Consumer Unit Size Calculator for UK Homes

How the calculation works

How to size a consumer unit for your home

The consumer unit is the heart of a home's electrical installation. It holds the main switch, which isolates the whole house, the RCDs that protect people against electric shock, and the circuit breakers that protect each circuit's cable against overload and short circuit. Choosing the right size from the start avoids an expensive change later. The installation should also meet BS 7671, the IET Wiring Regulations, which set the technical requirements for electrical installations in the UK.

This calculator works out how many modules those devices need and recommends the smallest standard board that leaves spare room. It uses one fixed rule: Modules = 1 (MS) + 4 (2 RCD) + N, rounded up with a 25% reserve, and then the next standard size up is selected. The formula counts modules only. It does not estimate the price of the board or the cost of fitting it, so the result tells you the size you need, not what it will cost.

The base circuits by home size

Every home starts from a base set of circuits that depends on its type. The reference values used in the calculator are six base circuits for a two-bed flat with a gas hob and eight base circuits for a three-bed house with a gas hob. Studios and one-bed flats need fewer circuits, while four-bed homes and larger start from a higher base. Larger homes often split lighting across two circuits and add socket circuits by zone, so the count rises with the number of rooms as well as the floor area.

In UK practice the base set usually covers lighting, general sockets for living rooms and bedrooms, kitchen and bathroom sockets, and separate supplies for larger appliances such as the washing machine, dishwasher and hot water cylinder. BS 7671 requires 30 mA RCD protection for most socket-outlet circuits in homes, not only those in kitchens and bathrooms, so the RCDs in the board are not optional extras.

How the module count works

Each single-pole circuit breaker takes one module, which is the standard width of one slot on a DIN rail and is often called a way in UK terms. Miniature circuit breakers are made to BS EN 60898-1, and the standard DIN rail mounting dimensions are set by BS EN 60715. In the calculator's model the main switch takes one module. The two RCDs take two modules each, which adds four modules, and every circuit takes one more module. The 25% reserve is applied to the total module count.

A two-bed flat with six base circuits therefore uses 1 + 4 + 6 = 11 modules before the reserve. With the 25% reserve, 11 modules becomes 13.75, which is rounded up to 14. The smallest standard board that holds 14 modules is the 18-module board, so that is the result.

Two worked examples

A three-bed house with a gas hob has eight base circuits. That gives 1 + 4 + 8 = 13 modules in use. The reserve raises this to 16.25, so at least 17 modules are needed, and an 18-module board is the smallest standard size that fits.

If the same house adds an electric hob, air conditioning for one unit and a 16 A EV charger, the circuit count rises by three to 11. That gives 1 + 4 + 11 = 16 modules in use, which the reserve raises to 20 modules. The 24-module board is then the right choice, and it leaves room for a later extension.

Why two RCDs

An RCD, or residual current device, compares the current going out through the live conductor with the current returning through the neutral. If the two differ by the rated residual current, which is 30 mA for the protection used in homes, the device trips and cuts the supply. The product standards BS EN 61008 and BS EN 61009 set a maximum disconnection time of 300 milliseconds at that rated current. A fault that leaks current to earth is detected this way even when the fault current is too small to trip a circuit breaker.

Two RCDs on separate groups means that a fault on one group leaves the other group live. A split like this is common practice in UK installations. Lighting and sockets on the healthy group keep working while the fault is traced, which matters at night or when the fault is in one room. The calculator assumes 40 A, 30 mA devices, each taking two modules.

Optional circuits and their effect

• Electric hob: an induction, ceramic or other electric hob needs a dedicated circuit, usually 6 mm² cable on a 32 A breaker. No other equipment should share that circuit. Selecting an electric hob adds one circuit and 2.50 kVA to the estimate.
• Electric heating: panel heaters, storage heaters or a heat pump each add one dedicated circuit. The cable size depends on the heater's power, so check the manufacturer's data before the electrician sizes the circuit.
• Air conditioning: the calculator asks for one or two units. Each unit needs its own separate circuit, usually 2.5 mm² cable, sized to the manufacturer's rating.
• EV charger: the calculator models a single-phase 16 A charger, which adds one dedicated circuit and 3.68 kVA. Many 7 kW chargers need a 32 A circuit and larger cable, so check the charger rating before you buy.

Connected load and supply capacity

Alongside the module count, the calculator adds up the estimated loads of the selected circuits and gives a connected load in kVA. Use this figure for comparison only. The capacity of your supply is set by your network operator, and the main switch rating on your existing board shows the current limit of that installation. If the estimate comes close to what your supply can provide, ask your network operator or an electrician before you add heating or an EV charger.

Edge cases

Many modern consumer units give each circuit its own RCBO, which combines a circuit breaker and an RCD, instead of two shared RCDs. This changes the module count, so the result is an approximation for that layout. The calculator also assumes a single-phase 230 V supply, which covers almost all UK homes. A three-phase supply needs a different design.

If you are only adding one circuit to an existing board, the calculator still shows the total size you would need, but the condition of the existing board and the free modules in it matter more than the estimate. Storage heaters and heat pumps can need larger circuits than the calculator models, so check their ratings before you rely on the result.

Common mistakes

• Sizing the board for today's circuits only. A board that is full on the day it is fitted forces a second board or a replacement later.
• Forgetting the main switch and the two RCDs when counting, which understates the module count by five.
• Sharing the hob or the air conditioning circuit with other equipment.
• Assuming RCD protection is optional for sockets. BS 7671 requires it for most socket-outlet circuits in homes.
• Counting modules without checking the real width of each device, because widths vary between manufacturers.

When to use this calculator, and when not to

Use it to plan a new consumer unit, a rewire or a major extension, or to compare board sizes before you get quotes. Do not use it as a design. It does not size cables for specific circuits, check voltage drop, choose protective device ratings for a particular load or cover commercial premises. Those tasks need a design by a qualified person under BS 7671.

Regulations and sign-off

In England and Wales, many types of new electrical work in a home are notifiable under Part P of the Building Regulations, which is set out in Approved Document P. Work must either be notified to building control or carried out by a registered competent person who self-certifies it. The person doing the work should issue an Electrical Installation Certificate, which confirms that the installation complies with BS 7671. For an existing installation, an Electrical Installation Condition Report (EICR) records its condition after inspection. Before you start any work, check the requirements with your local building control team.