Sauna Subpanel Installation: What Your Electrician Needs to Know

Installing an electric sauna heater is one of the most rewarding home wellness upgrades you can make — but it is also one of the most electrically demanding. Unlike plugging in a space heater or wiring a ceiling fan, a traditional sauna heater typically draws 30 to 60 amps of continuous 240-volt power. That kind of load demands careful planning, and in many residential installations, the best solution is a dedicated electrical subpanel.

This guide is written for homeowners who want to give their electrician the right information upfront — and for electricians who may be wiring their first sauna project. Whether you are building a DIY sauna room in your basement or installing an outdoor sauna in your backyard, the electrical work must be done correctly for safety, code compliance, and long-term performance.

What Is a Sauna Subpanel and Why Would You Need One?

A subpanel (also called a sub-panel or auxiliary panel) is a smaller electrical distribution panel that receives power from your home's main service panel via a feeder circuit. It contains its own set of breaker slots, allowing you to distribute multiple circuits from a single location closer to the equipment being served.

In sauna installations, a subpanel serves several important purposes. First, it moves circuit protection closer to the sauna, which reduces wire run lengths and associated voltage drop. Second, it provides a convenient location for a disconnect — something many jurisdictions require within sight of the equipment. Third, it creates room for multiple circuits when your sauna needs more than just the heater powered. A typical sauna installation may require separate circuits for the heater, interior lighting, an exterior receptacle, a ventilation fan, and possibly infrared heater panels in a hybrid setup.

Not every sauna installation requires a subpanel. If your main panel has available breaker slots, has sufficient amperage capacity, and is located reasonably close to the sauna, your electrician may be able to run a dedicated circuit directly. However, the following situations strongly favor installing a subpanel:

• Your main panel is full or nearly full with no open breaker slots
• The sauna is located far from the main panel (50 feet or more), such as in a detached garage, backyard structure, or separate building
• Your sauna requires multiple circuits (heater, lighting, fan, receptacles)
• Your local Authority Having Jurisdiction (AHJ) requires a visible disconnect near the sauna
• You want to future-proof the installation for potential upgrades like a higher-kW heater or additional accessories
• You are installing the sauna in a detached structure, which per NEC Article 225.30 generally requires a single feeder to serve the building — meaning a subpanel is the correct approach

Assessing Your Home's Electrical Capacity

Before any wiring begins, your electrician needs to perform a load calculation on your existing electrical service. This determines whether your home can handle the additional draw of a sauna without requiring a full service upgrade.

A standard residential electric sauna heater in the 4.5kW to 9kW range will draw between 19 and 37.5 amps at 240 volts. Larger heaters — 10kW to 18kW, common in bigger sauna rooms and commercial applications — can draw 42 to 75 amps. Under the National Electrical Code (NEC), sauna heaters are typically treated as continuous loads because they operate at maximum current for three or more hours. This means the circuit must be sized at 125% of the heater's rated current.

Here is what your electrician should evaluate during the initial assessment:

• Main service panel rating: Is your home on 100A, 150A, or 200A service? Most modern homes with 200A service can comfortably support a 30–50A sauna circuit. Older homes with 100A service may need a panel or service upgrade first.
• Existing load calculation: Account for all major loads — HVAC, electric range, water heater, clothes dryer, EV charger, hot tub, pool pump, and any other high-draw appliances.
• Available breaker slots: Count the open positions in the main panel. A 240V circuit requires two adjacent slots for a double-pole breaker.
• Wire run distance: Measure the total cable length from the main panel (or proposed subpanel location) to the sauna heater. Runs exceeding 50 feet may require upsizing the wire gauge to compensate for voltage drop.

If your electrician determines that your panel cannot support the additional load, the solution may range from adding a subpanel fed by an appropriately sized feeder breaker to upgrading your entire electrical service. In either case, this assessment should happen early — ideally before you purchase your sauna or heater. Use our Sauna Heater Size Calculator to determine the correct heater kilowatt rating for your room, which directly determines the electrical requirements your electrician will need to plan for.

Subpanel Sizing: Getting the Amperage Right

The subpanel must be sized to handle the total connected load it will serve, with appropriate headroom for the continuous-load multiplier. Here is a practical sizing framework your electrician can use:

For a small to mid-size sauna (4.5kW to 8kW heater), the heater circuit alone will require 30 to 40 amps. Add a 20A lighting and receptacle circuit, and total demand comes to roughly 50–60A. A 60A subpanel with a 60A feeder breaker is typically sufficient.

