Electrical Connection for Saunas and Hot Tubs
An electric sauna, an infrared sauna and a hot tub with electric heating all need a correctly sized electrical supply before delivery. This page explains which voltage, circuit breaker, residual current device (RCD) and cable to choose according to the output of the equipment, how to route the supply to an outdoor sauna or hot tub, and what must be ready on the day of commissioning. You can forward this page directly to your electrician.
The values given are based on current calculations, heater manufacturers’ instructions and the relevant standards. For a specific installation, the instructions of the chosen heater manufacturer and the design of a qualified electrician, who will also assess the condition of your existing installation, are decisive.
Who prepares the electrical connection
The supply from the main distribution board to the location of the sauna or hot tub is provided by the customer before delivery, through an electrician with the appropriate qualification. The electrician designs the protection and the cable, installs the cable route and the protective devices in the distribution board, and hands over the new part of the installation with an inspection and test certificate.
During commissioning we connect the sauna to the prepared supply, start and test the heater, explain how to operate it and carry out an overall check of the sauna on site. Commissioning is free of charge, see Delivery and Installation.
Overview: which supply for which equipment
| Equipment | Supply | Current per phase | Circuit breaker (type B) | Minimum cable cross-section |
|---|---|---|---|---|
| Sauna with 6 kW electric heater | 400 V, three-phase | 8.7 A | 3 × 10 A | 5 × 1.5 mm² |
| Sauna with 9 kW electric heater | 400 V, three-phase | 13.0 A | 3 × 16 A | 5 × 2.5 mm² |
| Sauna with 6 kW heater, single-phase version | 230 V, single-phase | 26.1 A | 1 × 32 to 35 A according to the heater instructions | 3 × 6 mm² |
| Sauna with 9 kW heater, single-phase version | 230 V, single-phase | 39.1 A | 1 × 50 A | 3 × 10 mm² |
| Hot tub with 3 kW electric water heater | 230 V, single-phase | 13.0 A heater only | 16 A | 3 × 2.5 mm² |
| Hot tub with 6 kW electric water heater | 400 V, three-phase | 8.7 A heater only | 3 × 16 A | 5 × 2.5 mm² |
| Infrared sauna 1.3 to 2.1 kW | 230 V, single-phase | up to 9.2 A | 10 A, earthed socket | according to the socket circuit |
The minimum cross-section applies to short runs inside the house. For a supply to an outdoor sauna or hot tub laid underground across the garden, we recommend a larger cross-section, see Cable and cable route. For a hot tub, the consumption of the circulation and massage pumps, filtration and lighting is added to the heater. We will confirm the exact values for your configuration before delivery.
Single-phase or three-phase connection
Sauna heaters and water heaters are resistive loads, so their current can be calculated simply from their output:
- 230 V, single-phase: current = output ÷ 230 V. A 6 kW heater draws 6,000 ÷ 230 = 26.1 A, a 9 kW heater 39.1 A.
- 400 V, three-phase: current per phase = output ÷ (1.732 × 400 V). A 6 kW heater draws 8.7 A per phase, a 9 kW heater 13.0 A per phase.
A three-phase connection spreads the load evenly across three phases. The conductors carry less current, a smaller circuit breaker and cable are sufficient and the household supply remains balanced. For 6 kW and 9 kW sauna heaters we therefore recommend a three-phase 400 V connection. The Harvia Top Steel heaters we supply are connected three-phase at 400 V. Harvia Vega heaters can be connected either three-phase or single-phase at 230 V.
Capacity of the main fuse
Before choosing a heater, check the rating of the main fuse ahead of the electricity meter. A single-phase 6 kW heater (26.1 A) would on its own exceed a 25 A main fuse on one phase, and a single-phase 9 kW heater (39.1 A) would exceed it even more. A three-phase 9 kW heater draws 13 A per phase, which is normally manageable with a 3 × 20 A or 3 × 25 A main fuse. With a 3 × 16 A main fuse, very little capacity would remain for the other appliances in the house. An increase in supply capacity is requested from your distribution network operator, so arrange it well in advance.
Circuit protection and residual current device
Dedicated circuit with circuit breaker
The sauna and a hot tub with electric heating each have their own circuit from the main distribution board, with no other appliances connected to it. The circuit is protected by a type B circuit breaker rated according to the table above and the instructions of the heater manufacturer.
RCD in the sauna
Under IEC 60364-7-703 and its national adoptions for rooms containing sauna heaters, all sauna circuits, such as lighting, sockets and other equipment, must be protected by an RCD with a rated residual operating current not exceeding 30 mA. The sauna heater itself is exempt from this requirement. Whether the heater is connected via an RCD is determined by its manufacturer’s instructions. Harvia, the manufacturer of our heaters, states that an RCD should not be installed in the heater circuit, as the heater is protected by earthing.
There is also a practical reason. The heating elements of the heater are filled with magnesium oxide insulation, which absorbs moisture from the air during longer periods out of use in a damp environment. When the heater is switched on, a small leakage current flows to the protective conductor, which can trip an RCD even though the heater is not damaged. After the elements have dried out briefly, the leakage current decreases.
RCD for the hot tub
A hot tub with electric heating, hydromassage, filtration or lighting is electrical equipment in direct contact with water. Its circuit must be protected by a dedicated RCD with a rated residual operating current not exceeding 30 mA. We recommend type A. A dedicated RCD prevents leakage currents from other appliances in the house adding up and causing unnecessary tripping.
