Consumer unit and RCBO planner
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Board layout | All RCBOs |
| Circuits | Kitchen sockets: type Sockets, rating 32 A; Downstairs sockets: type Sockets, rating 32 A; Upstairs sockets: type Sockets, rating 32 A; Downstairs lights: type Lighting, rating 6 A; Upstairs lights: type Lighting, rating 6 A; Cooker: type Cooker, rating 32 A; Shower: type Shower, rating 45 A; Immersion: type Immersion, rating 16 A; Smoke alarms: type Other, rating 6 A; Garage: type Outbuilding, rating 20 A |
| Surge protection device | Yes |
| Arc fault detection | No |
| Spare ways | 20% |
| Width of each RCBO | 1 modules |
| Width of the main switch | 2 modules |
| Width of the SPD | 2 modules |
16 modules needed (indicative)
| Result | Value |
|---|---|
| Modules needed | 16 |
| Spare ways | 2 |
| Way | Circuit | Rating (A) | Protection |
|---|---|---|---|
| 1 | Kitchen sockets | 32 | RCBO |
| 2 | Downstairs sockets | 32 | RCBO |
| 3 | Upstairs sockets | 32 | RCBO |
| 4 | Downstairs lights | 6 | RCBO |
| 5 | Upstairs lights | 6 | RCBO |
| 6 | Cooker | 32 | RCBO |
| 7 | Shower | 45 | RCBO |
| 8 | Immersion | 16 | RCBO |
| 9 | Smoke alarms | 6 | RCBO |
| 10 | Garage | 20 | RCBO |
Things to discuss with your electrician
- RCD protection for socket circuits up to 32 A (Reg 411.3.3).
- A non-combustible enclosure for the consumer unit in a home (Reg 421.1.201).
- Surge protection: the SPD is counted (Sections 443 and 534).
- Arc fault detection: ask whether AFDDs are recommended for your circuits (Reg 421.1.7).
Show the working
- Spare ways: 10 circuits × 20%, rounded up = 2
- RCBOs: 10 × 1 = 10 modules
- Main switch: 1 × 2 = 2 modules
- SPD: 1 × 2 = 2 modules
- Spare ways: 2 × 1 = 2 modules
- Modules needed: 16
How it works
- All RCBOs: modules = circuits × RCBO width + main switch + SPD + spare ways × RCBO width
- Dual RCD: modules = RCDs × RCD width + circuits × MCB width + main switch + SPD + spare ways × MCB width
- Spare ways = circuits × spare allowance, rounded up
- Enclosure = the smallest size in the maker's range with at least that many usable modules
Regulation notes (non-combustible enclosures, SPDs, RCD protection for sockets, AFDDs) are being checked against BS 7671:2018+A4:2026 by our researcher.
Indicative only: the electrician designs the board, chooses the devices and checks discrimination and fault protection.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Spare ways | 20 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 E6: spare ways allowance (assumption 20%). |
| Width of each RCBO | 1 modules | Common UK size Not yet checked against the source | From your product brief v2 E6 (CU1): single-module RCBO. Some RCBOs take 2 modules: check the maker's data. |
| Width of each MCB | Enter from the bag, pack or tin | Common UK size Not yet checked against the source | From your product Not stated in the brief. Researcher to fill from a consumer unit maker's data. |
| Width of each RCD | Enter from the bag, pack or tin | Common UK size Not yet checked against the source | From your product Not stated in the brief. Researcher to fill from a consumer unit maker's data. |
| Width of the main switch | 2 modules | Common UK size Not yet checked against the source | From your product brief v2 E6 (CU1): main switch width. |
| Width of the SPD | 2 modules | Common UK size Not yet checked against the source | From your product brief v2 E6 (CU1): surge protective device width. |
| Extra width per AFDD | Enter from the bag, pack or tin | Common UK size Not yet checked against the source | From your product Not stated in the brief. Researcher to fill from an AFDD maker's data. |
Regulations and standards
- BS 7671:2018+A4:2026 Reg 421.1.201 (non-combustible consumer unit enclosures)
- BS 7671:2018+A4:2026 Section 443 and Section 534 (surge protection)
- BS 7671:2018+A4:2026 Reg 411.3.3 (RCD protection for socket-outlets up to 32 A)
- BS 7671:2018+A4:2026 Reg 421.1.7 (arc fault detection devices)
Explained: BS 7671 (the IET Wiring Regulations), Approved Document P (electrical safety).
BS 7671 is used across the UK. In England and Wales, replacing a consumer unit is notifiable under Part P; Scotland and Northern Ireland have their own rules.
| Table | Clause | Nations | Status |
|---|---|---|---|
| Consumer unit enclosure sizes (usable modules) TODO: Researcher to add the maker or standard URL | Consumer unit makers' ranges (Researcher to fill from at least two UK makers' data sheets) | UK-wide | 0/6 values entered Not yet checked against the source |