For a larger sauna (9kW to 12kW heater), the heater may demand 40 to 50 amps. With auxiliary circuits, total demand can reach 60–70A. A 100A subpanel provides comfortable capacity and room for future expansion.

For commercial or large residential installations (12kW to 18kW or multiple heaters), demand can exceed 75A for the heater alone. A 100A or 125A subpanel is appropriate, and the feeder circuit from the main panel must be sized accordingly with properly rated conductors.

Remember the NEC continuous-load rule: since sauna heaters run continuously for extended sessions, the breaker protecting the heater circuit must be rated at 125% of the heater's full-load current. For example, a 9kW heater drawing 37.5A at 240V requires a minimum 47A breaker — which in practice means a 50A breaker. The wire gauge must also match: 6 AWG copper for 50A, 8 AWG for 40A, and 10 AWG for 30A circuits, with adjustments for long runs.

Wire Gauge, Conductor Type, and Run Length

Proper conductor selection is critical for both safety and heater performance. Undersized wiring leads to voltage drop, which causes the heater to underperform, run hotter than designed, and potentially void your warranty. Here is a reference guide for common residential sauna circuits:

Note: These are general guidelines based on copper conductors. Always verify against the manufacturer's installation manual and local code requirements. Wire gauge must be adjusted for insulation type, ambient temperature, conduit fill, and actual measured run length.

For conductor type, use NM-B (Romex) cable for interior residential runs through walls and ceilings. For runs through conduit — especially in exposed, outdoor, or underground applications — use individual THHN/THWN conductors rated for moisture resistance. Outdoor and underground runs for outdoor sauna installations require direct-burial-rated cable (such as UF-B) or THWN conductors inside Schedule 40 PVC conduit buried to the NEC-required depth, typically 18 inches minimum for 240V circuits.

One critical detail: most sauna heater manufacturers explicitly require copper conductors only. Aluminum wiring is generally not acceptable for sauna heater circuits. Confirm this in the manufacturer's installation manual before specifying materials.

Subpanel Installation: Step-by-Step Overview

While this is not a DIY guide — sauna electrical work must be performed by a licensed electrician — understanding the process helps homeowners communicate effectively with their contractor and know what to expect.

Step 1: Choose the subpanel location. The subpanel should be installed in a dry, accessible location as close to the sauna as practical. For indoor saunas, this might be in an adjacent utility room, hallway, or the room just outside the sauna. For outdoor saunas, a weather-rated panel enclosure mounted on the exterior of the sauna building or an adjacent structure is common. The panel must not be installed inside the sauna room itself due to the extreme heat and moisture.

Step 2: Run the feeder circuit. A feeder cable is run from the main panel to the subpanel location. The feeder breaker in the main panel is sized to match the subpanel's rating (for example, a 60A double-pole breaker for a 60A subpanel). The feeder cable typically consists of two hot conductors, one neutral, and one ground. Important NEC requirement: in a subpanel, the neutral bus and ground bus must be separated (not bonded together as they are in the main panel). A separate grounding electrode conductor may also be required, depending on the installation type and local code.

Step 3: Install the subpanel and branch circuit breakers. Mount the subpanel, terminate the feeder conductors, and install the branch circuit breakers for the sauna heater, lighting, receptacles, and any other loads. The heater circuit gets a dedicated double-pole breaker sized per the manufacturer's specifications and the 125% continuous-load rule.

Step 4: Run branch circuits to the sauna. Individual circuits are run from the subpanel to the heater junction box or contactor, interior lighting, exterior receptacles, and any other equipment. All wiring entering the sauna room must be routed away from high-heat zones, with cables and conduit kept clear of the heater and sauna stones. Many electricians route wiring below the bench level and through the wall behind or below the heater.

Step 5: Install the disconnect. Many local codes and most manufacturer manuals require an equipment disconnect within sight of the sauna heater. This can be a pull-out or switch-type disconnect, or in some jurisdictions, the subpanel itself may serve as the disconnect if it is within the line of sight. Always confirm with your local AHJ.