Protective equipotential bonding
Conductive metal parts near the sauna and hot tub, such as metal frames, stainless steel casings or structures, are connected to the protective equipotential bonding with a copper conductor sized according to the electrician’s design, typically 4 to 6 mm².
Cable and cable route
Supply to an outdoor sauna or hot tub
A copper cable, for example NYY-J or CYKY-J, is used from the distribution board to the sauna. On a longer underground run, the current is not the only factor: voltage drop and the conditions for reliable disconnection of the circuit breaker in the event of a fault also matter. A larger cross-section reduces losses in the cable and leaves capacity in case you later want to connect a hot tub with electric heating or other equipment.
| Length of run (9 kW heater, 400 V) | 5 × 2.5 mm² | 5 × 4 mm² | 5 × 6 mm² |
|---|---|---|---|
| 20 m | 3.2 V (0.8 %) | 2.0 V (0.5 %) | 1.3 V (0.3 %) |
| 40 m | 6.4 V (1.6 %) | 4.0 V (1.0 %) | 2.7 V (0.7 %) |
| 60 m | 9.6 V (2.4 %) | 6.0 V (1.5 %) | 4.0 V (1.0 %) |
The table shows the voltage drop on the supply at the full output of a 9 kW heater. For final circuits, a maximum of 3 to 5 % is permitted. For the supply to an outdoor sauna we recommend:
- 5 × 4 mm² for runs up to approximately 40 m,
- 5 × 6 mm² for runs over 40 m,
- for a single-phase connection or runs over 60 m, the electrician determines the cross-section by calculation.
Laying the cable underground
- Run the cable in a protective duct at least 700 mm below ground, in a sand bed and with warning tape above the cable.
- Route it away from places where digging is likely, such as planned trees or fence posts, and record the route on a plan.
- Leave a cable reserve of at least 3 m at the sauna or hot tub and protect the cable end from moisture.
- Lay the electrical duct together with the other utility connections before the foundation is finished. See Foundations for Saunas and Hot Tubs.
Wiring inside the sauna
Cables with PVC insulation are not used in the hot zone of the sauna. At temperatures above 70 °C they become brittle and lose their insulating properties. The connection from the junction box to the heater is made with SiHF silicone cable rated up to 180 °C. The junction box is placed low, no more than 50 cm above the floor, where temperatures are considerably lower. Electrical equipment in the sauna has a protection rating of at least IPX4, and conductors more than 1 m above the floor must withstand at least 170 °C.
Electrical connection for hot tubs
We offer hot tubs with a wood-burning heater or with an electric water heater of 3 kW or 6 kW. The electric heater warms the water flowing through it in the circulation circuit of the tub.
- 3 kW heater: usually single-phase 230 V, dedicated circuit with a 16 A circuit breaker.
- 6 kW heater: three-phase 400 V, dedicated circuit with a 3 × 16 A circuit breaker.
- Hydromassage, filtration and lighting increase the total consumption. The supply is sized for the sum of all loads of the tub.
- RCD not exceeding 30 mA, type A recommended, for the hot tub only.
- Supply termination next to the tub in a junction box suitable for outdoor use, at least IP44, out of reach of splashing water.
A tub with a wood-burning heater only, without hydromassage, filtration or lighting, does not need an electrical connection. If you plan to keep the tub tempered in winter, the supply must be reliable and independent of other appliances, as a tripped circuit breaker or a longer power cut in freezing conditions can damage the circulation circuit.
Connection for infrared saunas
Infrared saunas have a power input of approximately 1.3 to 2.1 kW depending on the model and are connected at 230 V to an earthed socket. The socket circuit should be protected by a 10 A circuit breaker and should not supply other high-power appliances at the same time. Do not connect the infrared sauna via an extension lead or multi-socket adapter. An outdoor infrared sauna needs a supply brought to its location in the same way as a Finnish sauna, with a smaller cross-section according to the electrician’s design.
Sauna with a wood-burning heater
A sauna with a wood-burning heater does not need a power supply for the heater. Electricity is only needed for lighting and other electrical equipment, for which a single-phase 230 V circuit protected by a 30 mA RCD is sufficient. If you are considering switching to an electric heater later, lay a 5 × 4 mm² or 5 × 6 mm² cable now. Digging a second trench later costs considerably more than a larger cable cross-section.
Checklist for your electrician
Before delivery of the sauna or hot tub, the following should be in place:
- The output and supply of the chosen heater or water heater are known from the order confirmation, and the protection is designed accordingly.
- The main fuse ahead of the meter has sufficient capacity, or an increase has been requested.
- The distribution board has a dedicated circuit with a type B circuit breaker.
- The sauna circuits are protected by an RCD of up to 30 mA, the heater according to its manufacturer’s instructions. The hot tub has a dedicated RCD of up to 30 mA.
- A supply cable of suitable cross-section is laid in a duct at least 700 mm deep and brought to the installation location with a reserve of at least 3 m.
- Conductive metal parts are connected to the protective equipotential bonding.
- An inspection and test certificate has been issued for the new part of the electrical installation.
During commissioning we connect the sauna and hot tub to the prepared and certified supply. Without it, the heater or heating cannot be started and tested.
Any questions?
If you are not sure which supply to choose for a particular model, write to us before work begins. Send us the rating of your main fuse, the distance from the distribution board to the installation location and the chosen heater or heating, and we will help you size the supply. Contact us.
