What cable type should I use for outdoor lighting?

SWA (Steel Wire Armoured) cable is the standard choice for outdoor and underground wiring. The steel armour provides mechanical protection and can serve as the circuit protective conductor (CPC/earth). For direct burial, SWA should be laid at minimum 450mm depth (600mm under roads) on a bed of sand with marker tape above. Alternative: surface-mounted in conduit for wall runs, transitioning to SWA for underground sections. Always use BS 5467 rated SWA cable.

Why use 3% voltage drop for lighting instead of 5%?

BS 7671 limits voltage drop to 3% for lighting circuits (6.9V on 230V) and 5% for other circuits. Lighting is more sensitive to voltage drop — a 5% drop causes noticeable dimming in LED fittings and can cause flickering or premature failure of electronic drivers. For outdoor lighting with long cable runs, the 3% limit often determines the cable size rather than the current capacity. Always check voltage drop for runs over 30m.

Do I need an earth rod for outdoor lighting?

For TT earthing systems: yes — you need a local earth rod because there is no supply earth. For TN-C-S (PME): BS 7671 Regulation 411.4.2 requires consideration of touch voltage if the PEN conductor is lost. An additional earth electrode is recommended for outdoor circuits on PME supplies. The SWA cable armour provides the CPC, but it must be properly terminated and bonded at both ends. An earth rod at the outdoor location provides additional safety.

What IP rating do outdoor light fittings need?

Minimum IP65 for exposed outdoor fittings — dust-tight and protected against water jets. IP44 may be acceptable for sheltered locations (under eaves, covered porches). Underground or ground-level fittings need IP67 (temporary immersion) or IP68 (continuous immersion). All junction boxes, connectors, and accessories in the outdoor circuit must also meet the required IP rating — a single weak point compromises the entire installation.

Should I use a photocell or timer for outdoor lighting control?

A photocell (dusk-to-dawn sensor) is preferred for security and pathway lighting as it automatically adjusts to seasonal daylight changes. A timer is better when you want specific on/off times. A combination photocell + timer (astronomical timer) is ideal — it uses GPS or programmed sunrise/sunset times. PIR motion sensors are used for security lighting. All control devices should be rated for the connected load and suitable for outdoor use (IP65+).

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Outdoor Lighting Calculator — SWA Cable & IP Rating

Design outdoor lighting circuits with SWA cable sizing, voltage drop check, IP65+ requirements, earth rod considerations for TT systems, and photocell/timer control.

Number of Light Fittings

Total outdoor fittings

Wattage per Fitting

LED wattage per fitting

Total Cable Run

CU to furthest fitting

Earthing SystemDetermines earth rod requirements

IP Rating RequiredIP65 standard for outdoor

Control MethodOutdoor lighting control

How We Calculate This

This calculator designs outdoor lighting circuits using SWA cable, checking current capacity, voltage drop, and IP/earthing requirements.

Design Considerations

Cable: SWA (BS 5467) for mechanical protection
VD limit: 3% for lighting (6.9V on 230V)
IP rating: IP65 minimum for exposed outdoor
RCD: 30mA required for all outdoor circuits
Burial depth: 450mm min (600mm under roads)
Earth rod: Required for TT, recommended for PME outdoor

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.