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How to install a twilight switch on an existing light

Updated: October 2026

You are adding a photocell to an outdoor light that is already on a circuit. How to install a twilight switch starts with whether you should start. A new circuit, conductors you cannot name, a bathroom, a pool, or a pond on the mains side are reasons to call an electrician before any terminal is opened. The Products Compared editorial team wrote this on 16 October 2026 for the United Kingdom. It is not one wiring diagram for every listing. A qualifying purchase can earn a commission at no extra cost to you. Voltage and IP are on the [twilight switch buying guide](/hub/electrical-equipment/twilight-switch/sub/twilight-switch-buying-guide). The rows are on the [twilight switch comparison](/hub/electrical-equipment/twilight-switch).

You are adding a photocell to an outdoor light that is already on a circuit. How to install a twilight switch starts with whether you should start.

Our top picks

ZYWUOY ZYWUOY Photocell Sensor, 12–48 V DC

Best overall

ZYWUOY ZYWUOY Photocell Sensor, 12–48 V DC
2.1/10about £10.59
View offer
Electraline Electraline 92283 Outdoor Twilight Switch, IP44, Set of 2

Rank 2

Electraline Electraline 92283 Outdoor Twilight Switch, IP44, Set of 2
8.1/10about £7.51
View offer
NINMORTH NINMORTH Twilight Switch IP54

Best value

NINMORTH NINMORTH Twilight Switch IP54
2.3/10about £12.63
View offer
On this page
  1. 1.Stop before you start
  2. 2.Isolate and prove dead
  3. 3.Point the eye away from the lamp
  4. 4.Three conductors, and no invented earth
  5. 5.The short Part P rule

Stop before you start

A photocell on an existing outdoor point is not a new cable to the consumer unit. In England and Wales a new circuit in a dwelling is notifiable. Stop as well for conductors you cannot name, a bathroom, a pool, a mains-side pond, or a flood bank that may need a contactor. On one 12 V listing, United Kingdom texts disagree on polarity, and one reports about 9 V at the lamp from 12 V. Continue only where you can name the conductors and the voltage covers the supply. A hard-wired unit has no BS 1363 plug. Do not fit a 120 V-only body because the title says twilight. Movement versus darkness is on twilight switch vs PIR.

Isolate and prove dead

Switch off the circuit that feeds the outdoor point and stop it being turned on again. Prove dead at the wiring point with a two-pole tester: between live and neutral, between live and earth, and between neutral and earth. All three should show no voltage. A neon screwdriver is not that test. If you have no two-pole tester, or you are not sure which breaker feeds the light, stop. A dark lamp is not proof. Prove dead again if you left the job.
ZYWUOY ZYWUOY Photocell Sensor, 12–48 V DC

Testsieger

ZYWUOY ZYWUOY Photocell Sensor, 12–48 V DC
2.1/10about £10.59
View offer

Point the eye away from the lamp

The photocell watches daylight. If its own lamp shines into the window, the load switches off, the eye sees night, and the lamp returns. Face the window at open sky, not under the lantern, not at a pale wall that bounces the beam, and not behind a clear cover that reflects the lamp. One forum case of an LED flashing about every 20 seconds stopped when a clear cover was removed. That is an illustration of reflection, not a measurement. While you are fitting, the action is position. A correct window still needs the right conductor on the right terminal, and these listings do not share one colour code.

Three conductors, and no invented earth

A mains photocell normally needs permanent live in, neutral for the cell, and a switched live out to the lamp. That is the function, not a colour. UK cable is brown, blue and green-yellow only when those are the colours in your hand. One maker sheet uses white for the load. One Electraline comment says the sheet marked green as earth when that wire was the neutral. If the sheet disagrees with conductors you can prove, stop. No listing here states Class I or Class II, so do not invent an earth the leaflet never asked for. IP44 does not make open terminals waterproof while you joint them.

The short Part P rule

Part P applies in England and Wales to electrical work in dwellings, including garden lighting, and it applies to DIY. A new circuit is notifiable. Since 6 April 2013, adding a light to an existing circuit outside a special location has been a non-notifiable example in England. The work must still comply with BS 7671:2018. Wales has diverged since 31 July 2014. Scotland and Northern Ireland are not that FAQ. If you are unsure which applies, ask before you energise. Flicker from the lamp lighting its own eye is also on the twilight switch LED load page.

Common questions

Can I add a twilight switch to an existing outdoor light myself?
You can consider it when the light is already on a circuit, the point is not a bathroom, pool, or mains-side pond, you can name the conductors, and the voltage covers the supply. A new circuit from the consumer unit, in England and Wales, is notifiable. A hard-wired photocell has no 13 A plug. That does not make a 120 V unit acceptable on UK mains.
How do I isolate the circuit and prove it is dead?
Switch off the breaker that feeds the outdoor point and stop it being turned on. At the wiring point, use a two-pole tester between live and neutral, live and earth, and neutral and earth. All three should show no voltage. A dark lamp is not a dead test. If you cannot identify the breaker, stop.
Why should I ignore a green wire just because a sheet calls it earth?
One Electraline comment says the sheet marked green as earth when that wire was the neutral. Other imported leads use white for a switched load. Green-yellow is protective earth on UK cable only when the cable is actually green-yellow. If the sheet and the proved conductors disagree, stop. Do not invent an earth the listing never states.
Is garden lighting notifiable under Part P?
In England and Wales, Part P covers fixed electrical work in dwellings, including garden lighting and DIY. A new circuit is notifiable. Adding a light to an existing circuit outside a special location has been a non-notifiable example in England since 6 April 2013, and BS 7671:2018 still applies. Wales has treated outdoors differently since 31 July 2014.
Does the same rule apply in Wales, Scotland and Northern Ireland?
The note used here is the England and Wales FAQ, and Wales has diverged since 31 July 2014. The Welsh Government page still puts outdoor lighting outside the minor-work exemption it describes. Scotland and Northern Ireland are not that FAQ. If the nation or the circuit is unclear, ask before you energise.

Related pages

Wireless switch

Twilight switch

Best twilight switch: who it suits and who should skip

Twilight switch buying guide: voltage, load and IP

Twilight switch LED load: read the LED wattage line

Twilight switch vs PIR: photocell, movement or timer

Sources

Public UK sources used for this comparison. Listing prices, stars, and review counts are from amazon.co.uk on 15 October 2026.

  1. IET — Part P FAQ, England and Wales (retrieved 15 October 2026)
  2. GOV.UK — Approved Document P (2013) (retrieved 15 October 2026)
  3. BSI — BS EN IEC 60669-2-1:2022+A11:2022 (retrieved 15 October 2026)
  4. IEC 60669-2-1:2021 (retrieved 15 October 2026)
  5. Professional Electrician — Knightsbridge outdoor power (retrieved 15 October 2026)
  6. Historic England — external lighting design and installation (retrieved 15 October 2026)
  7. Electric Point — Danlers TWSW photocell switch (retrieved 15 October 2026)
  8. amazon.co.uk — Electraline 92283 listing (retrieved 15 October 2026)
  9. Universal Lighting — outdoor dusk-to-dawn photocell timer (retrieved 15 October 2026)