Exhaust Fan Wiring Diagram

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Understand how to wire an exhaust fan with a standard wall switch, overrun timer, or humidity sensor — covering switched live, neutral, earth connections, and code compliance for bathroom installations.

An exhaust fan (also called an extractor fan) in a bathroom, kitchen, or utility area is wired as part of the fixed electrical installation and must comply with the applicable wiring regulations for the jurisdiction: BS 7671 in the UK (with specific requirements for zones in BS 7671 Section 701), NEC/NFPA 70 Article 210 and 422 in the USA, AS/NZS 3000 in Australia and New Zealand, or IEC 60364-7-701 for international applications. In all cases, installation in bathrooms and wet areas is restricted by zone classifications that determine which types of equipment and wiring methods are permitted.

Basic wiring (switch-controlled): The fan is wired in the same manner as a switched lighting circuit. A switched live (line after the switch) and a neutral feed are taken to the fan. The fan operates only when the switch is on. Earth must be connected to the fan's earthing terminal where the motor or casing has exposed accessible metalwork.

Overrun timer: Many exhaust fans incorporate an internal overrun (run-on) timer that keeps the fan running for a set period (typically 2–20 minutes, adjustable on some models) after the switch is turned off, ensuring adequate ventilation post-shower. These fans require a permanent live feed (unswitched) to power the timer electronics, in addition to the switched live to trigger the fan. The wiring requires a three-core cable or a two-core plus neutral from the ceiling rose or junction box: permanent live, switched live, and neutral (plus earth).

Humidity sensor: Fans with integral humidity sensors automatically activate when relative humidity in the room rises above a set threshold (typically 65–80 % RH) and shut down when it falls below. Wiring requirements are similar to the overrun timer: permanent live, neutral, and earth. Some humidity-sensor fans also incorporate a switch override input.

Light-switch interlock: In the most common UK bathroom fan wiring, the fan is wired from the same switch feed as the bathroom light — the fan activates when the light is switched on. This requires only a switched live and neutral feed at the fan (same two-core-and-earth cable arrangement as the ceiling rose).

For all bathroom installations, verify the fan's IP rating is adequate for its installation zone (Zone 1: IP44 minimum; Zone 2: IP44 minimum in UK; products must be appropriate for the zone).

How to wire exhaust fan wiring diagram

Confirm the installation zone and fan IP rating Measure the installation position relative to the bath or shower tray. Confirm the position falls within the correct zone as defined by BS 7671 Section 701 (or equivalent national standard). Verify the fan's IP rating is appropriate for the zone. For ceiling mounting above a shower enclosure, IP44 minimum is required. Check the fan manufacturer's installation instructions.
Isolate the circuit at the consumer unit Switch off the relevant lighting circuit MCB at the consumer unit. Lock or secure the MCB to prevent unintentional re-energisation. Prove the circuit is dead with an approved voltage indicator at the intended connection point (ceiling rose, junction box, or switched outlet) before touching any conductors.
Determine the cable requirement based on fan type For a basic on/off fan or a fan with an overrun timer powered by the switched live: two-core-and-earth cable (switched live + neutral + earth). For a fan with an overrun timer requiring permanent live, or a humidity sensor fan: three-core-and-earth cable (permanent live + switched live + neutral + earth), or two separate two-core cables.
Route the cable to the fan mounting location Run the cable through the ceiling void, using appropriate clips, staples, or conduit as required by the wiring regulations. Where the cable passes through timber joists, drill holes at least 50 mm from the top or bottom edge of the joist (UK regulations) to protect against nail penetration. Use appropriate grommets where the cable enters the fan housing.
Connect cables at the fan Connect the switched live (brown, or blue with brown sleeve for a switch-live core) to the fan's live input terminal. Connect the neutral (blue) to the fan's neutral terminal. Connect the earth (green-and-yellow) to the fan's earth terminal. For timer fans requiring permanent live, connect the permanent live to the designated timer live terminal per the fan's wiring diagram label — do not interchange the switched and permanent live feeds.
Connect at the source (ceiling rose, junction box, or consumer unit) At the source connection point, connect the cable to the existing circuit in accordance with the fan wiring diagram and the relevant wiring regulations. In a loop-in ceiling rose: switched live connects to the switch terminal, neutral to the pendant/neutral terminal, and permanent live (where needed) to the loop/line terminal. Ensure all joints are made in an accessible enclosure with adequate IP rating.
Test and commission Restore the circuit at the consumer unit. Test the fan operation: switch on and verify the fan runs. Switch off and verify overrun timer keeps the fan running for the set period. Test humidity sensor function if fitted by applying steam from a kettle briefly near the sensor and confirming the fan activates. Verify the fan switches off at the correct humidity level. Perform polarity and earth loop impedance tests as required by the applicable regulations.

