What MCB type should I use — B, C, or D?

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.

What size MCB do I need for a 9.5kW electric shower?

A 9.5kW shower draws approximately 41.3A at 230V. You need a 45A Type B MCB on a dedicated circuit with 10mm² twin and earth cable. For a 10.5kW shower (45.7A), a 50A MCB with 10mm² cable (clipped direct) or 16mm² (in conduit) is required. Always check voltage drop for longer cable runs.

Why is a ring final circuit protected by a 32A MCB when it uses 2.5mm² cable?

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.

Can I use a higher-rated MCB than calculated?

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.

What MCB do I need for an EV charger?

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.

Circuit Breaker Calculator — What MCB Size and Type Do I Need?

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 load current, then cross-references with typical configurations per BS 7671 for the selected circuit type.

Frequently Asked Questions

Related Calculators

Cable Sizing Calculator

Calculate the correct cable size based on BS 7671 18th Edition regulations.

Voltage Drop Calculator

Check voltage drop complies with UK wiring regulations.

Maximum Demand Calculator

Calculate maximum electrical demand for domestic installations.

Socket Spacing Calculator

Plan socket layouts with Part P guidance on spacing and positioning.

Lighting Layout Calculator

Calculate lighting requirements based on lux levels for each room type.

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Circuit Breaker Sizing Calculator — MCB Rating Guide