Step 6: Inspection and testing. Before energizing the system, a licensed inspector should review the installation for code compliance. Your electrician should verify voltage at the heater under load to confirm there is no excessive voltage drop — many manufacturers will not honor warranty claims if voltage at the heater falls below their specified minimum.

GFCI Protection: A Nuanced Topic for Saunas

Ground Fault Circuit Interrupter (GFCI) protection is one of the most debated topics in sauna electrical installation, and it is an area where your electrician needs to understand the specific requirements rather than defaulting to standard practice.

The NEC generally requires GFCI protection for circuits in damp or wet locations — which can include basements, garages, and outdoor installations where saunas are commonly placed. However, many sauna heater manufacturers explicitly state in their installation manuals that the heater circuit should not be connected through a GFCI breaker. The reason is practical: when water is poured over heated stones (a normal part of traditional sauna use), steam and moisture can create minor conduction paths that cause GFCI breakers to nuisance-trip, shutting down the heater mid-session.

NEC Article 110.3(B) requires that listed equipment be installed according to the manufacturer's instructions. When a manufacturer's manual explicitly prohibits GFCI on the heater circuit, most inspectors will defer to that instruction — but only if the documentation is available. This is one reason why purchasing heaters from reputable, UL-listed brands like Harvia, HUUM, or Saunum matters — their documentation and compliance records are thorough.

The practical solution when a subpanel is involved: install the sauna heater on a non-GFCI breaker per the manufacturer's instructions, and put other circuits (lighting, receptacles) on GFCI-protected breakers as required by code. This gives you the best of both worlds — shock protection where it is appropriate and reliable heater operation without nuisance tripping. Your electrician should always document the manufacturer's instruction and keep it accessible for the inspector. For a deeper dive into how these rules vary by location, see our guide on sauna electrical code by state.

Special Considerations for Outdoor and Detached Sauna Buildings

Outdoor saunas — including barrel saunas, cabin saunas, and custom-built backyard sauna buildings — introduce additional electrical requirements that go beyond a simple interior circuit run.

Underground feeder runs: When power must travel from the house to a detached sauna structure, the wiring typically goes underground. NEC requirements for burial depth vary based on the wiring method: direct-burial UF-B cable requires a minimum 24-inch trench depth for 240V circuits. THWN conductors in Schedule 40 PVC conduit require 18 inches minimum. Rigid metal conduit (RMC) can be as shallow as 6 inches in some applications. Your electrician should verify local requirements, as some jurisdictions exceed NEC minimums.

Separate grounding electrode: Detached buildings served by a feeder typically require their own grounding electrode system — usually two ground rods driven at least 6 feet apart and bonded together. This is in addition to the equipment grounding conductor in the feeder cable. The subpanel in the detached structure must have separate neutral and ground buses.

Weatherproof enclosures and conduit: All exterior electrical components — the subpanel, disconnect, junction boxes, and receptacles — must be rated for outdoor use. Use weatherproof (WP or "in-use") covers on receptacles and NEMA 3R or better rated enclosures for the subpanel and disconnect.

NEC Article 225.30 compliance: This article generally limits a detached building to a single feeder or branch circuit from the main structure. There are exceptions (notably 225.30(D) for "different uses"), but the cleanest approach for a detached sauna is a single feeder to a subpanel in the sauna building, with all branch circuits originating from that subpanel. This avoids code interpretation issues and passes inspection smoothly.

If you are planning an outdoor installation and need help selecting the right equipment, explore our outdoor sauna collection or request a custom sauna design and quote for installations that require specific planning.

Control Panel and Contactor Box Placement

Most 240V sauna heaters from brands like Harvia, HUUM, and Saunum use either a built-in control unit or an external control panel (sometimes called a contactor box or relay box). Where these components are located matters for both safety and code compliance.

External control panels must be installed outside the sauna room in a dry, protected location. The heat and moisture inside a sauna will damage sensitive electronics, void the warranty, and create a safety hazard. Many electricians mount the control panel just outside the sauna door, in an adjacent hallway, or in a nearby utility space.

If the contactor box or relay will be mounted inside the sauna room (some heater models allow this), it should be positioned away from the heater, typically under a bench. For saunas with a standard 7-foot ceiling, the contactor box should be mounted between 12 and 28 inches from the floor. The electrician must ensure that the bench structure does not block access to the contactor for maintenance or inspection.

