Picking the right consumer unit size sounds straightforward until you're on site, lid off, and realise you're one way short. This guide covers how to calculate the number of usable ways you actually need, what eats into your module count, and how to future-proof a board without significantly increasing the cost.
What "ways" actually means
Consumer units are sold by the number of module spaces in the enclosure — not by usable ways. A 12-module board does not give you 12 circuits.
Here is how the maths works on a modern board:
- 100A main switch: 2 module spaces
- Type 2 SPD (pre-wired): 2 module spaces
- Each MCB or RCBO: 1 module space
So a 12-module board with main switch and SPD leaves you with 8 usable ways. An 18-module board leaves 14. This is why the headline "18 way" on a product can mislead — always check the usable way count in the product spec, not just the module count.
| Module size | Usable ways (MS + SPD config) |
|---|---|
| 8 module | 5 |
| 12 module | 9 |
| 16 module | 13 |
| 18 module | 15 |
| 22 module | 19 |
| 28 module | 24 |
Our WME/SP range lists usable ways clearly for every size in the product description.
A starting point for common domestic scenarios
Standard 3-bed house rewire (no EV, no solar)
You will typically need circuits for: cooker, shower, immersion, ring main ×2 (upstairs/downstairs), lighting ×2–3, garage or outbuilding, smoke alarms and a spare. That is 10–12 circuits before anything else. A 16-module SPD board (13 usable ways) is usually the right call — not a 12-module that leaves no headroom.
New build with EV charger
Add a dedicated 32A or 40A RCBO circuit for the EV supply, and a separate way for a timed heating or load management circuit if required. You are now at 12–14 circuits minimum. An 18-module board (15 usable ways) gives sensible headroom; a 22-module (19 usable ways) gives comfortable room for future expansion.
Larger property or house with annex
Properties with an annexe, workshop or outbuilding fed from the same board regularly hit 16–20 circuits. A 22-module or 28-module SPD board is the right starting point. Going up a size costs very little compared to a site return and partial rework when the homeowner later wants another circuit.
Upgrade or partial rewire
Where you are replacing a split-load board like-for-like, check what is currently installed and ask whether the homeowner has plans for EV charging or solar in the next few years — worth the conversation before ordering.
Why SPD-fitted boards save time and reduce errors
Since BS 7671:2018 Amendment 2, surge protection is required for virtually all new domestic installations and rewires. You can wire an SPD into a plain enclosure yourself, but there are good reasons to choose a pre-fitted board:
- The SPD is factory-wired and tested — one less thing to verify at inspection
- It occupies two dedicated module spaces already accounted for in the usable way count, so there are no surprises when you open the box
- SPD cable lengths are optimised for the enclosure, which matters for protection performance — long leads reduce effectiveness
Twin RCD vs RCBO boards — which to specify
Twin RCD boards divide circuits into two groups, each protected by a single RCD. They are cheaper to build out because MCBs cost less than RCBOs, but a single fault on one circuit trips half the board. In a property with vulnerable occupants, a disabled person or home medical equipment, this is not acceptable.
RCBO boards use a main switch plus individual RCBOs for each circuit. A fault trips only that circuit. They are now the default specification for most domestic work — the cost difference has narrowed considerably, and the liability picture is much cleaner.
Future-proofing: always go one size up
The incremental cost between an 18-module and a 22-module enclosure is typically £5–£10 on the enclosure alone. The cost of a site return, new enclosure and rework when the homeowner adds solar or a second EV charger later is substantially more. If you are on the fence between two sizes, go larger.
The same logic applies to SPD specification. Even on a cost-conscious job, the pre-fitted SPD boards are worth the small uplift — they satisfy Amendment 2 out of the box and give the client documented transient overvoltage protection, which matters increasingly with sensitive EV equipment in homes.