What's involved in wiring a sauna heater electrical supply in the UK

Helpful video reference. We use The Sauna Heater channel's "Electrical Requirements Of Electric Sauna Heaters" as the video reference here. It covers the key supply sizing and protection decisions that apply to UK domestic sauna installations, including circuit amperage, dedicated radial circuit requirements, and the importance of using the correct cable type inside the sauna room. Worth watching before you speak to an electrician so you can have an informed conversation about what the job involves.

1. Check your sauna heater's electrical requirements

Before calling an electrician, gather the technical information from your sauna heater or from the manufacturer's data sheet. You need to know:

• The rated wattage (common UK domestic ratings: 3.5 kW, 4.5 kW, 6 kW, 8 kW, 9 kW).
• Whether it is single-phase (230 V, which is what most UK domestic supplies are) or three-phase.
• The recommended fuse or breaker rating from the manufacturer.
• Whether the control unit and the heater element are separate (many modern saunas have a separate wall control box that also needs a supply).

Three-phase heaters above about 9 kW are unusual in a domestic property and may require a discussion with your District Network Operator (DNO) before work can start. Single-phase heaters up to 9 kW are straightforward on a standard 100 A domestic supply.

2. What circuit size does your heater need?

The calculation is simple: divide the wattage by 230 to get full-load current in amps.

• 3.5 kW heater: 15.2 A. Protected by a 16 A or 20 A RCBO, typically 2.5 mm² cable.
• 6 kW heater: 26.1 A. Protected by a 32 A RCBO, typically 6 mm² cable.
• 8 kW heater: 34.8 A. Protected by a 40 A RCBO, typically 6 mm² or 10 mm² cable depending on run length and installation method.
• 9 kW heater: 39.1 A. Protected by a 40 A or 45 A RCBO, typically 10 mm² cable.

The electrician will confirm cable size after calculating voltage drop over the actual cable run. A long run to an outbuilding or garden sauna may need a larger cable than the table above suggests.

3. Cable inside the sauna room must be heat-resistant

This is the part that surprises most people. Standard PVC-insulated and sheathed cable (the grey twin and earth used throughout most of a house) has a maximum permitted conductor temperature of 70°C. Inside a sauna room, the air temperature in the upper zones reaches 80°C to 100°C during use, and the zone directly above the heater can reach 125°C.

Any cable inside the sauna room must be silicone rubber insulated, rated to at least 170°C. This applies to the cable from where it enters the room, along any visible or concealed runs, and to the connection at the heater terminals. Standard cable hidden behind cladding inside a sauna is a fire risk and a code failure on inspection.

Outside the sauna room, in normal temperature areas, standard PVC cable is fine.

4. The Section 703 zone requirements

BS 7671 Section 703 (rooms and cabins containing sauna heaters) defines zones inside the sauna room based on how close the equipment is to the heater:

• Zone A: directly above the heater. Minimum temperature rating for equipment: 125°C.
• Zone B: surrounding Zone A. Minimum temperature rating: 90°C.
• Zone C: the rest of the room interior. Minimum temperature rating: 90°C.

Switches, thermostats, timers and socket outlets must not be installed inside the sauna room. The heater control panel is normally located outside the room or in the changing area. The heater's own built-in thermostat and thermal safety cutout are the exception, but these are part of the heater unit itself and supplied by the manufacturer.

5. The protective device and isolator

The circuit at the consumer unit will be protected by an RCBO (a combined MCB and 30 mA residual current device). The RCBO provides both overload/short circuit protection and protection against earth faults. BS 7671 Section 703 does not strictly mandate RCD protection for the heater circuit in all cases, but fitting an RCBO is standard UK practice and strongly advisable.

A double-pole isolator or switch rated to the circuit current should be fitted outside the sauna room. This allows the heater to be safely isolated for maintenance without having to go to the consumer unit. It also provides a clear off position, which the built-in controller on the heater may not.

6. Outdoor and garden sauna installations

If your sauna is in a garden building, outbuilding or a separate structure in the garden, the cable run from the house to the building needs to be armoured cable (SWA, steel wire armoured). SWA cable buried at the correct depth, or run overhead with the correct spans and fixings, protects the circuit from accidental damage. The depth for buried SWA in a domestic garden is typically 0.5 metres in a protective duct, or 0.65 metres without duct if there is no likelihood of mechanical damage.

The outbuilding will also need its own isolating switch or sub-board to allow the electrical supply to be turned off at the building. This is all part of the same Part P notification as the sauna circuit itself.

7. Part P notification and the completion certificate

In England and Wales, installing a new circuit is notifiable work under Part P of the Building Regulations. Your electrician must either:

• Be registered with a government-authorised competent person scheme (NICEIC, NAPIT, ELECSA, etc.) and self-certify the work. They will notify the local authority on your behalf and issue you an Electrical Installation Certificate (EIC).
• Notify your local authority Building Control before starting work. Building Control will inspect and issue a completion certificate when satisfied.

Keep the certificate safe. You will need it when you sell the property, and it may be required by your home insurer if you ever make a claim relating to the installation.

How much does sauna wiring cost?

A straightforward single-phase sauna circuit with a short cable run within the house typically costs in the range of £250 to £450 for labour and materials, depending on the cable route and whether the consumer unit needs any work. An outbuilding installation with a buried SWA cable run will cost more. The best approach is to get a fixed-price quote once the electrician has seen the consumer unit and the planned cable route.