Heaters with WiFi-enabled digital controllers often require a low-voltage connection between the control unit and the heater. This low-voltage wiring should be run separately from the high-voltage power conductors to prevent interference. Consult the heater's installation manual for specific routing instructions.

Permits, Inspections, and Why They Matter

In most jurisdictions, installing a new 240V circuit — whether from the main panel or through a subpanel — requires an electrical permit and subsequent inspection. This is not bureaucratic red tape; it is a critical safety checkpoint. Common inspection failure points for sauna installations include:

• Incorrect wire gauge for the circuit amperage or run length
• Missing or improperly installed equipment disconnect
• Improper grounding or bonding, especially in detached structures
• GFCI protection issues — either missing where required or present on a heater circuit where the manufacturer prohibits it
• Insufficient burial depth for underground feeder runs
• Neutral and ground buses bonded together in a subpanel (they must be separate)
• Wiring routed through high-heat zones inside the sauna room

A qualified electrician who has experience with sauna installations will anticipate these issues and ensure the work passes inspection on the first visit. If you need help finding someone experienced, our Sauna Heater Electrician Database connects homeowners with licensed professionals who specialize in sauna electrical work.

What to Tell Your Electrician Before They Start

Communication between homeowners and electricians is where many sauna installations either succeed or hit unnecessary delays. Here is a checklist of information you should have ready before your electrician's first site visit:

• Heater make and model: This determines voltage, amperage, wire gauge, breaker size, and whether GFCI is permitted on the heater circuit. If you have not selected a heater yet, our guide to the best electric sauna heaters and the heater sizing calculator can help you narrow down the right model.
• Manufacturer's installation manual: This is the single most important document for the electrician. It specifies exact electrical requirements, clearances, wiring entry points, and any restrictions (such as GFCI prohibitions). Every heater sold through Haven of Heat includes manufacturer documentation.
• Sauna location and distance from main panel: Indoor basement sauna 20 feet from the panel? Simple. Detached backyard building 120 feet away? That changes everything — wire gauge, conduit type, trenching, grounding, and likely a subpanel.
• Additional circuits needed: Interior lighting, exterior receptacle, ventilation fan, audio system, or supplemental infrared panels all require their own circuits and must be planned upfront.
• Timeline and inspection requirements: If you need the installation completed before the sauna is delivered, coordinate scheduling with both the electrician and your sauna vendor.

Cost Expectations for Sauna Electrical Work

Electrical costs for sauna installation vary significantly based on the scope of work, geographic location, and existing infrastructure. Here are general ranges to help set expectations:

A simple dedicated circuit from an existing panel with available capacity, run less than 30 feet to an indoor sauna, typically costs between $400 and $800 for labor and materials.

A subpanel installation with a feeder circuit from the main panel, multiple branch circuits, and a disconnect runs between $1,000 and $2,500 depending on complexity and local labor rates.

An outdoor or detached building installation that includes underground conduit, trenching, a subpanel, separate grounding electrode, and weatherproof enclosures can range from $2,000 to $5,000 or more.

A full service or panel upgrade (from 100A to 200A, for example) adds $2,000 to $4,000 on top of the sauna-specific wiring.

These are estimates, and your actual costs will depend on your specific situation. Always get multiple written quotes. For more detailed cost breakdowns, see our guide on typical sauna installation costs. You can also use our Sauna Electrical Cost Calculator to estimate ongoing operating costs once your sauna is up and running.

120V vs. 240V: When a Subpanel Is Not Necessary

Not every sauna requires a 240V hardwired circuit or a subpanel. Smaller infrared saunas — particularly 1- to 2-person models — often operate on a standard 120V, 15A or 20A household circuit. These saunas plug into a dedicated outlet and do not require electrical panel modifications in most homes.

However, there is a critical size limitation. 120V plug-in sauna heaters are only suitable for very small sauna rooms — typically up to about 88 cubic feet (roughly a 3'×3'×6' to 4'×4'×6' room). For any sauna larger than this, a 240V heater is necessary for adequate heating performance, and that means a dedicated circuit (and potentially a subpanel) is part of the equation.

If you are still deciding between sauna types and want to understand the full picture of what electrical and plumbing work your project will require, our guide on electrical and plumbing requirements for saunas breaks it down by sauna type.

