Back-feeding a 5 kW hybrid into a full 16th edition board
A customer has ordered a 5 kW single-phase hybrid inverter with a declared maximum AC output current of 21.7 A. The supply is TN-C-S with a 100 A cut-out. The existing consumer unit is a 16th edition split-load board with a 100 A main switch, a 63 A 30 mA Type AC RCD as the main switch, and outgoing ways of 32 A cooker, 32 A ring, 16 A immersion, 6 A lights, 6 A lights and 6 A boiler - all ways are occupied, but the installer says he can 'double up' the 6 A lighting circuits or fit a 20 A MCB in place of the boiler circuit and back-feed through it, with a 32 A rotary isolator next to the inverter. The DNO has no record of generation at the address. The customer intends to add a battery to the same hybrid inverter next year.
1. Determine the connection notification route for this inverter and what must happen before the system is energised.
Read the worked answer
21.7 A on a single phase is well above the 16 A per phase G98 threshold, so this is a G99 application, not a G98 notification, whatever the kWp of the array happens to be. The DNO's criteria are based on the inverter's rated AC current, not on the panel count.
Actions before energising:
- Submit the G99 application with the inverter's G99 type-test certificate, single-line diagram and proposed protection settings, and obtain written acceptance before the system is connected or energised.
- A 5 kW single-phase inverter on a 100 A single-phase service can be refused or approved with conditions (export limitation, or a requirement to move to a three-phase connection). That possibility must be raised with the customer before the equipment is ordered and installed.
- The installation must be notified to the local authority building control / registered with the competent person scheme as a new installation and as a connection to an existing installation, and the EIC plus the DNO acceptance kept with the handover pack.
Judgement point: the installer's plan to 'double up' lighting circuits to free a way suggests the notification question has not been considered at all. Fix the regulatory position first; the board arrangement comes second.
2. Assess the proposed method of connection to the existing consumer unit and state what you would do instead.
Read the worked answer
Problems with the proposal:
- Rating of the assembly. The commonly applied rule (IET guidance) is that the sum of the ratings of the outgoing protective devices should not exceed the rating of the main switch. Existing ways total 32 + 32 + 16 + 6 + 6 + 6 = 98 A; adding a 20 A PV way gives 118 A on a 100 A main switch. Replacing the boiler circuit does not solve this - the boiler still needs a supply.
- Device rating. A 21.7 A inverter needs a device rated at least 25 A (typically a 25 A or 32 A MCB); a 20 A device is undersized and will nuisance-trip at full output.
- RCD type. A 30 mA Type AC RCD is not suitable for a transformerless inverter, which can impose DC components and high-frequency leakage. Type A as a minimum is required, and the RCD must be rated for bidirectional current and for the total leakage of the installation plus the inverter.
- Position on the busbar. The PV way must be at the end of the busbar furthest from the incoming main switch, so that the outgoing load current and the inverter current are not both flowing through the same section of busbar.
- Labelling. Dual-supply warning notices at the origin, meter position and inverter, plus a circuit schedule identifying the PV way.
Instead: install a separate generation consumer unit (or a dedicated enclosure) fed from the tails via Henley blocks and its own main switch and Type A RCD, with a 25 A MCB for the inverter, a lockable AC isolator adjacent to the inverter, and correctly sized cable. This avoids the busbar/rating problem entirely, keeps the existing board untouched and gives a clean point for the battery later.
3. The customer wants a battery added to the same hybrid inverter next year. What must be designed in now so that the addition does not require reworking the AC side?
Read the worked answer
Decisions that must be made at this stage, because they change the hardware and not just the paperwork:
- Current rating of everything on the AC side. The inverter's AC terminals and the battery's charge/discharge path share the same connection. Size the cable, isolator, MCB and generation consumer unit for the higher of the inverter's maximum output and its maximum input (charge) current - a hybrid can draw close to its full rating from the grid to charge, so a circuit sized only for 21.7 A export may be inadequate.
- Export limitation and metering. If the DNO approves with an export limit, the current transformer must be fitted on the correct conductor at the origin now, and its wiring route left accessible, otherwise the battery will later push the site over the approved limit.
- Backup/EPS circuits. If the customer wants essential-supply backup, that means a separate backed-up consumer unit and a changeover arrangement. Retrofitting this later means rewiring circuits - agree now whether it is wanted.
- Labelling and documentation that anticipate the battery, and space in the enclosure for the extra device and its isolator.
Judgement: it is far cheaper to agree the battery's AC requirements before the board is built than to discover next year that the 25 A circuit and 100 A assembly cannot carry a charge current as well as the load.