Circuit Breaker Sizing Calculator
Select the right protective device the way BS 7671 demands it: the breaker must carry the design current, must not exceed what the cable can withstand under its real installation conditions (Ib ≤ In ≤ Iz), must ride through the load’s starting current without nuisance tripping, and must provide the right earth-leakage protection for the circuit type. Enter the load, its inrush, and the cable it protects, and the calculator returns a recommended MCB, MCCB or RCBO rating, the pole configuration, and trip-curve guidance with reasons.
The cable itself should be sized first — use the Cable Sizing and Voltage Drop Calculator, then verify the breaker here.
Protective device selection CALC-06 · REV 2026-07
| Design current, Ib | – |
|---|---|
| Cable capacity, tabulated Iz | – |
| Derating, Ca × Cg | – |
| Cable capacity after derating, Iz | – |
| Recommended device | – |
| Coordination check, Ib ≤ In ≤ Iz | – |
| Pole configuration | – |
| Trip curve | – |
| Estimated inrush | – |
| RCD / earth-leakage protection | – |
| Breaking capacity | – |
Guidance tool only. The selection assumes BS EN 60898 / 61009 / 60947 devices and typical cable data; the full design must also verify earth-fault loop impedance (Zs) against the chosen curve, discrimination with upstream devices, and short-circuit energy let-through — work for a registered engineer.
How this calculator works
- Design current. Entered directly or computed from kW, voltage, phase and power factor, exactly as in the cable calculator.
- Coordination, Ib ≤ In ≤ Iz. The breaker rating In must sit between the design current and the cable’s current-carrying capacity after derating for ambient temperature and grouping (Tables 4B1 and 4C1). The companion condition I2 ≤ 1.45 Iz is met automatically by BS EN 60898 and 61009 devices once In ≤ Iz. If no standard rating fits between the two, the cable is the problem — the calculator says so and suggests the minimum compliant size.
- Trip curve. Curves B, C and D trip magnetically at 3–5, 5–10 and 10–20 times In respectively. To avoid nuisance tripping, the starting current should sit below In times the curve’s lower threshold; the calculator computes the inrush-to-rating ratio and selects accordingly, never going below the practical minimum for the load type. Inrush beyond 10× points to an MCCB with adjustable settings or a soft starter.
- Poles and RCD. Single-phase final circuits use 1P (or 1P+N RCBO); bathroom and EV circuits get double-pole isolation; three-phase motors take 3P and mixed three-phase loads 3P+N. Thirty-milliamp additional protection follows BS 7671: sockets up to 32 A (Reg 411.3.3), bathrooms (Section 701), EV charging with Type A + 6 mA DC detection (Section 722), and domestic lighting in new work (Reg 411.3.4). Feeders instead take time-delayed upstream earth-leakage for discrimination.
- Breaking capacity. Guidance scales from 6 kA (typical domestic, BS EN 60898) through 10 kA commercial boards to verified fault-level MCCB selection near transformers (BS EN 60947-2).
Assumptions and limits
- Cable capacities use the same typical PVC multicore tables as the cable calculator; XLPE and single-core installations need manufacturer data.
- Earth-fault loop impedance (Zs) verification per curve, discrimination studies and let-through energy checks are outside this tool and mandatory in a full design.
Building a full distribution board schedule? Our consultancy services and design document library cover it, or learn the method in a training program.