13.3 AC Cables, Voltage Drop and RCDs

The AC cable between consumer unit and inverter is a generator circuit, not a spare lighting radial. City & Guilds 2922 handbook v1.3 4.4.2 deals with circuit length and voltage drop. 4.4.3 deals with residual current devices. 551.7.2 sits underneath both as the parallel-operation overcurrent reminder. Get voltage drop wrong and the inverter trips on overvoltage when it exports. Get the RCD wrong and you either blind a Type A device with DC residual current, nuisance-trip the kitchen, or fit an RCD that cannot actually provide additional protection.

Circuit length and voltage drop

4.4.2 asks you to consider circuit length to ensure voltage drop remains suitably low. The handbook recommendation is voltage drop ≤ 1% where practicable. Manufacturers might also specify a maximum permissible voltage drop to the inverter. That manufacturer limit can be tighter than 1% because export raises the voltage at the inverter terminals above the voltage at the service head: the inverter is pushing current through the cable, so volt drop appears as a voltage rise at the machine.

On a 230 V nominal supply, 1% is 2.3 V. Designers still start with current-carrying capacity (Ib, In, Iz, grouping, insulation, installation method) and then check voltage drop. A 4 mm² cable that is thermally fine can still fail the 1% figure on a loft run.

Worked example, single-phase G98 inverter at 16 A, 18 m one-way from hall consumer unit to loft inverter, two-core 70 °C copper, typical millivolt-per-ampere-per-metre values from BS 7671 Appendix 4:

• 4 mm² at 11 mV/A/m: VD = 11 × 16 × 18 / 1000 = 3.17 V → 3.17 / 230 = 1.38% (above the 2922 1% recommendation).
• 6 mm² at 7.3 mV/A/m: VD = 7.3 × 16 × 18 / 1000 = 2.10 V → 0.91% (within 1%).
• 10 mm² at 4.4 mV/A/m: VD = 4.4 × 16 × 18 / 1000 = 1.27 V → 0.55%.

So the 18 m loft job is a 6 mm² (or larger) conversation if you are chasing the handbook figure, even though 4 mm² might satisfy Iz. Recalculate if you use a different installation method or conductor temperature; the exam point is the method, not memorising one table row as universal law.

MCS 5.6.8 versus the 2922 1% figure

MCS 5.6.8 requires AC cables to be sized to minimise voltage drop as far as reasonably practicable. The MCS note states that voltage drop is taken between the service head (supplier's cut-out) and the AC input terminals of the inverter. Minimising drop improves yield and reduces nuisance overvoltage trips. For most domestic small-scale installations a drop of 1% is considered practicable, whereas for larger systems and/or long cable runs, up to 3% may be more practicable.

Teach both without inventing a conflict. The 2922 open-book figure to chase is ≤ 1% where practicable. The MCS 3% line is an MCS installer note for larger or long runs, not a permission to tell the 2922 examiner that 1.38% is automatically acceptable because MCS exists. If a manufacturer quotes a maximum voltage drop or impedance, that specification still applies. If 1% is practicable (short run, easy upsize), do 1%. If you are on a long farm run, you may have to explain MCS practicability — but you still know what 2922 printed.

RCD considerations (4.4.3)

Minimum Type A, and when Type B enters

If residual current protection is used, the handbook minimum is a Type A RCD. Type A detects sinusoidal AC and pulsating DC residual currents. Many transformerless inverters can impose a smooth DC component on a residual current. Type A devices can be blinded if that DC component is too large. Therefore 4.4.3(b): consider Type B where the manufacturer's guidance does not state the maximum DC component. If the inverter manual states that Type A is suitable and gives the maximum DC residual current the product can present, Type A on a dedicated PV circuit can be the correct reading. If the manual is silent, do not assume Type A is enough — consider Type B.

Must not share an RCD with other circuits

Solar PV systems are not to share an RCD with other circuits (4.4.3(c)). The PV circuit is its own residual-current zone. A 16 A kitchen RCBO that already feeds sockets must not also pick up the inverter. Shared devices mix leakage (inverter plus kettle, induction hob, or fridge), so a harmless standing leakage can nuisance-trip and take out food and generation together. A socket fault then also disconnects the generator. The handbook rule is flat: no sharing.

Additional protection needs a system-referencing link

4.4.3(d) is the trap for people who add an RCD 'on the inverter output' and think they have bought 30 mA additional protection. RCDs on the AC output of an inverter will not provide additional protection without a neutral-earth (system referencing) link in the inverter. Additional protection assumes that a person touching a live conductor causes residual current back through the earthing system so the RCD can see an imbalance. Many transformerless inverters do not create that N–E reference on their AC output. Without the link, the RCD may never see the residual current the shock scenario needs. Do not claim additional protection from a floating inverter output.

Prefer not to feed the inverter from an RCD

4.4.3(e): where possible, design so that the inverter(s) are not fed from an RCD, to alleviate nuisance tripping. Standing leakage, moisture on the array, and EMC filters all create residual current. A dedicated MCB (bidirectional, correctly rated) on a PV circuit installed by a method that does not itself require 30 mA additional protection is the handbook preference.

'Where possible' is doing work. If the AC cable is concealed in a wall at less than 50 mm and is not in earthed metallic covering, BS 7671 additional-protection rules for that cable may still require an RCD. In that case use a dedicated PV RCD/RCBO of the correct type — still not shared with the kitchen — and accept that 4.4.3(e) was not practicable. Do not omit a required RCD and blame 4.4.3(e). Do not add a shared kitchen RCBO and blame 4.4.3(e) the other way.

Parallel operation and 551.7.2

4.4.3(f) requires additional thought on overcurrent protection of cables and enclosures where generator sets operate in parallel with the prosumer's supply, with reference to regulation 551.7.2. The PV AC cable and its enclosures can be energised from the public supply and from the inverter. Overcurrent protection must be suitable for that parallel operation: bidirectional devices, ratings based on the inverter's maximum AC current, and protection at the origin of the generator circuit so a fault is cleared whether the energy arrives from the grid or from the array through the inverter. An enclosure that was only ever fused as a '16 A socket spur' is not automatically safe because a generator now lands in it.

Type A versus Type B versus no RCD

Scenario: 16 A RCBO in the kitchen sharing with PV

A 3.6 kW inverter is 'temporarily' landed on a spare way that is actually a 16 A Type A RCBO already serving kitchen sockets. Voltage drop on a short kitchen cable might even look better than 1%. That does not rescue the design. The PV system shares an RCD with other circuits, which 4.4.3(c) forbids. Kitchen leakage plus inverter leakage will nuisance-trip. A toaster fault removes generation. The RCBO may not be characterised as a bidirectional generator protective device. There is no dedicated isolation story at the consumer unit for 'the PV circuit' because it is not a PV circuit — it is a socket circuit with an inverter hanging on it.

Correct the design: dedicated PV way, bidirectional OCPD, isolation per 4.4.1, voltage drop per 4.4.2 (≤ 1% where practicable), RCD type and sharing per 4.4.3, and 551.7.2 considered for parallel operation. The kitchen RCBO stays on the kitchen.