Saunas are special locations under BS 7671 Section 703. Heat-resistant cable, temperature zones, dedicated circuits, and correct zoning are essential. This guide covers everything from heater sizing to testing and certification.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
A sauna is a special location under BS 7671 Section 703. It needs a dedicated radial circuit sized to the heater (typically 20A to 40A), heat-resistant cable inside the room (silicone rubber rated 170°C to 180°C), and 30mA RCD protection on all circuits under Reg 703.411.3.3. Equipment must meet the zone rules of Regs 703.32.1 to 703.32.3. An EIC must be issued and the work notified under Part P.
No socket outlets are permitted in the location containing the sauna heater (Reg 703.537.5), and switchgear for lighting must be placed outside the sauna room.
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Domestic sauna installations have become increasingly popular in the UK. Whether it is a purpose-built sauna room, a barrel sauna in the garden, or a sauna cabin within a bathroom, the electrical installation requires careful attention to heat resistance, zoning, and safety.
Sauna electrical work falls under BS 7671:2018+A4:2026 Section 703 (rooms and cabins containing sauna heaters). This section imposes specific requirements on wiring methods, cable types, equipment zoning, and protective measures that go beyond a standard domestic circuit.
The most critical difference from standard installations is the temperature environment. Sauna rooms routinely reach 80°C to 100°C — well above the maximum operating temperature of standard PVC-insulated cable. Using the wrong cable type inside a sauna is a genuine fire risk.
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Sauna heater selection is based on the volume of the sauna room. The general rule is 1kW per cubic metre of sauna space, with adjustments for insulation quality and glazing:
| Sauna size | Heater | Approx. current | Circuit | Cable |
|---|---|---|---|---|
| Small (3 to 5m³) | 4.5kW | ~20A | 20A radial | 4.0mm² |
| Medium (5 to 8m³) | 6kW to 8kW | ~26 to 35A | 32A radial | 6.0mm² |
| Large (8 to 12m³) | 9kW | ~39A | 40A radial | 10.0mm² |
| Commercial (12m³+) | 12kW to 18kW+ | Per data plate | Three-phase supply — consult manufacturer | |
Always size from the data plate, not from kW. The rated current on the manufacturer plate is the definitive figure — single-phase current shown above is a guide only (kW ÷ 230V). Size the cable and protective device to the actual installation conditions using the cable sizing calculator so that grouping, ambient temperature and run length derating are applied correctly.
Verify the existing supply has sufficient spare capacity for the heater. A 9kW heater on a property with a 60A main fuse and existing loads of 40A during peak times may require a supply upgrade. Check maximum demand before committing to the installation.
BS 7671 Section 703 defines the requirements for rooms and cabins containing sauna heaters. Equipment selection turns on which of the three zones (Regs 703.32.1 to 703.32.3) it sits in. The zone definitions and the heat-resistance rules from Reg 703.512.2 are summarised below.
| Zone | Extent | What may be installed |
|---|---|---|
| Zone 1Reg 703.32.1 | Volume containing the heater, bounded by the floor, the cold side of the ceiling insulation and a vertical surface 0.5 m from the heater surface (or the cold side of a wall if the heater sits closer than 0.5 m). | Only the sauna heater and equipment belonging to the sauna heater (Reg 703.512.2). |
| Zone 2Reg 703.32.2 | Outside Zone 1, from the floor up to a horizontal surface 1.0 m above the floor, bounded laterally by the cold side of the wall insulation. | No special requirement concerning heat-resistance of equipment (Reg 703.512.2). Switchgear forming part of the heater or other fixed equipment may sit here per the manufacturer (Reg 703.537.5). |
| Zone 3Reg 703.32.3 | Outside Zone 1, above 1.0 m from the floor to the cold side of the ceiling and wall insulation. | Equipment must withstand a minimum of 125°C; cable insulation and sheaths a minimum of 170°C (Reg 703.512.2). |
The earlier guidance that supplementary equipotential bonding may be needed for accessible extraneous-conductive-parts (metal bench frames, metallic door fittings) still holds where such parts could introduce a potential — assess each installation on its merits and bond where required.
