Circuit Breaker Sizing Calculator — Select the Right MCB Every Time

Circuit breaker sizing is the process of selecting a miniature circuit breaker (MCB) with the correct nominal current rating and trip curve for a given circuit. The MCB must be large enough to carry the normal load current without nuisance tripping, yet small enough to protect the cable from overcurrent and to disconnect the supply rapidly under fault conditions.

BS 7671:2018+A4:2026 sets out the requirements for overload protection in Section 432 and Chapter 43 (Section 433). Note that A4:2026 deleted the former Regulation 433.1 sub-regulation; practitioners must consult the restructured provisions in Chapter 43 for the current overload protection requirements. The protective device must satisfy two fundamental conditions: the nominal rating (In) must be at least equal to the design current (Ib) of the circuit, and the cable's current carrying capacity (Iz) must be at least equal to the nominal rating. This ensures the cable is never subjected to a sustained current above its safe operating limit.

Selecting the wrong MCB rating can lead to two types of problems. If the MCB is too small, it will trip under normal load conditions, causing disruption to the circuit and the equipment it supplies. If the MCB is too large, it will fail to protect the cable from overload, allowing sustained currents that exceed the cable's thermal rating — leading to insulation degradation, overheating, and potentially fire. The cable sizing calculator and circuit breaker sizing calculator work together to ensure both components are correctly matched.

The most fundamental coordination requirement in BS 7671 is expressed by the inequality Ib ≤ In ≤ Iz. This simple rule governs the relationship between the load, the protective device, and the cable:

There is also a second condition that must be checked: I2 ≤ 1.45 x Iz, where I2 is the current causing effective operation of the protective device. For MCBs to BS EN 60898, I2 = 1.45 x In, which means this condition is automatically satisfied when In ≤ Iz. However, for BS 3036 semi-enclosed fuses, I2 can be up to 2 x In, which is why the Cf factor of 0.725 must be applied to ensure the cable is adequately protected.

The Elec-Mate calculator checks both conditions automatically and flags any circuits where the coordination requirements are not met.

MCBs have two protection mechanisms: a thermal element for overload protection (which responds to sustained overcurrents) and a magnetic element for short circuit protection (which responds to high fault currents). The trip curve defines the magnetic tripping threshold — the instantaneous current at which the MCB trips immediately.

In UK domestic installations, Type B MCBs are used for virtually all circuits. Type C is reserved for circuits supplying equipment with significant inrush current. Using a Type C where a Type B would suffice means the MCB requires a higher fault current to trip magnetically, which affects the maximum permitted earth fault loop impedance and the disconnection time verification.

Correct coordination between the MCB and the cable is essential for safety. The MCB must protect the cable from both sustained overload and short circuit fault current. For overload protection, the Ib ≤ In ≤ Iz rule ensures the cable is never subjected to a continuous current above its rating.

For short circuit protection, the cable must be able to withstand the thermal effects of the fault current for the time it takes the MCB to disconnect. This is verified using the adiabatic equation from BS 7671 Regulation 434:

In practice, for most domestic circuits, the standard cable sizes (1.5mm², 2.5mm², 4mm², 6mm², 10mm²) easily satisfy the adiabatic equation when protected by standard MCB ratings. The check becomes more critical on long cable runs, circuits with high prospective fault current, or where the cable CSA has been minimised. The adiabatic equation calculator in Elec-Mate performs this verification automatically.

Standard MCB ratings follow a preferred range defined in BS EN 60898: 6A, 10A, 16A, 20A, 25A, 32A, 40A, 50A, and 63A. The rating you select depends on the design current of the circuit. Common domestic and commercial applications include:

The Elec-Mate calculator suggests the appropriate MCB rating based on the design current you enter, and recommends the correct trip curve based on the load type. It also flags situations where the next standard rating up may be needed — for example, when the design current is very close to a standard rating and there is a risk of nuisance tripping under normal operating conditions.