Helpful video reference. The LEARN ELECTRICS video "BATHROOM FAN WIRING METHODS – 2 PLATE AND 3 PLATE METHODS – OVER-RUN TIMER – EASY TO FOLLOW DRAWINGS" covers both methods clearly with wiring diagrams for each. LEARN ELECTRICS produces UK-specific electrical tutorials on domestic installation topics, with supporting resources at learnelectrics.com.
1. Understand 2-plate versus 3-plate
The terms come from how many conductors are grouped at the junction box or connection point. In a lighting circuit using the 3-plate loop-in method, each ceiling rose or junction box has three groups of conductors: the incoming supply from the consumer unit, the switch wire, and the outgoing feed to the next fitting.
In a 2-plate bathroom fan connection, only two groups are used: the supply live and neutral from the junction box (either the switched live, or the unswitched supply depending on which method you choose). The fan is wired into this existing circuit at a convenient junction point.
In a 3-plate connection, a separate spur cable is run from an FCU back to the bathroom, giving the fan its own permanent live supply. The fan's internal electronics handle switching entirely.
2. Choose the right method for your installation
The 2-plate method is suitable when:
- You are replacing an existing fan that was already wired to the lighting circuit
- There is a convenient junction box in the void above the bathroom ceiling
- You want the fan to start when the light goes on and stop shortly after it goes off
The 3-plate method is more appropriate when:
- You want the fan to run independently, for example triggered by a humidity sensor rather than the light switch
- The existing lighting circuit wiring does not have a convenient junction point
- You need a new cable run from a spur on the landing or bedroom circuit above the bathroom
Both methods are equally acceptable under BS 7671. The 3-plate method gives more flexibility but requires slightly more work if there is no existing spur nearby.
3. Wire the 2-plate connection
Isolate at the consumer unit and verify dead at the junction box you will work at. In the junction box, identify the switched live: this is the conductor that is live only when the light switch is in the on position. In harmonised colours it is brown, often sleeved brown at both ends in older installations with grey cores.
Take the fan's live (brown) to the switched live terminal in the junction box. Take the fan's neutral (blue) to the neutral terminal. Connect the earth (green-yellow) to the earth terminal. The fan now mirrors the light: it runs when the light is on, and the overrun timer keeps it running for its set period after the light goes off.
If the fan has only two terminals with no overrun timer, it simply stops when the light goes off. Most modern fans include the timer built into the unit and the wiring method is the same regardless.
4. Wire the 3-plate connection
For the 3-plate method, a 3A fused connection unit (FCU) is installed on the permanent live from the nearby circuit. The FCU provides overcurrent protection and a switching point. From the load side of the FCU, a 0.75 mm² or 1.0 mm² flexible cable (or a fixed 1.0 mm² twin-and-earth run to the bathroom) connects to the fan terminals.
The fan's live (brown) connects to the FCU load-side live. Neutral and earth connect to their respective terminals in the junction box and are run to the FCU and fan. The fan's own internal controller takes over from here: it can switch on from a humidity sensor, from a proximity switch, or from a separate signal wire connected to the room's light switch, depending on the fan model.
Check the fan's wiring diagram before connecting. Many fans designed for 3-plate installation have a separate terminal for the switch live signal (marked SL or SW) that tells the fan the light has been turned off and starts the overrun period. Connect this terminal to the switched live from the room's light switch.
5. Programme the overrun timer
Most modern bathroom fans have an overrun timer adjustable via a small potentiometer on the fan body, accessible through the front grille with a small flat-blade screwdriver. Turn the potentiometer clockwise to increase the run-on time, anticlockwise to decrease it.
For a standard bathroom used mainly for hand-washing and light use, 10 to 15 minutes run-on is enough to clear residual moisture. For a shower room that generates significant steam, 20 to 30 minutes is better. Leave the timer set conservatively at first and adjust after a few weeks of use if condensation is still forming.
Some fans have separate run-on and delay-start timers. A delay-start timer keeps the fan off for the first minute or two after the light goes on, which avoids the fan pulling cold air in before the room warms up. This is useful in winter but optional.
6. Test the installation
Restore the supply at the consumer unit. Turn on the bathroom light and confirm the fan starts within a few seconds. For a 2-plate connection, the fan should start whenever the light is on. For a 3-plate connection with a separate signal wire, the fan should start when the light is on.
Switch the light off and watch the fan. It should continue running for the programmed overrun period and then stop cleanly. Listen for any unusual humming or vibration from the fan body, which can indicate a motor fault or an incorrectly seated grille.
Finally, check for any smell of heat from the fan or the junction box. A new installation occasionally has slight insulation burn-off on first use, but any persistent smell or visible scorch marks means the installation needs to be checked before regular use.
When to call us
A 2-plate connection at an accessible junction box above the bathroom ceiling is well within reach of a careful DIY person who has isolated the circuit correctly. Running a new spur for the 3-plate method, or routing new cable, is a different matter: that is Part P notifiable work and needs a registered electrician to certify it. Richard fits bathroom extractor fans across east Kent and carries out the notification where required.
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Richard wires and fits bathroom extractor fans, including overrun timers and humidity-controlled units. Small jobs at the £10 per 10-minute rate.
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