Home sauna electrical requirements: what supply does a sauna heater need?

A home sauna’s electrical requirement is set almost entirely by heater size. Heaters up to around 3 kW — which covers most infrared cabins and the smallest traditional heaters — will run on a standard 13A socket. Anything from 4.5 kW upwards needs a dedicated circuit run from the consumer unit, correctly sized, RCD or RCBO protected, with an isolator outside the cabin. In England and Wales that new circuit is notifiable work under Part P and must be certified.

The sequence matters: size the cabin, size the heater to the cabin, then size the supply to the heater. Doing it the other way round — buying the sauna and hoping the existing wiring copes — is how people end up with a £1,500 consumer unit upgrade they had not budgeted for.

Step one: heater size from cabin volume

The working rule of thumb for a well-insulated cabin is 1 kW of heater per cubic metre of internal volume. Internal volume, not external footprint.

That figure assumes proper sauna construction: 50–70 mm insulation, a foil vapour barrier and timber lining. Adjust upwards for:

• Glass. Count roughly 1.2 m³ of extra volume per square metre of glazing. A full glass front can add the equivalent of 3–4 m³ on its own.
• Uninsulated or masonry walls. A sauna built against a tiled or brick wall can need the equivalent of 1.2–2 m³ extra per square metre of that surface.
• Outdoor cabins in a UK winter. Exposed cabins lose heat faster than an indoor cabin sitting in a heated house.

Undersizing means a cabin that takes 90 minutes to reach temperature and never quite holds it. Oversizing wastes money on both the heater and the circuit, and — as our running costs page shows — on every session afterwards. Our sizes and occupancy guide works through cabin volume properly.

Step two: supply requirements by heater size

Heater	Approx. current (230V, 1φ)	Supply	Typical circuit
Up to 3 kW (most infrared)	~13A	13A plug or dedicated radial	2.5 mm², 16A or 20A MCB/RCBO
4.5 kW	~20A	Dedicated circuit	4 mm², 25A RCBO
6 kW	~26A	Dedicated circuit	6 mm², 32A RCBO
8 kW	~35A	Dedicated circuit	10 mm², 40A RCBO
9 kW	~39A	Dedicated circuit, or 3-phase where available	10 mm², 40–45A RCBO
10.5 kW and above	45A+	Usually three-phase	Specified by the electrician

These figures are indicative. Actual cable size is determined by the installer from the design current, the installation method, the run length, grouping, ambient temperature and the earth fault loop impedance — not from a table. A 25 m SWA run buried in the garden to an outdoor cabin behaves differently from a 6 m run in a loft, and voltage drop over a long run frequently pushes the cable up a size. Long garden runs to an 8 kW or 9 kW heater regularly end up in 10 mm² or 16 mm².

Three-phase supplies are common in commercial premises and rare in UK homes. Most domestic properties have a single-phase 60A, 80A or 100A service head, so a 9 kW heater is usually the practical single-phase ceiling in a house.

Step three: is there capacity in the consumer unit?

Two separate questions, both worth asking before ordering:

1. Physical spare ways. A dedicated circuit needs a free way in the consumer unit. If there isn’t one, you are looking at a sub-board or a board change.
2. Load capacity. A 9 kW sauna heater on an 80A supply alongside an electric shower, an oven and an EV charger can exceed the incoming capacity. A competent installer applies diversity under the wiring regulations, and may recommend load management or a smaller heater. This is also where an 8 kW heater on a smaller, better-insulated cabin beats a 9 kW heater on a big draughty one.

Older boards are the common surprise. A split-load board with rewireable fuses or no RCD protection at all will usually need replacing before a new sauna circuit can be added, and that is a several-hundred-pound job in its own right.

Protection, isolation and what goes inside the hot room

RCD or RCBO protection. Sauna circuits require additional protection by a 30 mA RCD. Most installers now use an individual RCBO per circuit rather than a shared RCD, so a fault in the sauna does not take out the rest of the house.

A double-pole isolator outside the cabin. Positioned by the door, in easy reach, so the heater can be isolated without entering a hot room. Isolators, switches and control panels are not rated for 90–110°C air temperature and must never be mounted inside.

Inside the cabin, only three things belong on the electrical side: the heater and its earthed enclosure, the temperature sensor, and IP-rated luminaires. Cable inside the hot room must be high-temperature rated — silicone-insulated cable is the norm — and standard PVC twin-and-earth is not acceptable in that environment because the insulation degrades.

Lighting. IP-rated, heat-tolerant, and low voltage in many designs. Fittings are usually placed under a bench or behind a shield rather than directly above the stones.

No socket outlets inside the cabin. Ever.

Outdoor cabins: SWA and the garden run

A garden sauna needs a permanent supply from the house, and that means steel wire armoured (SWA) cable, correctly buried at an appropriate depth with warning tape, or clipped along a suitable route. The armour is mechanically protective and can serve as the circuit protective conductor where the installer designs it that way.

The practical points that come up on outdoor installs:

• Run length drives cost more than heater size. A 30 m trench across a lawn is a day’s work and often the largest single line on the quote.
• Glanding and terminations. SWA needs the right glands at both ends, in enclosures rated for outdoor use.
• An outdoor enclosure or sub-board at the cabin end for the isolator, sometimes with a small distribution board if you are also running lighting or a cold plunge chiller.
• Earthing arrangement. TT, TN-S and TN-C-S supplies are treated differently for outdoor circuits, and the installer will confirm which applies.

Part P, certification and why it matters at resale

Installing a new circuit in a dwelling in England or Wales is notifiable under Part P of the Building Regulations. There are two compliant routes: the work is done by an electrician registered with a competent person scheme who self-certifies and notifies the local authority, or it is notified to building control before work begins and inspected.

Either way you should end up with an Electrical Installation Certificate and, via the scheme, a Building Regulations compliance certificate. Keep both. When you sell, the buyer’s solicitor asks for evidence of compliance for any work of this type; not having it typically means a retrospective regularisation application, an indemnity policy, or a price reduction. It is also what an insurer will want to see after any incident.

Scotland operates a building warrant system rather than Part P, and Northern Ireland has its own regulations — the certification expectation is similar but the route differs. Planning is a separate matter again, covered on our garden sauna planning permission page.

Getting a straight quote

When you ask installers to price the electrical side, give them: heater kW and manufacturer, cabin location, approximate cable run from the consumer unit, a photo of the consumer unit, and whether the run is indoor or buried outdoor. That is enough for a realistic figure rather than a placeholder.

Our fit checker puts a first estimate on cabin volume, heater size and supply type from your measurements, and the sauna installation guide covers what the rest of the job involves. A registered electrician makes the final determination on circuit design, cable size and protective devices — the figures here are for planning your budget, not for specifying an installation.