Specifications

Typical fan power rating (residential)	10–25 W at rated voltage
Supply voltage (UK/Australia)	230–240 V AC, 50 Hz
Supply voltage (USA/Canada)	120 V AC, 60 Hz
Minimum IP rating for bathroom Zone 1 and Zone 2 (BS 7671)	IPX4 (splash-proof); IP44 preferred and required by most fan manufacturers for Zone 1/2 use
Typical overrun timer range	2–20 minutes (adjustable on most modern timer fans)
Typical humidity sensor activation threshold	65–80 % relative humidity (adjustable on most humidity-sensor fans)
Cable type (UK domestic fixed wiring)	Flat two-core-and-earth or three-core-and-earth PVC to BS 6004; 1.0 mm² or 1.5 mm²

Safety warnings

All fixed wiring of exhaust fans must comply with the applicable electrical installation standard: BS 7671 (UK, including Section 701 for bathrooms), NEC/NFPA 70 Articles 210 and 422 (USA), AS/NZS 3000 (Australia/New Zealand), or IEC 60364-7-701 (international). In bathrooms and wet areas, work must be carried out by a qualified electrician and notified where required by building regulations.
Always isolate the circuit at the consumer unit and verify the circuit is dead using an approved two-pole voltage tester before making any connections. The fan cable may contain a permanent live conductor that remains energised even when the light switch is off.
Never fit an exhaust fan with an inadequate IP rating for the installation zone. Water ingress into an under-rated fan housing is a source of electric shock, short circuit, and fire risk.
Ensure the earth conductor is correctly terminated at the fan's earthing terminal and at the circuit earth. An unearthed metal fan casing presents a lethal electric shock risk if a live conductor makes contact with the casing.
Do not attempt to modify the fan's internal wiring or bypass its thermal cutout. Thermal cutouts protect the fan motor from overheating when airflow is obstructed — bypassing them creates a fire risk.

Tools needed

Approved two-pole voltage indicator (for proving circuit dead)
Insulated electrician's screwdrivers (flathead and cross-head)
Wire strippers for 1.0–1.5 mm² cable
Drill and hole saw or spade bit for fan mounting aperture
Stud/joist detector to avoid drilling into structure
Fish tape or cable rod for routing cable through ceiling voids
Continuity tester or loop impedance tester for commissioning

Common mistakes

Connecting the permanent live and switched live terminals in reverse on a timer fan — the timer circuit will not function correctly, often resulting in the fan running continuously or failing to run at all.
Omitting the earth connection at the fan when the fan has a metal casing or motor housing — leaving the casing unearthed is a shock hazard and a regulatory non-compliance.
Installing a fan with an insufficient IP rating inside a bathroom zone, particularly IP20-rated kitchen extractors fitted in Zone 1 or Zone 2 of a bathroom.
Failing to use an accessible, maintenance-accessible junction box for intermediate cable joints — connections inside ceiling voids must be accessible without damage to the building fabric.
Running the fan cable close to or through thermal insulation without considering the cable's current-carrying capacity reduction (derating) — an insulated ceiling void requires significant derating of cable capacity, potentially requiring a larger cable cross-section.

