Electrical Supply for Workshops and Outbuildings: Sub-board, SWA Cable and PME Earthing

Quick Answer: A workshop or outbuilding supply requires a maximum demand calculation, SWA submain sized for the load and voltage drop (typical 10mm² 3-core for 40A over 30m), a sub-consumer unit at the outbuilding with main switch and circuit protection, and careful earthing — the PME (TN-C-S) earth from the supply must NOT be exported to most outbuildings under BS 7671:2018+A2:2022 Regulation 411.4.5, so a local TT earth electrode with whole-supply 30mA RCD protection is usually required. The work is notifiable under Building Regulations Part P.

Summary

Powering an outbuilding — garage, workshop, garden office, summerhouse — is one of the most common electrical installation jobs after consumer unit replacement, and one of the most consistently done wrongly. The cable run is straightforward and the consumer unit at the destination is just a smaller version of a domestic board. What trips most installers is the earthing arrangement.

Most UK domestic supplies are TN-C-S (PME — Protective Multiple Earth), where the supplier combines neutral and earth into a single conductor in the supply cable. The DNO requires the neutral to be earthed at multiple points along the network. This system works well inside buildings, but exporting the PME earth to a detached outbuilding creates a serious risk: if the supply neutral becomes open-circuit (broken neutral fault), the consumer's earthed metalwork — including taps, radiators, gas pipework, and the outbuilding's metallic surfaces — can rise to mains potential. Anyone touching that metalwork while standing on earth outside is connected directly across mains.

This article walks through the design of an outbuilding supply: maximum demand calculation, SWA submain selection, sub-consumer unit configuration, and the critical earthing decision — when to use the PME earth, when to use a local TT electrode, and how to apply BS 7671:2018+A2:2022 Regulation 411.4.5 in practice. For cable specifics see armoured cable and for general sizing see cable sizing.

Key Facts

Quick Reference Table

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Workshop Type Estimated Demand Submain Cable Submain MCB Sub-CU Size
Garden shed — lights + 1 socket 6A 2.5mm² 2c SWA 16A 2-way
Garden office — lights + sockets + heater 16A 4mm² 3c SWA 20A 4-way
Small workshop — bench tools + 2x sockets + light 25A 6mm² 3c SWA 32A 6-way
Medium workshop — tablesaw + sockets + dust extractor 40A 10mm² 3c SWA 40A 8-way
Large workshop — multiple machines, welder, compressor 63A 16mm² 3c SWA 63A 12-way
Three-phase workshop — industrial machines 80–100A 3φ 25mm² 4c SWA 80A TPN 12-way TPN
Detached garage (no workshop) 16–32A 4–6mm² 3c SWA 20–32A 4–6 way

Detailed Guidance

Step 1: Maximum Demand Calculation

Maximum demand is the diversified estimate of the simultaneous load — not the sum of all rated loads. Lighting circuits rarely operate at full lamp count; sockets are not all in use at once. Apply diversity per IET On-Site Guide Table 1A and BS 7671 Appendix 1.

Worked example — small workshop:

Load Rated Diversity Diversified
Lighting (4× 60W LED) 1A 100% 1A
2× 13A socket spurs 26A 30% 8A
Dust extractor 1.5kW 7A 100% 7A
Bench grinder 300W 1.3A 50% 0.65A
Tablesaw 2.2kW 10A 100% 10A
Total 26.65A

Round up for headroom — specify a 32A submain. The sub-CU needs a 6/10A lighting circuit, 20A radial or 32A ring for general sockets, a dedicated 16/20A circuit for the tablesaw with workshop isolator, and a spare way for future expansion.

Step 2: Submain Cable Sizing

Voltage drop check (most common limiting factor):

For 32A over 30m on 6mm² SWA:

Better: 10mm² for the same circuit

Earth fault loop impedance:

Current rating check:

For full sizing methodology including grouping factors and ambient temperature corrections, see cable sizing.

Step 3: The Earthing Decision — PME vs TT

This is the most important decision in the design and the most commonly mishandled.

Outbuilding earthing — decision tree
┌─────────────────────────────────────────────────┐
│ What is the supply earthing arrangement at      │
│ the main building?                              │
└──────────────────┬──────────────────────────────┘
                   │
        ┌──────────▼──────────┐
        │ TN-S (separate      │
        │ earth conductor)?   │
        └──────┬──────────┬───┘
               │YES       │NO
               │          │
   ┌───────────▼──┐   ┌───▼──────────┐
   │ Can export   │   │ TN-C-S (PME) │
   │ earth to     │   │ or TT?       │
   │ outbuilding  │   └───┬──────┬───┘
   │ via SWA      │       │TN-CS │TT
   │ armour       │       │      │
   └──────────────┘       │      │
                  ┌───────▼──┐  ┌▼────────────┐
                  │Does the  │  │Already TT—  │
                  │outbuilding│  │export TT    │
                  │have any  │  │earth or     │
                  │outdoor   │  │provide local│
                  │earthable │  │electrode    │
                  │metalwork │  │             │
                  │people    │  │             │
                  │can touch?│  │             │
                  └───┬───┬──┘  └─────────────┘
                      │YES│NO
                      │   │
              ┌───────▼─┐ ▼────────────────┐
              │MUST use │ │Can export PME   │
              │TT —     │ │if no exposed    │
              │local    │ │extraneous       │
              │earth rod│ │metalwork — but  │
              │+ whole  │ │TT often safer   │
              │supply   │ │choice           │
              │30mA RCD │ │                 │
              └─────────┘ └─────────────────┘

The PME (TN-C-S) problem in plain English:

In a PME system, the supply neutral is also the earth. The DNO earths it at the substation and at multiple poles along the LV network. Under normal operation this provides a low-impedance earth path.

