Consumer unit capacity

How to assess physical space, assembly rating, protective devices, existing circuits and certification before adding low-carbon equipment.

Consumer-unit capacity is not just the number of empty slots. A board must have the physical space, current-carrying capability, protective-device arrangement and verified installation conditions needed for the proposed circuit or generating source.

A solar inverter, battery, EV charger and heat pump can each expose a different limitation. The installer must assess the complete assembly and the supply, not approve the job from a photograph of a spare way.

Six separate capacity checks

1. Physical space

A spare way is an unused circuit position. The proposed protective device may need more than one module, and surge protection, contactors, monitoring or control supplies can use additional space.

A blanked opening is not necessarily a usable way. Check the board model, busbar arrangement, manufacturer’s data and whether the correct compatible device is available. Devices from another product range must not be mixed merely because they fit the rail.

2. Assembly current rating

The main switch, RCDs, busbar, terminals and enclosure form an assembly with defined ratings and conditions. A “100 A” label on one component does not prove that every part of the board can carry 100 A in the proposed arrangement.

Generation and storage can feed current into the board from another point. The designer must consider current from the grid and local source, where it enters the busbar, and the rating of the affected section. This is a different question from the DNO head-fuse rating.

3. Protective-device suitability

The new circuit needs overcurrent protection, isolation and any required residual-current protection selected for the equipment and installation. Power electronics can produce residual currents with DC components and higher frequencies, so a legacy Type AC RCD is not automatically suitable.

There is no safe blanket rule that every solar or battery circuit simply takes a Type A RCBO. The required RCD type depends on the inverter or charger design, built-in residual-current detection, separation, earthing arrangement and manufacturer instructions. PV guidance can require Type B where an RCD is used unless the inverter or installation provides the specified alternative protection.

For a source capable of reverse power flow, protective devices and other components must also be suitable for the direction of current. A device that physically fits may be marked for supply and load terminals and may not be approved for bidirectional use.

4. Existing leakage and circuit division

Several circuits sharing one RCD can accumulate normal protective-conductor current. A new inverter, charger or heat-pump drive can increase it and cause unwanted tripping without having an insulation fault.

Individual RCBOs can improve division and fault discrimination, but they are not a cure for defective wiring. Measure, inspect and test the existing installation. Do not move a problematic circuit to less appropriate protection merely to make the new equipment stay on.

5. Maximum demand and source coordination

The board must be assessed for expected maximum demand as well as circuit space. A 32 An EV circuit added to a spare way still affects the service, tails, main switch and distribution.

Solar and battery circuits introduce generation. The design must consider overload under all operating modes, including grid-connected generation, battery charging, backup supply and any changeover arrangement. Load management can reduce import demand, but it does not make an underrated assembly adequate for local generation current.

6. Condition and documentation

The electrician needs to verify:

earthing and bonding
polarity and continuity
insulation resistance and fault protection
condition and rating of meter and consumer tails
signs of overheating or water entry
circuit identification
previous alterations and mixed protective-device ranges
access, working space and enclosure integrity

An old board is not condemned by age or a plastic enclosure alone. Existing installations are assessed against the standard and accepted practice appropriate to reporting, with departures and defects classified by competent judgement. A damaged, overloaded or unsuitable board still needs action regardless of age.

Current BS 7671 position

This is time-sensitive. As at 22 July 2026, BS 7671:2018+A4:2026 has been published and can be used. The preceding BS 7671:2018+A2:2022+A3:2024 remains valid during the transition until 15 October 2026.

The certificate should identify the edition used for the design and verification. A project should not combine selected requirements from the two editions without a documented engineering basis.

Amendment 4 includes a new chapter on stationary secondary batteries, so storage work should be designed by someone working from the applicable complete edition, not an old online checklist.

Surge protection

Surge-protective-device requirements depend on the installation, consequence assessment, supply arrangement and applicable BS 7671 edition. Low-carbon electronic equipment increases the value at risk, but an SPD cannot be specified from board space alone.

Check:

whether Type 1, Type 2 or a coordinated arrangement is required
the supply and lightning-protection context
overcurrent protection and conductor routing
the manufacturer’s maximum lead lengths and connection method
end-of-life indication and replaceable modules

An SPD may be housed in the main board or a suitable associated enclosure. Its presence does not replace equipment-specific DC or communications surge protection.

Adding a separate board

When the main board cannot accept a circuit, a separate distribution board can be a valid design. It is not an automatic loophole around the existing installation.

The installer must design the connection at the meter tails or upstream distribution point, including:

isolation
overcurrent and fault protection
tail and conductor rating and routing
enclosure and fire considerations
earthing and bonding
selectivity and RCD coordination
source-current paths where solar or storage is connected
labelling of multiple supplies and isolation points

Do not assume that “Henley blocks and a garage board” is a standard answer for every EV or battery installation. The actual equipment must form a compliant assembly and the responsibility for seals, meter tails and isolation must be agreed.

When replacement is proportionate

Replacing the main consumer unit may be sensible where several problems combine, such as:

no compatible spare ways
obsolete or unavailable devices
inadequate circuit division
unsuitable RCDs for the intended equipment
damaged enclosure or signs of overheating
inadequate assembly rating or source-current arrangement
poor circuit identification and extensive undocumented alterations

An adjacent board may be better where the existing installation is safe and serviceable and the new equipment benefits from separate protection. The design should explain why the chosen option is appropriate; solar, storage, an EV charger or a heat pump does not automatically require a replacement consumer unit.

Notification and certificates

Building-control rules differ across the UK. In England, installing a new circuit or replacing a consumer unit in a dwelling is notifiable work under Part P. A registered competent person can normally self-certify within the scope of the scheme; otherwise the appropriate building-control route is needed.

The electrical paperwork is separate from the building-control compliance record. Depending on the work, the handover includes the appropriate Electrical Installation Certificate or Minor Electrical Installation Works Certificate, schedules of inspection and test results, an updated circuit chart, equipment instructions and the building-regulations certificate where required.

An EICR records the condition of an existing installation. It is not a substitute for the installation certificate for new work.