A mains-powered smoke alarm needs a 230V supply taken from a circuit that is on whenever the consumer unit's main switch is closed. The standard choice — and the one BS 5839-6 endorses — is the lighting circuit. Sockets are not allowed because a tenant or guest can unplug them. The feed is a 1mm² twin-and-earth spur from the nearest ceiling rose or junction, terminated at the alarm base.
For interlinking, the standard is a 3-core-plus-earth cable that runs from base to base. Live and neutral power every alarm; the third core (typically the orange "interconnect" conductor) carries the trigger signal. When any alarm detects smoke it asserts the interconnect line and every linked alarm sounds within ten seconds. Where chasing the cable is impractical, radio-link modules give identical compliance.
Electrician London installs mains-powered Grade D systems at £180 per point — alarm, cable, interconnect, commissioning and certificate included. NICEIC Minor Works on the electrical side, BS 5839-6 commissioning on the alarm side. Lighting circuits sized and protected to BS 7671 18th Edition Amendment 2.
Why Electrician London
230V from the lighting circuit
Mains feed taken from the lighting circuit so the alarm is always live when the main switch is on. Never socket-fed — sockets can be unplugged.
BS 7671 cable sizing
1mm² twin-and-earth supply, 1.0mm² 3-core+E interconnect. Sized and protected to BS 7671 18th Edition Amendment 2.
3-core+E interconnect
Hard-wired interlink between every alarm head. One detector triggers every alarm within ten seconds — the BS 5839-6 standard.
NICEIC + BS 5839-6 dual certified
Minor Works Electrical Installation Certificate on the 230V side, BS 5839-6 commissioning certificate on the alarm side. Both emailed same day.
Mains-powered smoke alarm pricing
Per-point pricing including 230V supply, interconnect and certificates.
Mains-powered smoke alarm install
Grade D, lighting-circuit feed
£180 / point
Mains-powered heat alarm
Kitchens, plant rooms, lofts
£170 / point
Interlinked mains system (3-4 alarms)
Typical family home or 1-2 bed HMO
£550
Full HMO upgrade (5-7 alarms)
LD2 schedule, multi-storey
£750-£1,400
Replacement on existing wiring
Like-for-like swap, existing base
£180 / point
What's included
- Site survey and LD2 schedule
- 1mm² supply from the lighting circuit
- 3-core+E interconnect cabling
- Grade D alarm head with sealed battery
- Commissioning test under load
- BS 5839-6 commissioning certificate
- Minor Works Electrical Installation Certificate
- Logbook and install photographs
Frequently asked questions
Why is the feed taken from the lighting circuit and not a socket?
BS 5839-6 requires the mains supply to come from a circuit that is on whenever the consumer unit's main switch is closed. The lighting circuit qualifies because it has no user-accessible isolation point. Sockets are explicitly disallowed because a tenant can unplug them — defeating the entire purpose of a permanently-powered alarm.
What cable size do you use?
1mm² twin-and-earth for the supply spur and 1mm² 3-core+E for the interconnect run. Both sized to BS 7671 18th Edition Amendment 2 for the protective device on the lighting circuit (typically a 6A MCB). Heat-resistant cable is used in lofts and adjacent to the kitchen alarm.
What is the 3-core+E interconnect?
Three cores plus earth — live and neutral power the alarm, and the third conductor (usually orange) carries the interconnect trigger. When any alarm fires it asserts the interconnect line and every other alarm on the same loop sounds. Every Aico, FireAngel and Kidde mains alarm uses the same convention.
Can I add an alarm to an existing mains-powered system?
Yes — provided the existing system is BS 5839-6 compliant and uses a compatible interconnect convention (all major UK brands do). We extend the 3-core+E from the nearest base, fit the new alarm, commission and certificate the addition. £180 covers the point.
Does the install need an EICR?
No — the install itself is a Minor Works job under BS 7671 because it is a single new circuit or extension to an existing one. We issue the Minor Works certificate on completion. A full Electrical Installation Condition Report (EICR) is only triggered if the existing wiring shows deficiencies the installer is duty-bound to report.
How long does the power-off last during install?
Five to ten minutes per alarm point while the lighting circuit is isolated to tap in the spur. For a six-point HMO total power-off time across the day is usually under an hour. We coordinate with occupants to schedule isolations outside meal times.
Will the alarm work if the lighting circuit trips?
Yes — that is exactly what the sealed 72-hour battery is for. If the lighting circuit MCB trips, the battery takes over and keeps the alarm fully functional. The head displays a fault LED so the trip is noticed and reset. After reset the battery resumes its standby state.
Can mains-powered alarms be radio-linked instead of cabled?
Yes — radio-link is just a different way of carrying the interconnect signal. The mains feed is still hard-wired (Grade D mandates mains supply). What changes is whether the interconnect is a copper conductor or an RF signal. Both meet BS 5839-6 identically.
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