Installing a PIR Sensor to an Outside Light: Wiring, IP Ratings & BS 7671 Requirements

Adding a PIR motion sensor to an outside light is one of the most common jobs in domestic electrical work. Whether it’s a security floodlight on a driveway or a courtesy light by a front door, the principle is the same — the sensor detects movement and switches the light on automatically.

But getting it right means more than just connecting three wires. You need the correct IP rating for the location, the right cable type for outdoor use, RCD protection, and an understanding of how the PIR’s terminals actually work. This guide covers all of it.

How a PIR Sensor Works

A Passive Infrared (PIR) sensor detects changes in infrared radiation — essentially body heat — in its field of view. When a person (or animal) moves across the detection zone, the sensor triggers and closes an internal relay, sending a switched live to the light fitting.

Key characteristics:

Detection range: Typically 3–12 metres, adjustable via a sensitivity dial
Detection angle: Usually 110°–180° depending on the model
Recommended mounting height: 2.0–2.5 metres above ground for optimal coverage
Response: The sensor only triggers on moving heat sources — a stationary warm object won’t activate it

The PIR itself needs a permanent live and neutral supply to power its electronics. It then provides a switched live output (often labelled COM or SL) that goes to the light.

Basic PIR Wiring: Terminal Connections

Most standalone PIR sensors have three or four terminals:

L — Live In (permanent supply from the circuit)
N — Neutral
COM (or SL or L’) — Switched Live Out (goes to the light fitting)
E — Earth (on the sensor’s backplate or a separate terminal)

The wiring is straightforward: the supply live feeds into L on the sensor. The sensor’s internal relay switches this live on and off, outputting it from COM. Neutral and earth run continuously to both the PIR and the light fitting.

Basic PIR sensor wiring to an outside light

Important: The neutral wire goes directly to the light fitting as well as the PIR — it does not pass through the sensor’s relay. Only the live is switched.

Wiring Summary

Wire	From	To	Purpose
Brown (Live)	Lighting circuit	PIR terminal L	Permanent power to sensor
Brown (Switched Live)	PIR terminal COM	Light fitting L	Sensor-controlled power to light
Blue (Neutral)	Lighting circuit	PIR N + Light N	Continuous neutral to both
Green/Yellow (Earth)	Lighting circuit	PIR E + Light E	Protective conductor

Adding a Manual Override Switch

In many installations, it’s useful to have a switch inside the house that lets you control the outside light in three modes:

OFF — Everything off (sensor and light)
AUTO — PIR controls the light (normal operation)
ON — Light stays on permanently, bypassing the PIR

This is typically achieved with a 2-gang light switch wired as follows:

Switch 1 (master on/off): Supply live goes to COM. L1 feeds power to Switch 2 and the PIR’s L terminal.
Switch 2 (mode selector): In the AUTO position, power runs through the PIR sensor which switches the light. In the PERM position, power bypasses the PIR and feeds the light directly.

PIR with manual override switch

This arrangement means the homeowner can turn the light on permanently when needed (e.g., carrying shopping in) and then flick back to AUTO mode when they’re done.

Alternative: A single centre-off switch can provide the same three modes — OFF (centre), AUTO (position 1), and permanent ON (position 2).

IP Ratings: Choosing the Right Equipment

Any electrical equipment installed outdoors must have an appropriate Ingress Protection (IP) rating as required by BS 7671. Section 714 specifically covers outdoor lighting installations.

Regulation 714.512.2.105 states that outdoor lighting equipment must have a minimum IP rating of IP33. However, this is a bare minimum — in practice, you should choose a higher rating based on the location.

IP ratings for outdoor electrical equipment

Practical IP Rating Guide

Location	Minimum IP	Why
Under eaves / porch (sheltered)	IP44	Protected from direct rain but exposed to splashes
Open wall (exposed to weather)	IP65	Needs protection against wind-driven rain and dust
Near ground / driveway	IP65+	Risk of puddle splashes and debris
Areas cleaned with hose/jet wash	IP66	Must withstand powerful water jets
In-ground / recessed fittings	IP67–IP68	Submersion risk from pooling water

Both the PIR sensor and the light fitting must be rated for the location — it’s a common mistake to fit an IP65 light with an IP20 sensor and wonder why it fails after the first rainstorm.

