Why electrical prep matters more than buyers expect

Electric sauna heaters are not 13-amp appliances. A 9kW Finnish-style heater pulls roughly the same load as an electric shower — and runs for longer per session. Plugging one into a garden socket isn't an option; it needs its own circuit, its own protection, and its own isolator. Getting this wrong delays delivery, voids warranty, and in the worst cases creates a serious fire risk.

The good news: the work itself is routine for any qualified domestic electrician. The not-so-good news: it has to be done before the sauna arrives, because we commission the heater during handover and won't energise an uncertified supply. This page is here so you can have the right conversation with your electrician three to four weeks before delivery, not three days.

Heater loads — what your sauna actually draws

Heater output is rated in kilowatts (kW). Higher kW = larger sauna room it can heat, but also a larger electrical supply. Here are the loads for the heaters fitted across our cabin range, plus rough equivalents.

Circuit specification — what your electrician needs to install

A standard 9kW electric outdoor sauna needs:

For 3-phase commercial installations, the same logic applies but balanced across three phases, with the appropriate 3-phase isolator and protection. If your property is single-phase only, almost every domestic sauna we sell will work on single-phase.

Cable sizing & SWA explained

Outdoor circuits use SWA (steel-wire armoured) cable as standard. The "wire armour" is a layer of steel strands wrapped around the conductors inside the outer sheath — it gives mechanical protection if the cable is buried, pierced, or knocked, and it's the standard for any cable run outdoors or through walls to outbuildings.

Cable size depends on three things:

1. Load — bigger heater = bigger cable
2. Run length — longer cable run = bigger size to prevent voltage drop
3. Installation method — buried direct, in conduit, or clipped

As a rough planning guide for a 9kW single-phase sauna heater:

These are planning estimates only — your electrician calculates the actual cable size from the heater rating, run length, and installation method using BS 7671 cable tables. They won't be guessing; they'll have software for it.

The external isolator — non-negotiable

Every outdoor sauna needs a lockable external isolator switch mounted within line of sight of the sauna itself. This is the device used to fully de-energise the heater for maintenance, service calls, end-of-season shutdown, or in any kind of fault situation. Specifically:

• Rated for the circuit current (typically 40A for a 9kW heater)
• IP65 weatherproof if mounted externally (which it usually is)
• Lockable in the OFF position
• Mounted at a height accessible to an adult, typically 1.4–1.6m
• Within sight of the sauna so it can be visually confirmed off before maintenance

Some installers mount the isolator on the outside of the sauna itself; others mount it on a nearby external wall or fence post. Either is fine, provided the line-of-sight and accessibility requirements are met.

Part P, RCD protection & certification

Installing a new dedicated circuit to power an outdoor sauna is notifiable work under Part P of the Building Regulations in England and Wales (separate but similar regulations apply in Scotland and Northern Ireland). This means it must either be:

• Carried out by an electrician registered with a Part P competent person scheme (NICEIC, NAPIT, Stroma, ELECSA, etc.), who self-certifies and notifies Building Control on your behalf, or
• Notified to your local Building Control before the work starts, then inspected and certified by them

Either route, you must end up with an Electrical Installation Certificate (EIC) for the new circuit. Keep this — your sauna warranty depends on it, you'll need it if you ever sell the house, and it's the document that proves the supply is safe.

RCD protection at 30mA is mandatory for the circuit under BS 7671. This is usually provided by an RCBO (combined RCD + MCB) in the consumer unit, which is the cleanest install.

Can your existing consumer unit handle a new sauna circuit?

Three questions to ask your electrician at the survey stage:

1. Is there a spare way (slot) in the consumer unit for a new RCBO? If not, they may need to fit a small sub-board or upgrade the main unit.
2. Is the existing main supply (the cable from the meter) rated for the additional load? Most modern UK homes have a 100A main fuse, which comfortably handles a sauna alongside everything else. Older properties or those with multiple high-load items (EV charger, electric shower, electric hob, sauna) sometimes need a load assessment.
3. Is the consumer unit itself up to current standards? Pre-2008 plastic consumer units with rewireable fuses, or units without RCD protection, will typically need replacing to add a Part P-compliant new circuit.

None of these are dealbreakers. They're things to find out at the survey, not on the day the sauna arrives.

If you're choosing a wood-burning heater instead

Wood-burning Finnish sauna heaters (the Harvia Pro, Harvia 20 Pro, and Harvia Legend fitted to our cabin range) do not require an electrical supply for the heater itself. The flue draws naturally, the firebox is loaded manually, and there's no thermostat or control panel to power.

However, you may still want a small electrical supply to the sauna for:

• Interior LED lighting (low-voltage, often included with the sauna)
• An external sauna light or path light
• Power for sound systems, mobile chargers, or accessories
• An exterior socket for grounds maintenance

For these "lighting only" loads, a small 16A or 20A circuit on 2.5mm² SWA is usually sufficient — much cheaper and faster to install than a heater circuit. If you genuinely won't ever convert to electric, you may skip electrical entirely. Most buyers run a small supply anyway "just in case" because retrofitting later is more expensive than including it upfront.

Lights, controls, and the Wi-Fi heater question

A few extras worth raising with your electrician at the same survey:

• Interior sauna lights — usually low-voltage, low-load, often on the same supply as the heater with appropriate fusing
• Exterior lights — separate switched circuit for accent lighting around the sauna, on path lights, or for safety
• Wi-Fi remote control for the heater — the Harvia Cilindro PC90 supports an optional Wi-Fi module (around £400) that lets you preheat from your phone. This is a control wire only, not extra power, but the module needs mounting and bonding
• Smart home integration — if you want the sauna circuit on a smart sub-meter or load monitor, mention it now and they'll allow for it in the consumer unit layout

Cost & timeline expectations

Costs vary widely by region, property layout, and how much consumer-unit work is needed. As a planning guide for a straightforward UK domestic install with a 20–30m cable run from consumer unit to sauna:

• Materials (cable, RCBO, isolator, accessories): typically £150–£400
• Labour (1–2 days for most installs): typically £400–£800
• Consumer unit upgrade if needed: additional £400–£800
• Building Control notification (if not via Part P scheme): typically £100–£300

For complex installs — long buried cable runs, listed buildings, full consumer-unit replacement, or commercial 3-phase supplies — costs are higher and worth getting two quotes.

Timeline: Get your electrician quoting as soon as you order the sauna. From quote to certified install is usually 2–4 weeks. Don't leave it until the week before delivery — at peak season (summer) electricians get booked up.

How to brief your electrician (copy-paste-friendly)

Forward this paragraph to your electrician when getting a quote — it gives them everything they need to scope the job accurately.

If they ask which sauna it is, give them the product link from the Nuovo Luxury site — every product page lists the standard heater specification.

Frequently asked questions