A sauna is a special location under BS 7671 Section 703. Three zones, a 170°C cable figure in Zone 3, IPX4 as a minimum, 30mA RCD on all circuits, and a dedicated radial sized to the heater data plate. Every figure on this page is cited to its regulation.
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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.
The short answer
A sauna is a special location under BS 7671 Section 703. It needs a dedicated radial circuit sized to the heater data plate (typically 20A to 40A), 30mA RCD additional protection on all circuits of the sauna (Reg 703.411.3.3), and equipment to at least IPX4. In Zone 3, equipment must withstand 125°C and cable insulation and sheaths 170°C (Reg 703.512.2) — silicone rubber cable, not 70°C PVC. An EIC must be issued and the new circuit notified under Part P.
Socket-outlets shall not be installed within the location containing the sauna heater, and switchgear for lighting shall be placed outside the sauna room or cabin (Reg 703.537.5). Section 703 does not apply to prefabricated cabins complying with a relevant equipment standard (Reg 703.1).
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01 · Installation Guide
These are the figures you are most likely to be looking for. Everything below is taken from Section 703 of BS 7671:2018+A4:2026, which covers rooms and cabins containing sauna heaters.
| Zone 1 extent | Heater volume, out to 0.5 m from the heater surface | Reg 703.32.1 |
|---|---|---|
| Zone 2 / Zone 3 split | Horizontal surface 1.0 m above the floor | Regs 703.32.2 / .3 |
| Zone 3 equipment | Withstand a minimum of 125°C | Reg 703.512.2 |
| Zone 3 cable | Insulation and sheaths withstand a minimum of 170°C | Reg 703.512.2 |
| Degree of protection | At least IPX4; at least IPX5 where cleaning by water jets may reasonably be expected | Reg 703.512.2 |
| RCD | 30mA additional protection for all circuits of the sauna; heater exempt unless the manufacturer recommends it | Reg 703.411.3.3 |
| Socket-outlets | Shall not be installed within the location containing the sauna heater | Reg 703.537.5 |
| Heating appliance | Shall comply with BS EN 60335-2-53 and be installed to the manufacturer instructions | Reg 703.55 |
Regulation 703.1 sets the scope. The section applies to sauna cabins erected on site, and to the room in which the sauna heater or heating appliances are installed — in that case the whole room is considered as the sauna, not just the cabin. It does not apply to prefabricated sauna cabins complying with a relevant equipment standard, though you still have to establish that the cabin actually meets one before relying on that. Where facilities such as showers are installed, Section 701 applies as well.
The environment is what sets sauna work apart. Rooms routinely run at 80°C to 100°C, well above the 70°C maximum conductor temperature of standard PVC-insulated cable, and BS 7671 responds with explicit heat-resistance figures rather than general wording. Getting the cable type wrong inside a sauna is a genuine fire risk.
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02 · Installation Guide
Sauna heater selection is based on the volume of the sauna room. Manufacturers generally work to around 1kW per cubic metre of sauna space, adjusted for insulation quality and glazing. The circuit ratings below follow the overload coordination rule of Regulation 433.1.1 — design current Ib ≤ device rating In ≤ cable capacity Iz.
| Sauna size | Heater | Approx. current | Circuit | Cable |
|---|---|---|---|---|
| Small (3 to 5m³) | 4.5kW | ~20A | 20A radial | 4.0mm² |
| Medium (5 to 8m³) | 6kW | ~26A | 32A radial | 6.0mm² |
| Large (8 to 12m³) | 8kW to 9kW | ~35 to 39A | 40A radial | 10.0mm² |
| Commercial (12m³+) | 12kW to 18kW+ | Per data plate | Three-phase supply — consult manufacturer | |
The rated current on the manufacturer plate is the definitive figure. The single-phase currents above are a guide only (kW ÷ 230V) and the cable sizes assume a favourable installation method. Size the cable and protective device to the actual conditions with the cable sizing calculator so that grouping, ambient temperature, thermal insulation and run length are all applied before you commit to a size.
Check maximum demand before quoting. A 9kW heater on a property with a 60A main fuse and existing loads of 40A at peak times may need a supply upgrade, and that is a conversation to have at survey stage rather than on the day of the install.
03 · Installation Guide
Regulation 703.32 requires that the zones of Regulations 703.32.1 to 703.32.3 are taken into account, and refers to Figure 703 for the dimensions. Equipment selection then turns on which zone the item sits in, under Regulation 703.512.2.
| Zone | Extent | What may be installed |
|---|---|---|
| Zone 1Reg 703.32.1 | The volume containing the heater, limited by the floor, the cold side of the ceiling thermal insulation and a vertical surface circumscribing the heater 0.5 m from its surface. If the heater is closer than 0.5 m to a wall, the cold side of that wall insulation is the limit instead. | Only the sauna heater and equipment belonging to the sauna heater. |
| Zone 2Reg 703.32.2 | Outside Zone 1, limited by the floor, the cold side of the wall thermal insulation and a horizontal surface 1.0 m above the floor. | No special requirement concerning heat-resistance of equipment. Switchgear forming part of the heater, or of other fixed equipment installed here, may stay inside the cabin per the manufacturer instructions (Reg 703.537.5). |
| Zone 3Reg 703.32.3 | Outside Zone 1, limited by the cold side of the ceiling and wall thermal insulation and a horizontal surface 1.0 m above the floor. | Equipment must withstand a minimum of 125°C; cable insulation and sheaths a minimum of 170°C. |
Note the wording used throughout: boundaries are the cold side of the thermal insulation, not the visible internal lining. Where insulation sits behind timber cladding, an accessory that looks recessed in the lining may still be inside the zone.
