How to wire a backup generator changeover switch

Helpful video reference. The video above is from the UK Electrician channel: "Installing a Back Up Generator". It demonstrates a domestic UK installation with the correct interlock approach and shows the inlet socket, changeover wiring and labelling in a typical British consumer unit setup.

1. Understand the back-feed danger

When you lose mains power, the DNO sends engineers out to work on the street cables. If a generator is connected without a changeover interlock, it can energise those cables from your house. A linesman who touches a cable he thinks is dead could receive a fatal electric shock.

This is why BS 7671 Regulation 551.6 requires that where a generator is installed as a back-up source, means must be provided to prevent the generator and the mains supply from being live simultaneously. The interlock is not optional.

2. Decide how many circuits you want to back up

Most portable generators produce between 2 kW and 6 kW. Your whole house at full load will far exceed that, so the practical approach is to back up only essential circuits: a few sockets, the boiler, the fridge and possibly the freezer. Lighting is also sensible. Showers, ovens and EV chargers are not realistic candidates.

The simplest installation uses a manual changeover switch to back up a sub-board with the essential circuits on it, or a purpose-made interlock kit that fits onto your existing consumer unit and blocks the main switch and generator input from being both on.

3. Choose your changeover method

Three options are common in domestic UK installations:

• Consumer unit interlock kit: A sliding mechanical block fits over the existing main switch and a new generator input MCB. The slider physically prevents both from being on. These kits are available from Wylex, Crabtree and others to suit specific board types. They are the simplest retrofit but require access to the consumer unit and are Part P notifiable.
• Manual changeover switch (MCO): A separate enclosure with three positions: mains, off and generator. The enclosure is wired between the meter tails and the consumer unit. This gives a clear visual indicator of which source is connected.
• Reliance Controls transfer panel: A small sub-consumer unit with its own changeover switch and a limited number of circuit breakers for the backed-up circuits. The transfer panel wires in parallel with the main board, not in series with it, so it does not interrupt the main supply.

For most domestic installations, the Reliance Controls approach or a consumer unit interlock kit offers the best balance of cost and simplicity.

4. Size the generator inlet socket

The generator connects to the house wiring via a BS EN 60309 commando socket fitted externally or in a garage. For a 3 kW generator, a 16 A blue inlet socket is sufficient. For a 6 kW machine, use 32 A. The inlet socket must face downward or be recessed so water cannot enter the socket when the generator is not connected, and it must be on its own RCD-protected circuit or have an RCD integral to the changeover panel.

The socket is an inlet, not an outlet: current flows into the house from the generator through the socket, so the socket's male pins face outward and the generator's flex has a female connector. Many people get these the wrong way round when ordering parts.

5. Run the cable from the inlet to the changeover

Use 4 mm² three-core and earth for a 16 A circuit or 6 mm² for 32 A. The cable run must be protected against mechanical damage and, if it passes through a wall, through a conduit or trunking. Mark the cable at both ends with a durable label.

At the consumer unit end, terminate at the interlock kit's generator input MCB or at the changeover switch's generator terminal. Do this with the mains supply isolated and proved dead.

6. Check the interlock operation before restoring mains

With everything wired and the mains still off, test the mechanical interlock physically. Confirm that you cannot switch the generator on whilst the mains MCB is in the on position, and that you cannot switch the mains on whilst the generator position is selected. If either position allows both to be on simultaneously, the interlock is either fitted incorrectly or it is the wrong kit for the board.

Connect a generator (or a dummy load if testing mechanically only) and confirm the backed-up circuits receive power only when the changeover is in the generator position.

7. Label everything clearly

The inlet socket, the changeover switch and any generator-backed circuits in the consumer unit must be permanently labelled. The consumer unit should also carry a notice stating that the installation includes an alternative source of supply and that the changeover switch must be in the mains position before the DNO is called to restore power.

Notify the work through a Part P competent person scheme or submit it to building control. Without certification, you may have difficulty selling the property and your insurer may decline a claim related to the installation.

When to call us

Generator changeover installations are squarely within Part P notifiable work. Richard can assess your consumer unit, specify the correct interlock or transfer panel, wire the inlet socket and changeover switch, carry out the correct tests and issue a Minor Works Certificate or Electrical Installation Certificate as required.