Connection to consumer units, inverters and electrical distribution
The AC interface
A grid-connected inverter needs a dedicated final circuit from the consumer unit, a lockable AC isolator adjacent to the inverter, and a method of isolation acceptable to the DNO. The inverter should not be in a bedroom, escape route or unventilated cupboard; follow the manufacturer's clearances. Check that the consumer unit has a spare way, adequate busbar rating and no shared neutral, and that the total load plus generation is considered.
RCD and protection choices
Residual current protection must follow the inverter manufacturer's instructions. Transformerless inverters can produce DC residual current, so a Type AC device must not be relied on where DC components are possible; Type B, or Type A with appropriate DC detection built into the inverter, is the usual solution. Never assume a Type AC RCD is adequate just because the circuit is single-phase.
Voltage rise and cable sizing
The inverter must not push the voltage at the point of common coupling beyond DNO limits. Long, thin AC runs are the usual culprit. Size the AC cable for current-carrying capacity, voltage drop and voltage rise, and check the DNO's stated limit - around 1 % of nominal is the figure commonly used as a design target.
Worked example - AC voltage drop
4 mm² twin and earth, 18 m run, 16 A, tabulated mV/A/m = 11. Voltage drop = 11 × 16 × 18 ÷ 1000 = 3.17 V, which is 3.17 ÷ 230 = 1.4 %. That is acceptable for voltage drop, but on a long rural run it can be enough to trigger over-voltage trips at the inverter.
Traps
Wrong CT orientation on export-limited systems; forgetting the dual-supply label at the meter and consumer unit; assuming the DNO has been told because the installer notified Part P; sizing the AC cable on current alone.