Section 703 contains no supplementary bonding requirement of its own. Where simultaneously accessible extraneous-conductive-parts are present — a metal bench frame or metallic door fittings that could introduce a potential — supplementary protective equipotential bonding is assessed under Regulation 415.2 in the normal way.
04 · Installation Guide
This is the single most important decision on a sauna job. Regulation 703.512.2 puts a hard number on it: in Zone 3, the insulation and sheaths of cables shall withstand a minimum temperature of 170°C. Regulation 703.52 then requires that any wiring on the warm side of the thermal insulation in Zone 1 or Zone 3 is heat-resisting.
| Cable / insulation | Max conductor temp | Use in a sauna |
|---|---|---|
| PVC twin-and-earth (6242Y) | 70°C | Does not meet the 170°C Zone 3 figure. Use only outside the sauna, up to the transition junction box. |
| XLPE | 90°C | Does not meet the 170°C Zone 3 figure either. Zone 2 carries no special heat-resistance requirement, so XLPE can be used there, but not in Zone 1 or Zone 3. |
| Silicone rubber | 170°C to 180°C | Meets the 170°C Zone 3 requirement. The standard choice throughout the sauna room, including the heater connection. |
Silicone rubber is available in single-core for conduit or trunking and in multi-core, and it must be supported with heat-resistant fixings — standard plastic clips will soften and drop the cable. Most electricians run silicone throughout the sauna room rather than switching cable type at a zone boundary that is defined by the position of insulation you cannot see once the cladding is on.
Outside the sauna room, from the consumer unit to the wall penetration, standard PVC cable is in a normal temperature environment and is fine. Make the transition to heat-resistant cable in a junction box outside the room. Better still, follow the preference stated in Regulation 703.52 and keep the wiring system outside the zones altogether, on the cold side of the thermal insulation, where the temperature question does not arise.
05 · Installation Guide
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06 · Installation Guide
Regulation 703.411.3.3 requires additional protection for all circuits of the sauna by one or more RCDs having the characteristics specified in Regulation 415.1.1 — RCDs with a rated residual operating current not exceeding 30mA. The combination of high temperature and moisture, particularly in a combined sauna and shower room, is exactly the scenario that regulation exists for.
The same regulation carries one exception: RCD protection need not be provided for the sauna heater unless such protection is recommended by the manufacturer. That makes the manufacturer documentation part of the design decision, not an afterthought — where the manufacturer recommends RCD protection for the heater, Regulation 703.411.3.3 makes it a requirement. Record which way you went and why, because the next person inspecting it will ask.
Regulation 531.3.3 is explicit: an RCD Type AC shall only be used to serve fixed equipment where it is known that the load current contains no DC components. A sauna controller with electronics in it does not satisfy that test, so Type A — which trips on alternating sinusoidal residual current and on residual pulsating direct current — is the appropriate minimum for a controlled heater. An RCBO on a dedicated way is the usual arrangement, as it keeps the rest of the installation up if the sauna circuit operates.
07 · Installation Guide
Initial verification follows Chapter 64 of BS 7671:2018+A4:2026. Two points are worth flagging on a sauna in particular: the insulation resistance sequence changed at A4, and the RCD test is a single test at IΔn.
An Electrical Installation Certificate (EIC) must be issued, with Section 703 referenced in the remarks and departures recorded if there are any. Installing the new circuit is notifiable building work under Part P in England and Wales, so notify it through your scheme or to building control.
08 · Installation Guide
| 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 (6kW), medium run | £550 to £700 | 32A circuit, longer cable run, heat-resistant cable and fixings |
| Large sauna (8 to 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 own conditions. They include materials, labour of half a day to a full day, testing and the EIC, and exclude the sauna heater, the cabin, and any building work.
09 · Installation Guide
Sauna work is specialist and prices accordingly. The differentiator is knowing Section 703 well enough to answer the customer's questions on the spot, and turning up with the right materials.
Keep a reel of silicone rubber cable and heat-resistant fixings in the van if you are marketing sauna installations. Neither is a standard stock item at most wholesalers and both may need ordering, so having them ready is the difference between one visit and two.
Use the quoting app to itemise the heat-resistant cable, silicone fixings, fireproof junction box and sauna-rated luminaires. They cost considerably more than the standard equivalents, and a quote built from a domestic radial template will not cover them.
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