By the UK Generator Guide — Home Petrol Generator Reviews & Advice Team · Updated May 2026 · Independent, reader-supported

How to Connect a Petrol Generator to Your House UK: Transfer Switch Guide

Connecting a petrol generator directly to your home's electrics without proper isolation is dangerous and illegal in the UK. Backfeeding—where generator power flows into the mains network—risks electrocuting utility workers and damaging equipment. A transfer switch solves this by ensuring your generator powers your home or the grid, never both simultaneously.

This guide covers the two main approaches: manual transfer switches (cheaper, manual operation) and interlock kits (automatic, more expensive), plus what the UK Building Regulations actually require.

Why You Need a Transfer Switch

Your home's electrical panel is permanently connected to the national grid. When you plug a generator into a socket or try to wire it directly without a transfer switch, current can flow backwards into the mains during an outage. This is illegal under the Electricity (Safety) Regulations 2016 and creates genuine hazard: a utility worker repairing the "dead" line could be electrocuted, and your generator could be destroyed by backfeed voltage.

A transfer switch physically isolates the grid connection when the generator is running, and reconnects it when you switch back to mains power. No overlap, no backfeed, no risk.

Manual Transfer Switches

A manual transfer switch is a large, heavy-duty changeover switch (usually 40–100A rated) that you install between your main consumer unit and the home circuits you want to run. When the mains fails, you:

1. Start the generator
2. Throw the switch lever to "Generator"
3. Restore power to selected circuits
4. When mains returns, switch back and stop the generator

Advantages:

• Significantly cheaper than interlock systems (£200–£500 fitted)
• Simple, reliable, no electronics to fail
• Clear visual indication of which power source is active
• Easy to understand and operate

Disadvantages:

• Requires manual intervention; you must be home and awake to switch
• Risk of human error (forgetting to switch back before mains restoration)
• Power is out until you physically throw the switch
• You can only run circuits connected to the transfer switch, not your whole house
• Doesn't work if you're away

Manual switches typically feed a sub-panel with essential circuits only (lighting, boiler, fridge, heating). You'd need to choose which circuits to protect before installation.

Interlock Kits

An interlock kit is an automatic mechanism that prevents you from accidentally running both mains and generator simultaneously. It's fitted to your main consumer unit and uses mechanical or electronic logic to:

• Detect mains failure
• Automatically switch to generator when voltage drops below a threshold
• Automatically switch back to mains when it returns
• Lock out the generator breaker when mains is live, and vice versa

Advantages:

• Fully automatic; no manual switching needed
• Faster response during blackouts
• Can protect more circuits, including your whole house if you want
• Safer—the interlock prevents accidental dual-power scenarios
• Better for people who are often away

Disadvantages:

• More expensive (£1,500–£3,000+ fitted by an electrician)
• Requires mains detection circuitry; more can go wrong
• Still need to start the generator manually (some kits add auto-start for extra cost)
• More complex to install and test; must be commissioned properly

UK Building Regulations and Wiring Standards

Any generator connection to your home's fixed wiring must comply with BS 7671 (17th Edition), the UK's standard for electrical installations. Key requirements:

Consumer Unit Protection: The interlock or manual switch must be installed as close as practical to your main consumer unit (ideally as part of it or integrated into the main switchboard). This minimises the length of unprotected wiring.

Protective Devices: Your generator circuit needs appropriate overcurrent protection, usually a 63A or 80A dual-pole switch rated for the generator's output. Cables must be sized for the generator's maximum current without overheating.

Earthing: A petrol generator's neutral and earth must be bonded together (at the generator itself), but when connected to mains-derived systems via an interlock, the earthing arrangement becomes more complex. An electrician must design this correctly to avoid dangerous voltage differentials.

G83 Notification: If your generator exceeds 16A per phase (roughly 3.6 kW), and you want to run circuits when mains is live at the same time (G83 embedded generation), you must notify your Distribution Network Operator (DNO) and install protective relaying. Most domestic setups won't do this—they use the generator only during outages. Check with your DNO; notification is usually free but mandatory.

Installation: Do It Yourself or Hire?

Manual transfer switch: An experienced DIYer with electrical knowledge can install one, but it must be:

• Sized and installed to current wiring regs
• Tested with a Phase rotation meter and multimeter
• Inspected by a qualified electrician before use (strongly recommended)

Interlock kit: Installation is beyond straightforward DIY. It involves modifying your main consumer unit, integrating detection circuitry, and ensuring proper earthing. An electrician certified in Part P (Building Regulations) should do this. Expect £1,500–£2,500 for labour plus materials.

Common Mistakes to Avoid

• No transfer switch: Connecting a generator via a standard socket or directly to the panel risks backfeed and is illegal.
• Wrong cable size: Undersized cables overheat; use 10 mm² or 16 mm² twin-and-earth, sized for your generator's maximum output.
• Ignoring earthing: Petrol generators are prone to earthing faults. Your installation must isolate faults safely.
• Forgetting to switch: With a manual switch, humans forget. Label it clearly and keep a checklist.
• No load shedding: Petrol generators are easily overloaded. Run essential loads only (lights, heating, fridge—not an electric shower and oven simultaneously).

Summary

For most UK homeowners, a manual transfer switch protecting essential circuits is the practical starting point: it's legal, affordable, and safe when operated correctly. If you need automatic switching or whole-house backup, an interlock kit installed by a qualified electrician is the proper choice.

Either way, get the design checked by a Part P electrician before installation, and always notify your DNO if the generator exceeds 16A per phase.