What's involved in wiring a greenhouse electrical supply

Helpful video reference. We use Electrician's Life UK's video "Installing Power To Garden Building (Studio Build Episode 2)" as the video reference here. It covers a real UK installation of a buried supply to a separate garden building: cable selection, SWA gland termination, the sub-board layout and the earthing decision for a separate structure. The principles apply directly to a greenhouse supply.

1. List everything you want to power

Write down every electrical load you plan to run in the greenhouse now and in the future: LED grow lights, propagation heating mats, a tubular frost-protection heater, a small greenhouse fan, power tools, a hosepipe timer controller. Add the wattages and divide by 230 to get the amps. Add a 20% safety margin for starting currents and future additions.

A typical domestic greenhouse supply is rated at 32 A. If you only want a frost heater and a couple of lights, 20 A is sufficient. If you are planning serious growing lamps or a large heating installation, think about 40 A and size the cable accordingly.

2. Understand why greenhouse wiring is different from a garden room

A garden room or summerhouse is a dry or damp location. A greenhouse is damp to wet. The practical difference is the IP rating required for every electrical accessory:

• Garden room: IP44 minimum for any exposed accessories.
• Greenhouse: IP65 minimum throughout, because condensation drips on everything and hosepipes splash everywhere.

IP65 means the accessory is protected against dust ingress entirely, and against water jets from any direction. Socket outlets, junction boxes, luminaires, fuse holders, timer controllers - all of them need to be IP65 rated in a greenhouse. This is not the same as buying a standard outdoor socket with a hinged cover lid; the whole assembly, lid open or closed, must carry the IP65 rating.

Avoid metal-bodied accessories where possible. Metal junction boxes and steel-wire conduit rust quickly in the greenhouse environment, which compromises earthing continuity over time. Use UV-stabilised plastic enclosures rated for damp or wet conditions.

3. Notify building control

Installing a new supply to a separate structure - including fitting a sub-board in the greenhouse - is notifiable under Part P of the Building Regulations in England. Your options:

Registered electrician: they self-certify and notify the local authority after the work, and issue you an Electrical Installation Certificate. This is the straightforward route for most homeowners.

Building notice: submit a notice to your local authority before starting, pay the inspection fee (typically £150 to £300), and arrange a building control inspection at completion. You still need the electrical installation certificate from whoever does the testing.

Either way, the certificate matters for your insurer and for any future buyer of the property. Do not skip it.

4. Size and order the cable

Measure the actual cable route from the house consumer unit to the greenhouse sub-board. This is not the straight-line distance across the garden - it includes the run from the consumer unit to the point where the cable exits the house, across the garden underground, and up into the greenhouse.

For a 32 A supply, use 6 mm² two-core-and-earth SWA for runs up to about 30 metres. For longer runs, or if you want 40 A protection, use 10 mm² SWA. Always check voltage drop using the cable manufacturer's tables: the limit for a final circuit under BS 7671 is 3% of the nominal voltage (about 6.9 V on a 230 V supply).

5. Dig the trench

SWA cable must be buried at the correct depth:

• Open garden soil: 600 mm minimum.
• Under a path or patio: 450 mm minimum with cable protection tiles above and a marker tape 150 mm below the surface.
• Under a driveway: 900 mm, or run in heavy-duty conduit.

Keep the trench straight where possible and sketch a plan with measurements from fixed reference points. Record the depth at any points where it changes. File the plan with the property papers. When someone digs a cable in a greenhouse border years later, the plan is what saves the cable.

6. Lay and terminate the SWA cable

Where the cable exits the house foundation, protect it with a length of grey conduit from the trench up through the external wall. The conduit must be sealed at both ends to stop moisture and vermin entering the wall cavity. Fit an SWA cable gland where the cable enters the conduit and another where it exits inside the house.

At the greenhouse end, the cable rises up through the floor or wall in conduit again. Fit a suitable IP65 enclosure for the gland entry point, and terminate the SWA into the greenhouse sub-board through a cable gland. Connect the steel wire armour to the sub-board earth terminal at both ends.

7. Earth the greenhouse correctly - this is the most important rule

If your house has a TN-C-S (PME) supply - which most UK homes built after about 1970 do - you cannot extend that PME earth to a separate structure. A PME earth combines the neutral and earth conductors at the distribution transformer. If the neutral conductor fails between the transformer and your house, the combined neutral-earth can rise to a dangerous potential, raising any metalwork connected to it to a voltage that can kill.

For a greenhouse, the solution is a TT arrangement: drive a local earth rod (typically 1.2 m or 2 m steel rod) into the ground outside the greenhouse, and connect it to the earth terminal at the greenhouse sub-board. The house earth remains entirely separate. The electrician will also fit a 30 mA RCD at the sub-board to provide the fast-trip protection that a TT installation depends on, since TT earth fault loop impedance is typically higher than TN.

If you are unsure whether your supply is TN-C-S or TN-S, the electrician will check at the meter. Do not assume: the earthing type determines how the greenhouse must be earthed.

8. Install the sub-board and accessories

Inside the greenhouse, fit a small consumer unit (sub-board) rated for the supply current. An IP55 or IP65 consumer unit is available specifically for outbuilding use - use one of these rather than a standard domestic board, which is not suitable for the damp environment.

Wire individual circuits inside the greenhouse in 1.5 mm² T&E for lighting and 2.5 mm² T&E for socket circuits, all protected with RCBOs at the sub-board. Keep the cable clipped to the greenhouse frame away from soil level, and use IP65-rated wiring accessories at every point.

9. Test and certify

Before energising, the electrician will carry out the required tests: polarity check on every circuit, insulation resistance between live conductors and earth (with the supply disconnected), and earth continuity. After energising: earth fault loop impedance and RCD trip time tests. Results go on an Electrical Installation Certificate.

When to call us

The full job - trench, SWA cable, earth rod, sub-board, accessories and Part P certification - is electrician work. You can dig the trench yourself and have it ready for the cable installation day, which saves time on site. Richard covers Sandwich and east Kent and handles outbuilding supplies regularly, from the consumer unit RCBO through to the last socket in the greenhouse.