Consumer unit size calculator
Each circuit needs a protective device in the consumer unit (fuse box), usually one module wide. The calculator adds the circuits, the main switch or RCD and a 20% reserve.
Your measurements
Result
- Total circuits
- 13 pcs
- Consumer unit modules (incl. 20% reserve)
- 18 pcs
Includes 20% spare modules for future circuits; the final design must be checked by a qualified electrician.
How the calculation works
- Total circuits = lighting + sockets + dedicated circuits for large appliances.
- Modules = (total circuits + 2 for the main protection) × 1.2, rounded up — a 20% reserve.
Worked example
4 lighting circuits, 6 socket circuits and 3 for air conditioning, oven and water heater make 13 circuits. The consumer unit needs (13 + 2) × 1.2 = 18 modules.
What the calculator assumes
- Each single-phase circuit breaker is one module wide.
- The 2 extra modules are for the main switch or residual-current protection.
When to adjust the result
- A two-pole RCD takes 2 modules and a surge protector 2–4 — count the real modules of the devices chosen.
- Circuits in bathrooms and outdoors require residual-current protection. Electrical work must be designed, carried out and tested by a qualified electrician under local rules.
Planning circuits sensibly
Give high-load appliances their own circuits — cooker or hob, oven, washing machine, dishwasher, heat pump, EV charger — and split lighting and sockets across several circuits so one fault does not leave a whole floor dark. Leave spare ways in the consumer unit for future additions.
Residual-current protection (RCD or RCBO) is required for most socket and bathroom circuits in modern installations. Work on the consumer unit is dangerous and, in many countries, must be carried out or certified by a qualified electrician — in England, for example, it is notifiable under Part P of the Building Regulations.
Frequently asked questions
How many ways does my consumer unit need?
One per circuit plus the main switch/RCDs, and about 20% spare for future circuits.
Why leave spare ways?
So you can add circuits later (EV charger, heat pump, outdoor power) without replacing the unit.
RCD or RCBOs?
RCBOs protect each circuit individually, so a fault trips only that circuit; a shared RCD is cheaper but takes several circuits off at once.
How many spare ways should I leave?
Two to four spare ways is a sensible margin for future circuits such as an EV charger, heat pump or garden supply.