Outdoor Electrical Installations: Armoured Cable, IP Ratings, RCD Protection and Part P

Quick Answer: Outdoor electrical work is notifiable under Approved Document P when it involves a new circuit, a consumer unit change, or any installation in a special location. Buried cables to garden buildings should be SWA (steel wire armoured) to BS 6724 at a depth of 600 mm minimum or sleeved in steel duct at lesser depths, with 30 mA RCD protection for any socket-outlet rated up to 32 A and an enclosure rating of IP44 minimum (IP65 for exposed positions). All work must be tested and certified to BS 7671:2018+A2:2022.

Summary

Outdoor circuits look superficially like indoor circuits with weatherproof boxes on the end, but they sit under a different set of pressure points: ingress, mechanical damage, earth-fault current paths over wet ground, and the prospect of an inexperienced user pushing a metal hedge trimmer through the supply cable. The Wiring Regulations treat outdoor installations as a hazard category because each of those failure modes individually has a poor history.

The dominant cable choice is SWA (steel wire armoured) — its armour gives both mechanical protection from spades and rodents and a metallic earth path that bonds the cable's earth conductor to a defined route. The choice of accessory enclosure depends on exposure: under cover, IP44 may suffice; in driving rain or pressure-washed environments, IP65 or better is required. RCD protection (30 mA for sockets, anywhere persons might come into contact with cables when working outdoors) is the final defence.

For homeowners, the question is normally either "can I run a power cable to the shed myself?" — and the legal answer is no, this is notifiable work under Part P — or "why does the garden socket trip every time it rains?" — almost always a sign that the IP rating of the accessory or the cable termination has been compromised.

Key Facts

• Part P — outdoor work involving a new circuit, new consumer unit, or work in a special location is notifiable under Approved Document P. Special locations include rooms containing a swimming pool or sauna and the outdoor zones around them.
• SWA cable — Steel Wire Armoured to BS 5467 (XLPE insulation) or BS 6346 (PVC insulation, increasingly displaced); both also tested to BS 6724 LSZH for low-smoke applications.
• Cable burial depth — at least 600 mm in arable land and 450 mm under hard surfaces, with cable warning tape 150 mm above the cable per BS 7671 Section 522.8.10. Inside lawful curtilage, 600 mm is the safe default.
• Direct burial — SWA can be buried directly without a duct, provided it is correctly bedded in sand or fine soil to prevent stones cutting the outer sheath.
• Sleeving and duct — where depth cannot be achieved (e.g. crossing a paved area), the cable must be enclosed in a steel duct or run through a concrete-encased twin-wall ducting compliant with BS EN 50086.
• Termination — SWA glands (e.g. CW-type for outdoor, CWL with low-smoke versions, A2 brass for plastic enclosure earthing) — manufacturer-specified torque, with earth tag bonded to the enclosure earth point.
• IP ratings — IP44 for general outdoor use under cover; IP55 for exposed positions; IP65–IP66 for direct rain and washdown; IP67 for occasional immersion.
• RCD protection — 30 mA RCD or RCBO on every socket up to 32 A used outdoors (411.3.3) and on cables buried in walls without mechanical protection (522.6.202).
• Earthing system — TT earthing (with local earth electrode) is sometimes required for outbuildings supplied from a TN-C-S installation where the PME earth cannot safely extend (e.g. swimming pools, EV chargers in some configurations).
• Earth electrode — copper-bonded steel rod, 1.2 m or 2.4 m typical; Ra (electrode resistance) target ≤ 200 Ω, ideally < 100 Ω for reliable RCD operation.
• Bonding — outdoor metalwork, fences, swimming pool grids may require supplementary equipotential bonding per Section 705 (agricultural) or Section 702 (swimming pools).
• Outbuilding circuits — typically a single sub-main from the main consumer unit feeding a small distribution board (CU) inside the outbuilding; protected by an upstream RCBO at the main board and locally by MCBs in the outbuilding CU.
• Photovoltaic and EV charging — separate sections of BS 7671 (Section 712 for PV; Section 722 for EV charging); these introduce additional earthing and SPD requirements beyond standard outdoor work.
• Garden lighting — extra-low voltage (12 V SELV) lighting via a transformer/driver to BS EN 61558-2-6 simplifies many outdoor installations and is not notifiable under Part P provided the circuit is not in a special location.

