MCB Ratings Explained – 6A, 10A, 16A, 20A, 32A, 40A & More

Miniature Circuit Breakers (MCBs) are available in a wide range of current ratings.

The number printed prominently on an MCB — such as 6, 16, 20 or 32 — indicates its rated current in amps (A).

Understanding this number is important when identifying an existing MCB or looking for a replacement.

What Does the Amp Rating on an MCB Mean?

The rated current, often shown as In, is the current the MCB is designed to carry under its specified operating conditions.

Common MCB ratings include:

Other ratings are also available depending on the manufacturer and product range.

The correct rating is determined by the design of the electrical circuit. You should not simply choose a higher-rated MCB because it can carry more current.

What Does B32 Mean?

MCBs commonly have a letter immediately before their amp rating.

For example:

B32

The B identifies the MCB's trip characteristic.

The 32 identifies its rated current as 32A.

Similarly:

B6 = Type B, 6A

B16 = Type B, 16A

C20 = Type C, 20A

C32 = Type C, 32A

The letter and number should both be considered when identifying a replacement.

What Are Different MCB Ratings Used For?

Different circuits require different levels of overcurrent protection.

For example, domestic installations may contain lower-rated MCBs supplying lighting circuits and higher-rated devices supplying socket circuits or larger fixed loads.

However, an MCB rating should not be selected solely according to the type of appliance or circuit.

Factors including cable size, installation method, design current, grouping, ambient conditions and required disconnection characteristics can affect circuit design.

The correct protective device therefore needs to be determined from the electrical installation rather than copied from a generic list.

Can I Replace a 32A MCB With a 40A MCB?

Not simply because the 40A device physically fits.

Increasing the MCB rating can allow the circuit to carry more current before overload protection operates.

If the circuit and cable have been designed for a 32A protective device, fitting a 40A MCB without establishing that the circuit is suitable could leave the wiring inadequately protected.

A change in protective-device rating should only be made after the circuit has been properly assessed.

Can I Replace a B32 With Another B32?

The matching B32 marking is only part of what needs to be checked.

Two B32 MCBs can have different:

Manufacturers
Product ranges
Breaking capacities
Dimensions
Busbar connections
Terminal arrangements
Product standards
Compatibility requirements

A Wylex B32 MCB, for example, should not automatically be treated as interchangeable with a Hager, Crabtree or Schneider Electric B32 MCB.

Even two MCBs carrying the same brand may come from different ranges or generations.

What Is MCB Breaking Capacity?

Another important marking is the MCB's rated short-circuit breaking capacity.

Depending on the device, you may see markings associated with ratings such as 6kA or 10kA.

This relates to the fault current the protective device is designed to safely interrupt under specified conditions.

It is separate from the normal amp rating.

For example, a B32 6kA MCB and a B32 10kA MCB share the same 32A current rating and Type B characteristic, but they are not identical devices.

The required breaking capacity depends on the prospective fault current and the design of the installation.

Why Does the Exact MCB Rating Matter?

The MCB forms part of the protection designed for the electrical circuit.

Changing its rating can affect:

Overload protection
Short-circuit protection
Cable protection
Fault disconnection
Coordination with other protective devices

For this reason, the rating should not be increased or changed simply to prevent an MCB from tripping.

Frequent tripping can indicate an overload or electrical fault that needs investigating.

Identifying a Replacement MCB

When replacing an existing MCB, record more than just its amp rating.

Look for:

Manufacturer
Model or part number
Rated current
B, C or D trip characteristic
Breaking capacity
Number of poles
Consumer unit or distribution board range

For older installations, the exact original MCB may have been discontinued.

ReCircuit Resale stocks a range of new, surplus and discontinued MCBs from major electrical manufacturers, which can be particularly useful when maintaining older consumer units and distribution boards.

Always confirm that a replacement device is suitable for the particular installation before fitting it.

Electrical work within consumer units and distribution boards should be carried out by a competent person.

Next Guide: B Curve vs C Curve vs D Curve MCBs