The two numbers a sauna build needs before you buy anything
Both tools run the arithmetic in your browser, in full view — no single confident number standing in for a trade that hasn't agreed on one. Every figure is attributed to the retailer, manufacturer or code source it came from.
Every number these tools use is named where it's used, with the source it came from. Where the trade genuinely disagrees — the heater's glass-load constant is the clearest case — you get every published figure side by side, not one of them presented as the answer.
Sauna heater sizing, with the disagreement showing
Measure the room, and this works out the heater it asks for — step by visible step. It will not give you one confident number, because the trade does not have one: the constant for glass alone ranges from 1 to 4.8 depending on which retailer you ask, and every sizing calculator you have used silently picked one. This one shows you all of them, with names attached.
This room computes to 7.7 kW. Rounded up to the next standard size, that is a 8 kW heater — about 4% of headroom over the calculated figure.
Change nothing but the constant, and the same room asks for 6.58 – 11.58 kW. That spread is not measurement error — it is four named retailers publishing incompatible numbers for the same room. Sauna Republic's outlier constant is not in that range; it would put this room at 10.27 kW.
The arithmetic, in full
Nothing here is hidden behind a form post. Every line is one operation, and the lines add up to the figure above — check it with a pocket calculator if you like.
| Interior volume — length × width × height | 294 cu ft |
| + glass allowance, at 1.5 cu ft per sq ft | 53 cu ft |
| Glass, under this constant | adds kW, not volume |
| × wall construction | ×1.0 |
| × siting and climate | ×1.0 |
| × ceiling above 7 ft, ~10% per foot | ×1.0 |
| Effective volume | 347 cu ft |
| ÷ baseline, 45 cu ft per kW | 7.7 kW |
| + glass, at 1 kW per 2 sq ft | 0.0 kW |
| Calculated requirement | 7.7 kW |
| Rounded up to a standard size | 8 kW |
Two lines in that table deserve a word. Height is counted twice — once inside the volume, and again in the ceiling factor. That is not a slip: the convention treats a tall ceiling as a heat-loss and stratification penalty on top of the extra air it holds. And a ceiling below 7 ft gets no discount here, because the sources publish an increase above 7 ft and say nothing about going the other way — so this tool says nothing either.
347 cubic feet effective, through Northern Saunas / The Sauna Place — about 1.5 cu ft per sq ft, single-pane at 45 cu ft per kW: 7.7 kilowatts calculated, a 8 kilowatt heater rounded up. Across the mainstream constants the same room ranges from 6.58 to 11.58 kilowatts.
Every published glass constant, run against your room
Same room, same baseline of 45 cu ft per kW, same everything — only the glass constant changes from row to row. Each one is attributed to the retailer that publishes it. We are not adjudicating between them, and neither should a calculator that hides which one it used.
| Glass constant | Published by | What it adds | Calculated | Heater size | vs your lens |
|---|---|---|---|---|---|
| Sauna Supply Co — about 1 cu ft per sq ft Your lens | Sauna Supply Co | Equivalent volume | 7.31 kW | 8 kW | −0.39 kW |
| Northern Saunas — about 1.2 cu ft per sq ft, double-pane Your lens | Northern Saunas | Equivalent volume | 7.47 kW | 8 kW | −0.23 kW |
| Northern Saunas / The Sauna Place — about 1.5 cu ft per sq ft, single-pane Your lens Matches your single-pane glazing | Northern Saunas; The Sauna Place | Equivalent volume | 7.7 kW | 8 kW | your lens |
| The Sauna Place, alternative method — about 1 kW per 11 sq ft Your lens | The Sauna Place (alternative method) | Kilowatts, added after the multipliers | 9.72 kW | 10.5 kW | +2.02 kW |
| Sauna Republic — 4.8 cu ft per sq ft Outlier Your lens | Sauna Republic | Equivalent volume | 10.27 kW | 10.5 kW | +2.57 kW |
The disagreement costs you a heater size. Across the mainstream constants your room lands anywhere from a 8 kW to a 10.5 kW heater. No source in our set resolves this, so treat the top of that range as the safer buy and check it against the heater's own published room range before ordering.
And the baseline constant is disputed too
Same room, same glass constant (Northern Saunas / The Sauna Place — about 1.5 cu ft per sq ft, single-pane) — only the cubic-feet-per-kilowatt baseline changes. This disagreement is narrower than the glass one, but it is the figure every sizing chart on the internet quietly assumes.
| Baseline | Published by | Calculated | Heater size |
|---|---|---|---|
| 50 cu ft per kW Your lens | SaunaKits; Cedar Barrel Saunas | 6.93 kW | 8 kW |
| 45 cu ft per kW Your lens | Sauna Supply Co; Sauna Republic | 7.7 kW | 8 kW |
| 35 cu ft per kW Your lens | Öli Saunas (low end of a published 35–50 range) | 9.9 kW | 10.5 kW |
The three published baselines put your room between a 8 kW and a 10.5 kW heater — so on this room, the baseline you pick is doing real work, not just rounding.
Every figure above is a convention — retailer rules of thumb, not a manufacturer's engineering. The number that actually governs your purchase is the room range the specific heater publishes on its own spec sheet, in cubic feet. Take the effective volume this tool computed and check it against that range before you order anything. Where the two disagree, the manufacturer wins — they tested the unit, and their warranty is written against their own range, not against ours.