Future-Proofing Your Installation

One of the smartest things your electrician can do is plan for where your sauna setup might go in the future — not just where it is today. Here are a few ways to future-proof:

• Oversize the subpanel slightly. A 100A subpanel costs marginally more than a 60A panel but gives you room to add circuits later without starting from scratch.
• Run conduit even if you are using NM-B cable. Conduit makes future wire pulls easier and protects against physical damage.
• Size the feeder for a potential heater upgrade. If you are installing a 6kW heater now but your sauna room could support an 8kW or 9kW heater in the future, sizing the feeder and branch circuit for the larger load now saves a complete rewire later.
• Add an extra circuit or two to the subpanel. A spare 20A circuit for future accessories — a cold plunge pump, exterior lighting, a sound system — costs almost nothing to add during initial installation and can save hundreds later.

Choosing the right heater from the start helps avoid costly re-work. Our sauna heater sizing chart and calculator walks you through exactly how to match heater output to your room dimensions and construction materials. And if you need guidance on the overall sauna build, our free sauna layout drawings and designs provide floor plans for a wide range of room sizes.

Common Mistakes Electricians Make on Sauna Projects

Even experienced electricians can run into trouble on sauna installations if they are unfamiliar with the specific requirements. Here are the most common mistakes to avoid:

Using the wrong wire gauge for the run length. A 10 AWG circuit that is adequate for a 30-foot run to a 6kW heater may be undersized for a 75-foot run to the same heater. Voltage drop over long distances is a real performance issue, not just a code technicality. The heater will heat slowly, strain its elements, and the manufacturer may deny warranty claims if voltage at the heater is below spec.

Installing GFCI protection against manufacturer instructions. As discussed above, many sauna heater manufacturers explicitly prohibit GFCI on the heater circuit. Installing one anyway — even with good intentions — can lead to constant nuisance tripping and a frustrated homeowner.

Routing wiring through high-heat zones. Wiring that passes near the heater, through the ceiling directly above the heater, or along the upper walls of the sauna room is exposed to temperatures that can degrade standard insulation. Cables must be routed below bench level or through the wall behind the heater at the lowest practical point.

Bonding neutral and ground in the subpanel. This is a fundamental code violation that happens more often than it should. In a subpanel, neutral and ground must be on separate, isolated bus bars. The bonding screw (or strap) that connects them in a main panel must be removed in a subpanel.

Forgetting the disconnect. Many jurisdictions require a visible disconnect within sight of the sauna heater. Skipping this step means a failed inspection and a return trip.

Not reading the manual. This sounds obvious, but the manufacturer's installation manual is the authoritative source for every electrical specification. Heater models vary — even within the same brand — and assumptions based on a previous installation can lead to errors.

Choosing the Right Heater Makes the Electrician's Job Easier

The electrical requirements for your sauna installation flow directly from the heater you select. Choosing the correct heater for your room size, construction materials, and desired sauna experience simplifies everything downstream — from breaker sizing to wire selection to subpanel planning.

Haven of Heat carries electric sauna heaters from the industry's most respected manufacturers, including Harvia, HUUM, Saunum, Scandia, and Finlandia. Each of these brands provides detailed installation manuals with clear electrical specifications that your electrician can reference directly.

Start with our Electric Sauna Heater Size Calculator to find the right kilowatt rating for your room. From there, browse the matching models in our electric sauna heaters collection. If you want help selecting a heater or have questions about your specific electrical situation, our team is available to assist — contact us or visit our Sauna Heater Learning Center for additional guides and resources.

For more detailed information on general sauna electrical requirements, home wiring best practices, and hiring the right professional, explore these additional resources:

• Home Sauna Electrical Requirements
• Sauna Electrical and Plumbing Requirements: Everything You Need to Know
• Best Places to Hire an Electrician for Sauna Heater Installation
• Saunas and Your Electric Bill: What Is the Real Cost?
• Best Sauna for Home: How to Choose the Right Type, Size, and Setup

*Haven Of Heat and its affiliates do not provide medical, legal, electrical, building, financial, or professional advice. All content published on this website is for general informational and educational purposes only and should not be relied upon as a substitute for advice from qualified professionals. Always consult a licensed medical provider regarding health-related questions, and consult licensed contractors, electricians, inspectors, or local authorities for installation, electrical, building code, zoning, HOA, or safety requirements. Local codes and regulations vary by jurisdiction.