If you're putting a sauna in a gym, apartment building, office or any commercial space, there's a good chance your panel gives you 208V, not 240V. Most residential sauna heaters are built for 240V. They'll run on 208V, but they won't make the heat you paid for.

This is one of those details that gets missed until the sauna is built and sits at 150°F. Here's how it works and what to do about it.

Why 208V and 240V Are Different

Residential service in the U.S. is split-phase, which gives you 240V between the two hot legs. Many commercial buildings use three-phase power, where the voltage between any two hot legs is 208V.

The two numbers sound close. They aren't, for a heater, because heating elements are basically resistors, and the power they produce depends on voltage squared.

The practical rule we use: wiring a 240V single-phase electric heater to a 208V commercial system, using two hot wires, reduces its power output by about 25%. That has to be built into heater sizing from the start. Always confirm with your electrician before you buy.

The Math

For a resistive heater, power is voltage squared divided by resistance. The resistance of the element doesn't change, so the output scales with the square of the voltage ratio:

(208 ÷ 240) squared is about 0.75.

So an 8 kW heater rated for 240V delivers roughly 6 kW on 208V. A 6 kW heater delivers about 4.5 kW. A 9 kW heater delivers about 6.75 kW.

Current drops too, but not as much as the power does. That 8 kW heater draws about 33 amps at 240V and about 29 amps at 208V. You're still on a dedicated two-pole circuit with the same wire and breaker size, because the circuit has to be sized from the heater's nameplate, not from what it draws on a weak supply. Your electrician will size it from the nameplate and local code.

How to Size a Heater for 208V

Start with the normal baseline: 1 kW per 45 cubic feet for a well-insulated, wood-lined room. Then divide by 0.75 to get the nameplate rating you need.

For larger heaters, check what's available. Many 15+ kW commercial heaters are offered in true three-phase 208V versions, which are rated for that voltage and don't take the penalty. Where a 208V-rated model exists, it's usually the better answer than oversizing a 240V one.

Adjust the same way you normally would. Go up for glass, exterior exposure, cold climates and concrete walls, and go down only for unusually good insulation. Our heater sizing guide has the full set of adjustment factors.

Option 1: Use a Heater Rated for 208V

The cleanest fix is to buy a heater rated for 208V in the first place. Many manufacturers sell the same model in 208V and 240V versions, and the 208V version has a different element rating so it delivers the stated kW at your voltage. Tell the supplier your supply voltage and phase before you order.

Option 2: Oversize a 240V Heater

If the model you want only comes in 240V, size up using the table above. This works, but it has costs. The larger heater has a bigger footprint and bigger clearances, and a heavier electrical load on paper. Check the heater's room-volume range too, since some are not intended for rooms outside a given size.

Option 3: Get a 240V Supply

Sometimes the right answer is a small transformer, or a different panel with true split-phase 240V. That's an electrician and building-owner decision, and it can cost more than it's worth, particularly in a leased commercial space. Ask before you assume.

What to Check Before You Buy

• Supply voltage: measure it, or ask the building. Don't assume 240V because the panel says "240."
• Phase: single-phase or three-phase. This changes which heater models you can use.
• Panel capacity: a 9 kW or larger heater is a serious load.
• Heater nameplate: confirm the rated voltage and wattage. For 240V-only models, plan for the 25% reduction.
• Local code and inspection: commercial installs usually need a permit and an inspection.

How to Tell If 208V Is Your Problem

If your sauna is slow to heat or tops out too low, and it's in a commercial building, voltage is a prime suspect. Heat-up times for electric heaters are normally 30-60 minutes depending on the room size and insulation. If you're far past that, check the supply voltage at the heater terminals with a meter, under load. Our sauna not getting hot enough post covers the other common causes, such as undersized heaters and air leaks, so you can rule them out.

Gyms and studios run into this often. Our commercial sauna design article covers what else to get right in those builds.

A Worked Example: An 8x10 Gym Sauna

Say you're adding an 8x10 sauna with a 7.5-foot ceiling to a small gym. Volume is 600 cubic feet, so the baseline is about 13.3 kW on 240V. The gym has 208V three-phase service.

If you buy a 240V-only 14 kW heater and run it across two legs, you get about 10.5 kW. That's roughly 10 kW of heat for a room that needs 13. The sauna will heat, but slowly, it will struggle to recover after the door opens, and the löyly will be weak. Members will say it "doesn't get hot," and they'll be right.

The better path is to price a 208V-rated 15-16 kW heater, or to confirm with the manufacturer how the 240V model performs at 208V. Either way, the decision happens before the heater is on site, not after.

Controls, Contactors and Other Gotchas

A few things catch people on commercial jobs:

• Control voltage. Some heater controls and contactors have their own voltage ratings. Confirm they accept 208V.
• Fan and light circuits. These are 120V, and in a three-phase building they come off a different leg than the heater. Your electrician should plan the panel layout.
• Phase balancing. In a three-phase building, a large single-phase load can unbalance the panel. Large heaters often come in three-phase versions for this reason.
• Warranty. Running a heater outside its rated voltage can affect the warranty. Ask in writing.

None of this is hard. It just needs to be settled early, because it changes what you order.

Why This Matters for Heat-Up Time

A sauna that normally takes 30-60 minutes to reach temperature on 240V will take noticeably longer on an undersized 208V setup. In a commercial setting, that's not a minor annoyance. If the sauna is on a timer or a booking schedule, the heater has to be ready when people arrive. Sizing correctly for the supply voltage is what makes that work.

Frequently Asked Questions

Can I run a 240V sauna heater on 208V?

Yes, in most cases, by connecting it across two hot legs. It will work, but output drops by about 25%. Confirm with the manufacturer and an electrician before wiring it.

How much power do I lose on 208V?

About 25%. An 8 kW heater rated for 240V delivers roughly 6 kW on 208V. That's why we size up the nameplate rating by about a third when the supply is 208V.

Do I need a bigger breaker for a 208V heater?

Not because of the voltage. The circuit is sized from the heater's nameplate and local code, and your electrician determines wire and breaker size. Most 6-9 kW heaters use a dedicated 40-50 amp circuit, and 10 kW and up usually need 60 amps.

Is it better to buy a 208V-rated heater?

Usually, yes. If one exists for the model you want, it delivers its rated output on your supply with no oversizing. Ask the supplier for the 208V version when you order.

Next Steps

If you're designing a sauna for a commercial or multi-family space and want the electrical, heater size and ventilation worked out together, our remote sauna design service can do that. Our electrical requirements page is a good place to start before you talk to your electrician.