If the supply neutral becomes open-circuit (a "broken neutral" fault — a corroded joint at a pole, a cable fault, sometimes upstream of multiple properties), then:

Inside a building, this risk is mitigated by main protective bonding (Reg 411.3.1.2) — all extraneous-conductive parts are bonded together so everyone touching them is at the same potential. Outside, you cannot bond grass and tarmac. The hazard is real and has caused fatalities.

Regulation 411.4.5:

BS 7671:2018+A2:2022 prohibits the use of TN-C-S (PME) earthing where there is a significant risk of broken neutral creating a touch voltage hazard. The judgement call is whether the outbuilding has external metalwork that a person could touch while standing on true earth.

Practical guidance:

TT installation requirements:

A common error: running 3-core SWA with full gland bonding at both ends, then "adding" a local earth rod. This creates a parallel earth path — a broken neutral fault energises the local TT earth via the supply armour, rendering the TT earth useless. The supply earth MUST be electrically separated from the local TT earth at the outbuilding for TT to function correctly.

Step 4: Sub-Consumer Unit at the Outbuilding

The sub-consumer unit (sub-CU) is a small consumer unit at the outbuilding, fed from the submain.

Configuration for TT outbuilding (most common): 63A/80A DP 30mA RCD as main switch (provides shock protection given the TT earth), standard DIN rail busbar, final circuits comprising 6A/10A Type B MCB for lighting, 32A or 20A Type B for sockets, and a 16A/20A MCB with workshop isolator for dedicated machines. Earth bar connects to the local electrode by 16mm² G/Y. Neutral bar is NOT bonded to earth — bonding it would defeat the TT arrangement.

Configuration for TN-S exported earth (rare): plain DP isolator as main switch, RCBOs (Type A/F) on individual final circuits, earth bar from the incoming PE conductor.

Configuration for TN-C-S exported earth (only where external touch-voltage risk is genuinely absent): plain DP isolator or 100mA time-delayed RCD; comprehensive main protective bonding of all extraneous-conductive parts (water, gas, structural steel). TT is usually the safer choice — TN-C-S export is rarely the right answer.

Step 5: Three-Phase Considerations

Most domestic outbuildings are single-phase. Three-phase becomes necessary for:

Three-phase supply requirements:

Step 6: Part P Notification

In England, installing a submain to an outbuilding constitutes a new circuit and is notifiable under Building Regulations Approved Document P:

For Wales: equivalent under Welsh Approved Document P. For Scotland: Building (Scotland) Regulations Section 4.5. For Northern Ireland: Building Regulations Part F.

Specifically notifiable for outbuildings:

Frequently Asked Questions

Why can't I just use the PME earth like I do inside the house?

Inside a house, all earthed metalwork is bonded together via main protective bonding — water pipe, gas pipe, structural metalwork. Anyone inside touching earthed metal is at the same potential as everything else. Outside, you cannot bond the grass and the patio to the earth bar. If the PME neutral breaks, the earthed metal of a detached garage rises to mains potential, but the ground around it does not. A person standing on the ground and touching the garage becomes the current path. This has caused several UK fatalities — BS 7671 Reg 411.4.5 exists specifically to prevent it.

What earth resistance do I need on a TT earth electrode?

BS 7671 Reg 411.5.3 sets a maximum of 1666Ω for a 30mA RCD (RA × IΔn ≤ 50V) but recommends much lower in practice — typically <200Ω for reliable RCD operation across all fault scenarios. In good soil, a single 1.2m rod will achieve <100Ω. In rocky or sandy ground, multiple rods or a deeper rod may be needed. Test with an earth electrode resistance tester after installation and record on the Electrical Installation Certificate.

Can I run the SWA above ground on cable cleats instead of burying it?

Yes, SWA can be surface-mounted on a wall or fence with cleats at appropriate intervals (manufacturer guidance, typically ~600mm cleat spacing for 6mm² and below, 400mm for larger sizes). However, exposed SWA needs UV-stable sheath — BS 6346 standard PVC sheath has limited UV resistance; specify UV-stable PVC or PE outer sheath for sun-exposed runs. Mechanical protection from impact is also a consideration — buried is generally more robust long-term.

How long can a submain be before voltage drop becomes an issue?

For 32A on 10mm² SWA, voltage drop is about 1.4V per 10m. Allowable drop on submain alone might be 2.5–3% (5.7–6.9V) to leave headroom for the final circuit, so 40–50m on 10mm². Beyond that, step up to 16mm² (mV/A/m = 2.8) which extends the run substantially. For very long runs (>100m) consider 3-phase to reduce currents by √3, or simply oversize the cable significantly. There is no regulatory maximum length — voltage drop is the practical limit.

Do I need an external isolator at the outbuilding for the DNO?

The DNO requirement is for an isolator accessible to them at the supply origin — that is the main consumer unit, not the outbuilding. However, a clearly-labelled external isolator at the outbuilding (or just inside the door) is good practice for the user — for maintenance, emergency, or in case of fire. A DP 63A switch-fuse outside the building, locked when not in use, is a sensible specification.

Regulations & Standards