Values follow IEC 60364-5-52 as used by BS 7671 — not the US NEC/AWG system — and installation belongs with a qualified electrician.
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| Cable size | Current rating (B2) | Breaker 230 V | Max. load 230 V | Breaker 400 V (3-phase) | Max. load 400 V | In insulation (A2) |
|---|---|---|---|---|---|---|
| 1.5 mm² | 16.5 A | 16 A | 3,680 W | 13 A | 9,007 W | 13 A |
| 2.5 mm² | 23 A | 20 A | 4,600 W | 20 A | 13,856 W | 16 A |
| 4 mm² | 30 A | 25 A | 5,750 W | 25 A | 17,321 W | 25 A |
| 6 mm² | 38 A | 32 A | 7,360 W | 32 A | 22,170 W | 32 A |
| 10 mm² | 52 A | 50 A | 11,500 W | 40 A | 27,713 W | 40 A |
| 16 mm² | 69 A | 63 A | 14,490 W | 50 A | 34,641 W | 50 A |
| 25 mm² | 90 A | 80 A | 18,400 W | 80 A | 55,426 W | 63 A |
| 35 mm² | 111 A | 100 A | 23,000 W | 80 A | 55,426 W | 80 A |
| 50 mm² | 133 A | 125 A | 28,750 W | 100 A | 69,282 W | 100 A |
Single-phase, yes: 16 mm² carries 69 A in conduit (B2) and 85 A clipped direct (C). Three-phase in conduit, no: 62 A is below 63 A. Clipped direct (76 A) it works; in insulation (52 A) it does not.
20 A in conduit (method B2), 25 A clipped direct (C) and 16 A in a thermally insulated wall (A2) — single-phase, 30 °C, not grouped.
6 mm² in conduit (B2), 4 mm² clipped direct (C) and 6 mm² in insulation (A2), single-phase. Check the voltage drop on longer runs.
3,680 W at 230 V, 1,920 W at 120 V and 11,085 W on a 400 V three-phase supply (power factor 1).
3,500 W draws 15.2 A at 230 V, so a 16 A breaker with at least 1.5 mm² cable (in conduit) — more on long runs because of voltage drop.
Whether you're planning a new home build, renovating an existing property, or simply adding a new circuit for an appliance, choosing the right circuit breaker size and cable cross-section is essential for electrical safety. Our free Circuit Breaker Calculator makes this process simple, fast, and accurate – no electrical engineering degree required.
An undersized circuit breaker will trip repeatedly under normal load, causing frustration and interrupting power. An oversized breaker, however, won't protect the cable adequately – this can lead to overheating, insulation damage, or even electrical fires. Getting the sizing right is not just about convenience; it's a fundamental electrical safety requirement under IEC 60364 and the national codes based on it, such as BS 7671. The US NEC uses a different, AWG-based system that this calculator does not cover.
Our circuit breaker calculator helps homeowners, electricians, and construction professionals quickly determine the right breaker rating and minimum cable size before starting any electrical project.
The calculator uses Ohm's Law and the power formula P = U × I × cos φ together with the current ratings of IEC 60364-5-52. Simply enter the following details:
Based on these inputs, the calculator instantly outputs the load current, the recommended breaker amperage (the next standard rating), the minimum cable cross-section, the maximum load at that breaker and its utilisation in %. Voltage drop is not included – use our cable size calculator for that.
A standard electric range or cooker draws about 7,200 watts. On a 400V three-phase supply the calculator determines 10.4A per phase, a 13A breaker and a minimum of 1.5 mm² cable in conduit (method B2). If you protect it with a 16A breaker instead, you need 2.5 mm², because 1.5 mm² carries only 15A three-phase in conduit. On a single-phase 230V supply the same cooker draws 31.3A, which means a 32A breaker and at least 6 mm² copper cable in conduit.
A 3,500W split-system air conditioner running on 230V draws around 15.2A. The calculator recommends a 16A circuit breaker (95% utilisation) with a minimum of 1.5 mm² copper cable in conduit, or 2.5 mm² in a thermally insulated wall. Voltage drop is not checked here: at 20 meters, 1.5 mm² would already exceed 3%, so the cable size calculator recommends 2.5 mm².
An outdoor socket rated for up to 3,680W (16A at 230V) requires a 16A breaker and at least 1.5 mm² cable for short runs in conduit or clipped direct. This calculator does not warn about voltage drop: at 16A over 20 meters, 1.5 mm² gives 3.3%, so 2.5 mm² is needed – check longer runs with the cable size calculator.
Voltage drop refers to the reduction in electrical voltage as current travels along a cable. A limit of 3% is a common design value for final circuits in homes. Excessive voltage drop causes poor appliance performance, overheating, and increased energy waste.
Cable cross-sectional area directly affects current-carrying capacity. A 1.5 mm² copper cable carries 16.5A in conduit (method B2) and 19.5A clipped direct (C); a 2.5 mm² cable carries 23A and 27A (single-phase, 30 °C). That allows a breaker of up to 16A on 1.5 mm² and 20A (B2) or 25A (C) on 2.5 mm². The calculator automatically references these capacity tables and always shows the smallest cable size whose rating covers the breaker.
Not all circuit breakers are the same. The trip curve defines how quickly a breaker responds to overcurrent:
The calculator does not choose the trip curve: it sizes the breaker rating, and you pick the curve to suit the load – Type B is the usual choice for household sockets and lighting.
This tool is designed for DIY homeowners, licensed electricians, construction project managers, architects, and engineering students. While it provides accurate guidance based on standard formulas, it is intended as a planning aid. All electrical installations should be completed and inspected by a qualified electrician in accordance with local codes.
A 20A circuit needs a 20A circuit breaker and at least 2.5 mm² copper cable in conduit (23A) or clipped direct (27A). In a thermally insulated wall 2.5 mm² carries only 18.5A, so use 4 mm². The calculator works with metric sizes to IEC 60364-5-52, not AWG, and does not check voltage drop – use the cable size calculator for long runs.
Yes! The circuit breaker calculator fully supports both single-phase and three-phase electrical systems. Simply select your voltage configuration and enter the total load power.
Using undersized wire creates excessive resistance, which causes heat buildup. In severe cases, this can melt insulation and start a fire. Always use at least the minimum cable size shown by the calculator, and a larger one on long runs where voltage drop decides.
No. This calculator is a planning and educational tool. A licensed electrician must design, install, and sign off on all electrical work in compliance with local electrical codes and safety standards.
Absolutely. Our circuit breaker calculator is completely free to use, with no registration or subscription required. Use it as many times as you need for any project.