Cable Selection for Outdoor Runs

Standard PVC twin-and-earth cable is not suitable for permanent outdoor use — it degrades under UV exposure. BS 7671 requires cables installed outdoors to be appropriate for the external influences they’ll be exposed to.

For outdoor lighting circuits, your options include:

SWA (Steel Wire Armoured) cable — the most robust option, can be buried directly or surface-mounted with cleats
PVC cable in conduit — standard T&E can be used if protected inside UV-resistant conduit
UV-resistant flex — for short connections between a junction box and the fitting (check the manufacturer’s rating)

If burying cable, it should be at a minimum depth of 500mm (or as specified by the cable manufacturer) and protected by cable covers or suitable ducting.

RCD Protection

Regulation 411.3.3 of BS 7671 requires that circuits supplying outdoor equipment be protected by a 30mA RCD (or RCBO). This is mandatory, not optional.

Best practice is to run outdoor lighting on its own dedicated circuit with its own RCBO at the consumer unit. This way, a fault on the outdoor circuit doesn’t trip the RCD protecting your indoor lighting.

PIR Sensor Adjustments

Most PIR sensors have three adjustment dials (usually small potentiometer knobs on the back or underside). Getting these right makes the difference between a sensor that works perfectly and one that triggers constantly — or never at all.

PIR sensor adjustment settings

LUX (Light Level)

Controls the ambient light level at which the sensor becomes active.

Turn towards the sun symbol: Sensor operates day and night (useful for testing)
Turn towards the moon symbol: Sensor only operates when it’s dark (typical for security lighting)

TIME (On-Duration)

Controls how long the light stays on after the sensor is triggered.

Minimum (usually ~10 seconds): Good for testing; too short for practical use
1–5 minutes: Typical setting for entrance lighting
Maximum (up to 30 minutes): Useful for driveways or areas where you need extended illumination

SENS (Sensitivity / Range)

Controls how far away the sensor can detect movement.

Low sensitivity: Only detects movement very close to the sensor (3–4 metres)
High sensitivity: Detects movement at full range (up to 12 metres)

Tip: Start with maximum sensitivity and reduce it if you get false triggers from passing traffic, animals, or neighbouring footpaths.

Common Installation Mistakes

Mistake	Problem	Solution
Using an indoor-rated sensor outdoors	Water ingress causes failure or short circuit	Always check the IP rating for the location
Forgetting the override switch	No way to turn the light on manually	Wire a 2-gang switch inside for OFF/AUTO/ON
Running standard T&E cable outdoors	UV degradation causes insulation breakdown	Use SWA cable or protect cable in UV-rated conduit
Mounting the PIR too high	Reduced detection range and dead zones	Mount at 2.0–2.5m for optimal coverage
Pointing the sensor at a heat source	Constant false triggers from boiler flues, vents, etc.	Angle the sensor away from fixed heat sources
No RCD protection	Non-compliant and unsafe	Outdoor circuits must have 30mA RCD protection

Key BS 7671 Regulations

For quick reference, here are the main regulations relevant to this type of installation:

Section 714 — Outdoor lighting installations (special installation under Part 7)
Reg. 714.512.2.105 — Minimum IP33 for outdoor lighting equipment
Reg. 512.2.1 — Equipment must be suitable for the external influences at its location
Reg. 411.3.3 — 30mA RCD protection for circuits supplying outdoor equipment
Reg. 522.8 — Cables in locations exposed to direct sunlight must be suitable for UV
Part P Building Regulations — A new outdoor lighting circuit is notifiable work

Practice and Further Study

Outdoor lighting falls under Part 7: Special Installations or Locations and Part 5: Selection and Erection of Equipment of BS 7671. Test your knowledge with our practice quizzes:

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