Quick Reference Table

Detailed Guidance

Why SWA dominates outdoor work

Steel wire armoured cable does three jobs in one:

1. The conductors carry current.
2. The galvanised steel wires around the conductors provide mechanical protection — they will defeat a spade, a fork, and most rodent damage.
3. The steel armour, when properly terminated with a CW-type gland and earth tag, forms a parallel earth path that extends the protective conductor to the cable's outer surface — hugely simplifying earth fault loop impedance compliance over long runs.

The alternative — multi-core flexible cable in conduit — is more vulnerable to damage, requires the conduit to be both sealed and earthed, and has a higher fitting time. SWA is universally specified for buried garden runs.

Sizing the cable

Two factors drive cable size for an outbuilding sub-main:

• Current capacity — the load (lights, sockets, perhaps a small heater) determines the design current. A 32 A circuit comfortably feeds a typical garden office or workshop; a small shed might be served by a 16 A circuit.
• Voltage drop — 4% maximum for power circuits per BS 7671. Long runs to outbuildings (30 m+) push voltage drop quickly. A 16 A load over 30 m on 4 mm² SWA gives roughly 3% voltage drop on copper, just within limits; over 50 m the same circuit needs 6 mm² to stay within 4%.

Common sizes:

• 2.5 mm² SWA — runs under 25 m at modest load.
• 4 mm² SWA — workshop / garden office to 30 m.
• 6 mm² SWA — long runs, higher loads.
• 10 mm² SWA — EV charger sub-main, large workshop.

Burial, sleeving and routing

Direct burial is straightforward: dig a trench, lay the cable on a 50 mm bed of sand or fine soil, cover with another 50 mm of fine material, then backfill with the excavated soil. Place yellow cable warning tape 150 mm above the cable. Mark the route on the as-built drawing — it saves the next person doing landscaping years later.

Where depth cannot be achieved (e.g. crossing under a paved patio you do not want to lift), use a steel duct (50 mm or 75 mm) bedded in concrete to provide mechanical protection. Keep duct bends gentle (radius ≥ 8× cable outer diameter).

For runs that pass into the building, bring the SWA up against the wall and into a junction box at the building entry; from there transfer to T&E or singles inside the building. The transition must keep the cable's earth continuity intact and the gland torqued correctly.

Accessories and enclosures

The IP rating you need depends on exposure:

• IP44 is "splash protected" — adequate for under-cover sites such as porches and the underside of canopies.
• IP55 is "limited dust ingress and water jets" — appropriate for most exposed external positions.
• IP65 is "dust-tight and pressure water" — choose this for rain-exposed positions, garden walls and the outside of outbuildings.
• IP66/IP67 is "dust-tight and powerful jets / immersion" — used for ground-level enclosures or where a hose may impinge.

Practical mistakes:

• Using an "outdoor" socket from a DIY shed that turns out to be IP44 — fitted in driving rain, the seal fails inside two seasons.
• Mounting the enclosure with the cable entry pointing upwards — water tracks down the cable and into the box, even with a gland.
• Forgetting that pull cord, conduit fittings and accessory glands all need to be IP-rated to match the enclosure; a single lower-rated entry compromises the whole assembly.

RCD protection — outdoor is more demanding than indoor

Two things drive the 30 mA RCD requirement outdoors:

• 411.3.3 — additional protection for socket-outlets up to 32 A used by ordinary persons, which extends to outdoor sockets explicitly.
• The earth fault loop impedance to remote outbuilding circuits is often poor, and the wet ground around an electrode increases the risk that a fault current finds a path through a person to ground rather than through the protective conductor. RCDs are the primary defence in this scenario.

For an outbuilding fed via a sub-main, design the protection as:

• Main consumer unit: 32 A or 40 A type-A RCBO feeding the sub-main (Type A handles modern DC components from inverter loads).
• Outbuilding CU: MCBs for the local circuits, optionally with additional 30 mA RCBOs for socket circuits and any portable equipment supply.