This is the single most important aspect of sauna electrical installation. Standard PVC cable will fail in a sauna environment. The cable types suitable for sauna installations are:
The standard choice for wiring inside the sauna room, including the heater connection. Silicone rubber insulation withstands the high temperatures in Zone 1 and Zone 2. Available in single-core (for use in conduit or trunking) or multi-core. Must be supported with heat-resistant fixings — standard plastic cable clips will melt.
Cross-linked polyethylene insulation is rated to 90°C conductor temperature. It can be used in the lower zones of the sauna where ambient temperatures remain below 80°C, but it is not suitable for the upper zones or near the heater. In practice, most electricians use silicone rubber throughout the sauna room for simplicity and safety.
| Cable / insulation | Max conductor temp | Use in a sauna |
|---|---|---|
| PVC twin-and-earth (6242Y) | 70°C | Not inside the sauna. Acceptable only outside the room, up to the transition junction box. |
| XLPE | 90°C | Lower zones only, where ambient stays well below the rating. Not for Zone 3 or near the heater. |
| Silicone rubber | 170°C to 180°C | Standard choice inside the sauna, including the heater connection. Meets the 170°C cable requirement for Zone 3 (Reg 703.512.2). |
Outside the sauna room (from the consumer unit to the wall penetration), standard PVC cable is acceptable as it is in a normal temperature environment. The transition from standard cable to heat-resistant cable should be made in a junction box outside the sauna room.
Regulatory basis — Reg 703.52: Where a wiring system is installed on the warm side of thermal insulation in Zone 1 or Zone 3, it shall be heat-resisting. Metallic sheaths and metallic conduits shall not be accessible in normal use in those zones. Where practicable, wiring should be installed on the cold side of the thermal insulation (outside the zones). This is the explicit BS 7671:2018+A4:2026 requirement underpinning the choice of silicone rubber or equivalent heat-resistant cable inside the sauna room.
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BS 7671 Regulation 703.411.3.3 (the sauna-specific Part 7 rule) requires additional protection for all circuits of the sauna by one or more RCDs having the characteristics of Regulation 415.1.1 (30mA). The combination of high temperatures and moisture — particularly in combined sauna/steam rooms — makes RCD protection essential.
There is a specific exception for the sauna heater: RCD protection need not be provided for the sauna heater itself unless the heater manufacturer recommends it. Always check the manufacturer documentation before deciding whether to apply RCD protection to the heater circuit. If the manufacturer recommends it, RCD protection becomes mandatory for the heater.
An RCBO on the dedicated way at the consumer unit is the standard approach for sauna circuits. Type A is preferred over Type AC as sauna heater controllers may include electronic components that produce DC fault components.
The completed installation must be tested in accordance with BS 7671 Chapter 64 (Initial Verification, BS 7671:2018+A4:2026):
An Electrical Installation Certificate (EIC) must be issued. The remarks should reference BS 7671 Section 703. The work must be notified under Part P of the Building Regulations as it involves a special location.
| Scope | Indicative price | What it covers |
|---|---|---|
| Small sauna (4.5kW), short run | £400 to £550 | 20A circuit, RCBO, standard + silicone cable, testing, EIC |
| Medium sauna (6 to 8kW), medium run | £550 to £700 | 32A circuit, longer cable run, heat-resistant cable and fixings |
| Large sauna (9kW), long run | £650 to £850 | 40A circuit, 10.0mm² cable, more labour |
| Sauna lighting circuit (add-on) | + £150 to £300 | Heat-resistant luminaires and silicone cable |
| Consumer unit upgrade (add-on) | + £350 to £600 | If no spare ways are available |
These are indicative market figures for guidance, not a quote — price each job to its conditions. They include materials, labour (half a day to a full day), testing, and the EIC certificate, and exclude the sauna heater, sauna cabin, and any building work.
Sauna installations are specialist work that commands premium pricing. The key differentiator is knowing the Section 703 requirements and using the correct heat-resistant materials.
Keep a reel of silicone rubber cable in the van if you are marketing sauna installations. It is not a standard stock item at most wholesalers and may need to be ordered. Having it ready saves a return visit.
Use the quoting app to itemise the heat-resistant cable, silicone fixings, fireproof junction box, and specialist luminaires. These cost more than standard materials — make sure the quote reflects this.
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