Helpful video reference. This UK electrical tutorial "How to wire a 3 pole fan isolator switch -- extractor fan switch install wiring explained" walks through the connections for both a timer fan (using 3-core-and-earth) and a simple non-timer fan (twin-and-earth). The wiring diagrams are clear and the UK-specific terminal labelling is shown correctly. Worth pausing at the terminal shots and comparing against your own fan's installation instructions before you start.
1. Identify your fan and what cable you need
Open the fan body and look at the terminal block. A non-timer fan has two power terminals (live and neutral) plus an earth. A timer or overrun fan has a third terminal -- some manufacturers label it "T" or "overrun", others use a numbering scheme like 1, 2, 3.
The cable you need:
- Non-timer fan -- run 1.0 mm2 or 1.5 mm2 twin-and-earth from the isolator to the fan. 1.0 mm2 is adequate for runs up to about 15 m on a standard lighting circuit.
- Timer or overrun fan -- run 1.0 mm2 or 1.5 mm2 3-core-and-earth. The three live conductors carry live, neutral, and the timer signal. The earth is in addition to those three.
The supply side -- from the lighting circuit junction point to the isolator -- is always twin-and-earth: live and neutral only.
2. Confirm the supply point and isolator position
The fan isolator is supplied from the permanent live of the lighting circuit for that room. In practice this is usually a junction box above the ceiling, or a spur taken at the switch back box from the permanent live feed -- never from the switched live, because the fan needs to receive power whenever the bathroom is in use, not just when the light is switched on.
Choose the isolator position carefully. It must be outside the bathroom zones. Zone 2 ends 0.6 m from the edge of the bath or shower tray, so the ceiling immediately above the door is the most common location. Confirm the measurement before marking out.
3. Isolate the circuit
Switch off the relevant lighting circuit at the consumer unit. Confirm dead in the switch back boxes and at any junction boxes you plan to work in. Write a note on the consumer unit while you work.
4. Fit the back box and run the cables
For a surface-mounted isolator, fix a plastic surface box at the chosen position. For a flush-mounted isolator, cut and chase the pattress into the wall or ceiling. Run the supply cable from the junction point down to the back box, and run the load cable from the back box up through the ceiling void to the fan body. Leave 150 mm of cable inside each box -- you will need the slack for connecting.
Where cables pass through a joist or ceiling void they must be in a safe zone, clipped, and protected against mechanical damage per BS 7671 regulation 522.6.6 and 522.6.7.
5. Wire the isolator terminal connections
3-pole fan isolators have two sets of terminals -- IN and OUT (or supply and load, labelled by the manufacturer). Connect:
- Supply IN terminals: brown to L1, blue to L2, green-and-yellow earth to the earth terminal.
- Load OUT terminals (non-timer fan): brown to L1, blue to L2, green-and-yellow earth to earth terminal.
- Load OUT terminals (timer fan): brown to L1, blue to L2, grey (or black, re-sleeved as appropriate) to L3 (timer signal), green-and-yellow earth to earth terminal.
Terminal torque is usually 0.5 Nm on small fan isolator switches. Tighten firmly but do not crush the conductor insulation. Fit the cover plate and press the rocker to confirm it moves freely.
6. Connect the fan body
At the fan terminal block, connect the load cable conductors to match the fan manufacturer's terminal labelling. For a timer fan, the third conductor (grey or black, re-sleeved as appropriate for the circuit) connects to the overrun/timer terminal. Confirm with the fan's installation instructions -- terminal layouts vary significantly between Vent-Axia, Xpelair, Manrose and other brands.
7. Restore power and test
Restore the lighting circuit at the consumer unit. With the isolator in the ON position, the fan should run. Switch the isolator OFF -- a non-timer fan should stop immediately. A timer/overrun fan should continue running for the set period (typically 10-15 minutes) and then stop. If the fan hums but the impeller does not spin, switch off immediately and check the load-side connections for a polarity error.
When to call us
Adding a fan isolator to an existing extractor fan wired on the 3-plate method is a manageable job for an electrician-literate homeowner, but it involves working in a bathroom ceiling void and making connections in multiple boxes. If the fan is brand new, or if the existing wiring is not what you expected when you opened the back boxes, call Richard. He covers Sandwich and east Kent on a small-job rate.
Extractor fan wiring in Sandwich?
Richard can add a fan isolator, replace a bathroom fan, or sort out incorrectly wired bathroom electrics -- all on the local small-job rate.
Contact Richard