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Circuit Breaker Sizing Calculator — MCB Rating Guide
Determine the correct MCB rating and type for your circuit based on BS 7671 18th Edition. Covers all common UK domestic circuit types.
Enter the total load on this circuit (leave blank for typical values)
UK nominal supply voltage (default 230V)
Manual diversity multiplier for non-cooker circuits (default 1.0; ignored for cooker circuits, which use the statutory 10A+30% rule)
Additional headroom on the design current (default 0%)
Safety notice
Electrical work in dwellings can be notifiable under Part P of the Building Regulations. Treat these figures as planning guidance only: circuits must be designed, installed and certified to BS 7671 by a competent person, normally a registered electrician.
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How We Calculate This
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.
Working figures for this are collected in our UK Circuit Breaker Types reference table.
Frequently Asked Questions
Type B MCBs are standard for almost all domestic installations. They trip at 3-5 times rated current and are suitable for resistive loads (heaters, ovens, lighting) and small motors. Type C (5-10x) is used for circuits with moderate inrush current like air conditioning or small commercial motors. Type D (10-20x) is for heavy inductive loads like large motors and transformers. For UK homes, Type B is correct in virtually all cases. For combined RCD and MCB protection, see our RCBO Selection Calculator and RCD Selection Guide.
A 9.5kW shower draws approximately 41.3A at 230V, so it needs a 45A Type B MCB on a dedicated circuit. Use 10mm² twin and earth as the minimum for any shower 8kW and above — shower cables almost always run through joists and thermal insulation, where 6mm² (47A clipped direct) derates to around 23A and cannot protect a 40A or 45A device. For a 10.5kW shower (45.7A), use a 50A MCB; 10mm² (64A clipped direct) still comfortably exceeds 50A, and you may need to step up to 16mm² only if voltage drop or severe derating demands it on a long run. Every circuit serving a location containing a bath or shower must have 30mA RCD additional protection (BS 7671 Section 701, Reg 701.411.3.3).
In a ring final circuit, the cable forms a loop from the consumer unit and back, meaning current can flow in both directions. This effectively doubles the cable capacity. A 2.5mm² cable in a ring has sufficient capacity for a 32A MCB. This is a standard UK arrangement per BS 7671 — a radial circuit with 2.5mm² cable would only be protected at 20A.
No. The MCB must be sized to protect the cable, not just the load. Using an oversized MCB means the cable could overheat before the MCB trips. The rule is: Design current (Ib) <= MCB rating (In) <= Cable capacity (Iz). The MCB protects the cable from overload; the cable must be able to carry the MCB's rated current. Use our Cable Sizing Calculator to verify the correct cable size for your chosen MCB rating.
A 7kW (32A) home EV charger requires a 32A Type B MCB on a dedicated circuit. The cable must be 6mm² minimum (10mm² for longer runs). The circuit also requires Type A RCD protection (30mA). Some smart EV chargers have built-in Type A RCD protection — check the manufacturer's requirements. Use our EV Charger Cable Sizing Calculator for detailed cable sizing and voltage drop checks.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.