Outdoor Lighting Calculator — SWA Cable & IP Rating

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), provided it is correctly terminated. 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. Modern SWA is 90°C XLPE-insulated to BS 5467 (the older 70°C PVC type to the now-withdrawn BS 6346 is rarely stocked). This calculator sizes cables on the 70°C PVC ratings from BS 7671 Table 4D4A, Reference Method D2 (direct in the ground), so an equivalent 90°C XLPE cable will always have at least the stated capacity. If you instead pull the cable through a duct in the ground, use the slightly lower Method D1 ratings.

BS 7671 Appendix 12 recommends keeping voltage drop to 3% for lighting circuits (6.9V on 230V) and 5% for other circuits, measured from the origin of the installation. The tighter lighting limit preserves performance margin. Mains-driven LED fittings use constant-current drivers that regulate output over a wide input-voltage window, so a 5% mains drop generally does not make them visibly dim — voltage drop matters most for long low-voltage (12/24V) tails after the driver and for non-regulated loads. For long outdoor runs the 3% limit often determines the cable size rather than the current capacity. Always check voltage drop for runs over 30m. Use our Lux Level Calculator to determine the right lux for your outdoor space.

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. See our Earthing System Calculator for electrode sizing and resistance calculations.

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.

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+).