Circuit Breaker Sizing Calculator — MCB Rating Guide

This calculator determines the correct Miniature Circuit Breaker (MCB) rating for your circuit based on BS 7671 18th Edition wiring regulations. The MCB must be sized to protect the cable while being large enough to carry the design current without nuisance tripping.

The sizing rule

Ib ≤ In ≤ Iz

Where Ib = design current, In = MCB rating, Iz = cable current carrying capacity.

MCB types explained

• Type B: Trips at 3-5x rated current. Standard for domestic installations (lighting, sockets, showers, cookers).
• Type C: Trips at 5-10x rated current. For moderate inrush loads (some commercial, fluorescent lighting circuits).
• Type D: Trips at 10-20x rated current. For heavy inductive loads (large motors, transformers, welding equipment).

Standard UK MCB ratings

Available ratings: 6A, 10A, 16A, 20A, 32A, 40A, 45A, 50A. The calculator selects the smallest standard MCB rating that is equal to or greater than the design current, then cross-references typical configurations per BS 7671 for the selected circuit type.

Cooker diversity

For a cooker circuit, the calculator applies the IET On-Site Guide household-cooker rule (Table 1B) rather than a flat multiplier: 10A + 30% of the remaining full-load current, +5A if a socket is in the control unit. A 12kW cooker (≈52A) therefore assesses to 10 + 30% × (52 − 10) ≈ 22.6A, i.e. a 32A MCB, because heating elements rarely all run at once. BS 7671 itself (Reg 311.1) only requires the designer to assess maximum demand and leaves the diversity method to engineering judgement, so this figure comes from the On-Site Guide, not a BS 7671 appendix. Enter the cooker’s full (nameplate) rating — diversity is applied for you.

Cable size is a minimum

The cable shown is the smallest twin & earth that satisfies Ib ≤ In ≤ Iz for clipped-direct installation (Table 4D5, Method C), but never below the established trade minimum for the circuit type (10mm² for showers ≥8kW, 6mm² for cookers and EV chargers). The bare Table 4D5 figure is an ideal maximum — you must still derate for grouping, thermal insulation and ambient temperature (Reg 523 / Appendix 4) and check voltage drop, max 3% for lighting and 5% for other circuits (Reg 525 / Appendix 12), before final selection. The full set of twin and earth cable current ratings by size is on our UK cable sizes reference page.