Troubleshooting

Fan does not run when switch is turned on: Cause: No switched live reaching the fan, blown circuit fuse/tripped MCB, failed fan motor, or open circuit in the switch Fix: Isolate circuit, prove dead, and use a continuity tester to verify the switch is functioning (continuity when on, open when off). Restore supply and measure voltage at the fan's live terminal with the switch on — should be 230 V (or 120 V) relative to neutral. No voltage indicates an open circuit in the switched live feed. Replace switch or trace cable fault.
Fan runs continuously and does not stop: Cause: Switched live and permanent live terminals swapped on a timer fan (fan is receiving permanent live only); timer module faulty; or humidity sensor set too sensitively or sensor failed in activated state Fix: Isolate circuit and verify wiring against the fan's internal diagram. On a timer fan, confirm the switched live (from the switch) connects to the trigger input and the permanent live connects to the permanent supply terminal. Adjust the humidity sensor threshold if applicable. Test by turning the switch off — if the fan does not stop, the trigger and permanent inputs are likely swapped.
MCB trips when the fan is switched on: Cause: Short circuit in the fan motor windings, cable wiring fault (live and neutral bridged), or motor seized and drawing locked-rotor current Fix: Isolate the circuit. Disconnect the fan motor supply leads and megger test between live and neutral, and between each conductor and earth. A reading below 1 MΩ indicates insulation failure. Inspect for cable damage and bridged connections. Test the motor for freedom of rotation by hand — a seized motor draws locked-rotor current and will trip the MCB.

Frequently asked questions

What cable does an exhaust fan with an overrun timer require?

An exhaust fan with an overrun timer requires three conductors plus earth at the fan: a permanent live (unswitched line), a switched live (line from the switch), and a neutral. This is typically supplied as a three-core-and-earth cable (brown, black/grey, blue, and bare earth) or by running two separate cables to the fan — one providing permanent live and neutral, the other providing the switched live from the switch.

What IP rating is required for a bathroom exhaust fan?

In UK bathrooms under BS 7671 Section 701, Zone 1 (above the bath or shower tray to 2.25 m height) requires a minimum of IP44. Zone 2 (the area 0.6 m beyond Zone 1 horizontally) also requires a minimum of IP44. Ceiling fans above 2.25 m and outside Zone 1 and Zone 2 require a minimum of IP20 but IPX4 is good practice. Always verify the fan's IP rating on its datasheet against the specific installation zone.

Can I wire an exhaust fan on the same circuit as the bathroom light?

Yes. The most common domestic arrangement in the UK and many other jurisdictions is to wire the fan from the same switch live as the bathroom light so both activate together. The fan then uses its internal overrun timer to continue running after the light is switched off. The neutral supply must also be available at the fan, taken from the ceiling rose or lighting circuit neutral. This arrangement requires only two-core-and-earth cable at the fan when the overrun timer is powered by the switched live.

Does the exhaust fan need its own dedicated circuit?

In residential applications, a small exhaust fan (typically under 5 A) may be connected to the existing lighting circuit as a spur. In commercial applications, mechanical ventilation fans must comply with the applicable building regulations and may require a dedicated circuit with appropriate protection. Larger extract fans serving commercial kitchens require dedicated circuits, switched isolators, and in some cases fire damper interlocks.

What is the difference between a local wall switch and a pull cord for bathroom fan control?

In UK bathrooms, ordinary electrical switches are not permitted within 600 mm of a bath or shower (Zone 2 boundary). Control is achieved either by a pull cord switch (cord-operated insulated switch that can be mounted within Zone 2) or by a switch outside the bathroom controlling both the light and fan. The pull cord mechanism is entirely insulated and does not require a user to touch a metal switch plate, reducing shock risk in wet conditions.

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