TT earthing for outbuildings

TN-C-S supplies (the typical PME urban supply) can extend their earth to outbuildings only where the safety case can be made. Where the earth would extend across a wet pool deck, a swimming pool zone, or where there is a risk of broken neutral on the supply, an alternative is to provide a local TT earth at the outbuilding via an earth electrode rod and serve the outbuilding as a TT system from an isolated sub-main.

Test the electrode resistance on commissioning — most domestic outbuilding TT systems target Ra ≤ 200 Ω, ideally below 100 Ω, with an MFT (multi-function tester) loop measurement.

Garden lighting — when 12 V is the better answer

Many garden lighting jobs are simpler and safer at 12 V SELV (Separated Extra-Low Voltage). A waterproof transformer fitted under the soffit feeds 12 V cable to LED spike lights, deck lights and pond lights. The incoming side is mains and notifiable; the 12 V side is SELV and not notifiable — provided the SELV is properly separated and the transformer is to BS EN 61558-2-6.

Practical advice:

• Use 12 V LED rather than 230 V garden lights wherever the load and run length allow.
• Pick a transformer at twice the lamp wattage to allow for cable drop and future expansion.
• Run cable in 50 mm trench inside the curtilage of the garden; mark with tape.

Consumer-facing question — "can I install a garden socket myself?"

Single new outdoor socket addition (notifiable under Part P) — not legal DIY in England and Wales without LABC notification or a registered Competent Person Scheme electrician. In Scotland, Part P does not apply but the work is still required to be safe and tested. Replacing a like-for-like outdoor socket is permitted DIY but only where the installation is otherwise compliant.

Frequently Asked Questions

Can I run a 13 A flex through the wall to a garden socket?

No. A garden socket is a fixed installation requiring a dedicated circuit, RCD protection, and Part P notification. Running flex through a wall is non-compliant and a fire risk.

Does an outdoor socket need its own circuit?

Not necessarily — it can be a spur or extension of an existing ring or radial circuit, provided the upstream protection includes a 30 mA RCD and the cable type is suitable. In practice, a dedicated outdoor circuit is cleaner and avoids load problems.

What is the maximum length of SWA I can run from the consumer unit to the shed?

Limited by voltage drop, not by an absolute distance. With 4 mm² SWA at 16 A load, runs to 35 m are routine; 6 mm² extends that to 50 m+. For very long runs (60 m+), upgrade to 10 mm² or split the load.

Do I need a separate consumer unit in the outbuilding?

If there is more than one final circuit in the outbuilding (e.g. lighting and sockets), yes — fit a small CU. Single-circuit outbuildings can be served directly without a local CU.

What about a pond pump?

Pond pumps are normally fed from an outdoor RCD-protected socket in IP65 weatherproof housing. The pump itself must be IP68-rated and earthed via the supply cable. Avoid running the pump cable across a path; bury or duct.

Regulations & Standards

• The Building Regulations 2010 — Approved Document P (electrical safety in dwellings) — defines notifiable work.

• BS 7671:2018+A2:2022 — Section 522 (cable installation), 411 (automatic disconnection), 701 (bathrooms), 702 (swimming pools), 705 (agricultural premises), 712 (PV), 722 (EV charging).

• BS 5467:2016 — Cables with thermosetting insulation for fixed installations rated up to 600/1000 V (XLPE-insulated SWA).

• BS 6724:2016 — Armoured cables for fixed installations, having low-smoke, halogen-free properties.

• BS EN 50086 series — Conduit systems for cable management.

• BS EN 61558-2-6 — Safety isolating transformers for general use, used for SELV garden lighting.

• IET Guidance Note 1 — Selection and Erection — practical detail on outdoor cable runs and SWA termination.

• Approved Document P (gov.uk) — statutory guidance for electrical safety in dwellings.

• Electrical Safety First — Outdoor electrics guide — consumer-facing summary of outdoor safety duties.

• HSE — Electricity and outdoor work — workplace context for outdoor electrical work.

• BSI — BS 7671 standard — the IET Wiring Regulations.

• cable sizing for fixed installations — the calculations behind 4 mm² vs 6 mm² SWA.

• earthing systems and bonding requirements — TT vs TN-C-S decisions for outbuildings.

• EV charger installation — Section 722 rules and earthing arrangements.

• garden lighting wiring and SELV — 12 V garden lighting fundamentals.