To make that concrete: Harvia publishes a 100–425 cu ft room range across its KIP 4.5–8 kW line. That is the shape of number you are looking for on a spec sheet — named here as an example of the format, not as a recommendation, and worth re-checking against current manufacturer documentation, since nameplates change by revision. Harvia, manufacturer figure
One more thing worth knowing before you reach for a bigger unit: in North America, UL/CSA listing caps a sauna heater's element temperature at 194°F / 90°C. Sizing up buys you a room that reaches and holds its target more comfortably — it does not buy you a hotter element than the listing allows. Cedar Barrel Saunas, citing UL/CSA
This result is above about 8 kW, which is the point where a single-phase residential supply usually stops being the straightforward option — you are into a subpanel, a dual-feed heater, 208V three-phase, or a smaller room. That is a circuit conversation, not a sizing one, and it is the highest-liability part of this whole build: breaker and conductor sizing follow the heater's nameplate and the 125% continuous-load rule, and what your jurisdiction requires is your inspector's call, not a website's. Convention; NEC 210.19/210.20 for the 125% rule
Wood-fired stoves are sized differently — the convention runs about 1.5× the electric equivalent for the same room, so nothing on this page transfers directly. Infrared cabins are not sized by volume at all; they are specified by panel count and occupancy, which makes cubic feet the wrong question entirely. If you are shopping either, this calculator will give you a confident number that does not apply to you.
Every factor this tool applies
The full reference behind the dropdowns — useful if JavaScript is off, if you want to run the arithmetic by hand, or if you want to see which figures are published and which ones are us doing arithmetic on a published range.
| Wall construction | Applied | Source |
|---|---|---|
| Insulated, all wood | no change | House baseline |
| Uninsulated or exterior walls — low end | +10% | Haven of Heat |
| Uninsulated or exterior walls — middle | +15% | Haven of Heat published the band; the midpoint is our arithmetic |
| Uninsulated or exterior walls — high end | +20% | Haven of Heat |
| Log or masonry walls | +20% | No published constant in our source set |
| Siting and climate | Applied | Source |
|---|---|---|
| Indoors | no change | House baseline |
| Outdoors, not a cold climate | no change | No published constant in our source set |
| Outdoors, cold climate — low end | +15% | The Sauna Place |
| Outdoors, cold climate — middle | +20% | The Sauna Place published the band; the midpoint is our arithmetic |
| Outdoors, cold climate — high end | +25% | The Sauna Place |
| Glass constant | Shape | Value as published | Source |
|---|---|---|---|
| Sauna Supply Co — about 1 cu ft per sq ft | Adds equivalent volume | 1 cu ft per sq ft | Sauna Supply Co |
| Northern Saunas — about 1.2 cu ft per sq ft, double-pane | Adds equivalent volume | 1.2 cu ft per sq ft | Northern Saunas |
| Northern Saunas / The Sauna Place — about 1.5 cu ft per sq ft, single-pane | Adds equivalent volume | 1.5 cu ft per sq ft | Northern Saunas; The Sauna Place |
| The Sauna Place, alternative method — about 1 kW per 11 sq ft | Adds kilowatts directly | 1 kW per 11 sq ft | The Sauna Place (alternative method) |
| Sauna Republic — 4.8 cu ft per sq ft Outlier | Adds equivalent volume | 4.8 cu ft per sq ft | Sauna Republic |
| Baseline constant | Value | Source |
|---|---|---|
| 50 cu ft per kW | 50 cu ft per kW | SaunaKits; Cedar Barrel Saunas |
| 45 cu ft per kW | 45 cu ft per kW | Sauna Supply Co; Sauna Republic |
| 35 cu ft per kW | 35 cu ft per kW | Öli Saunas (low end of a published 35–50 range) |
The ceiling factor is not in a table because it is not a menu: roughly +10% per foot of ceiling above 7 ft, prorated for part-feet. The per-foot figure is published; the proration is our arithmetic, and a room 6 inches over 7 ft gets 5%, not 10%.
Standard heater sizes this tool rounds up to: 3, 4.5, 6, 8, 9, 10.5, 12, 15, 18 kW — the sizes commonly stocked, a convention rather than a standard. Your heater's own published range governs.
Sources
- Baseline constant — SaunaKits and Cedar Barrel Saunas state 50 cu ft per kW; Sauna Supply Co and Sauna Republic use 45; Öli Saunas publishes a 35–50 range. All Convention (retailer rules of thumb), not manufacturer engineering. This tool holds all three rather than averaging them, because an average of three incompatible conventions is not a fourth convention — it is a number nobody published.
- Glass adjustment — Sauna Supply Co, about 1 cu ft per sq ft; Northern Saunas, about 1.5 single-pane and 1.2 double-pane; The Sauna Place, 1.5 flat and, separately, about 1 kW per 11 sq ft; Sauna Republic, 4.8 cu ft per sq ft. All Convention. Sauna Republic's figure is flagged as an outlier everywhere it appears and is excluded from the range quoted at the top of the tool — and the tool says so rather than dropping it quietly.
- Uninsulated or exterior walls — +10–20%, Convention (Haven of Heat). The band is published; the 15% midpoint option is our arithmetic and is labelled as such in the tool. Log and masonry have no separate published figure in our source set; the tool applies the top of this band and says it is a substitution.
- Outdoor cold climate — +15–25%, Convention (The Sauna Place). Same treatment: band published, midpoint ours. Mild-climate outdoor siting has no published figure, so nothing is added and the result is called a floor.
- Ceiling above 7 ft — about +10% per foot, Convention (Haven of Heat / The Sauna Place). Prorated for part-feet by us. No reduction is applied below 7 ft, because none is published.
- Element temperature cap — 194°F / 90°C in North America, Manufacturer/Literature (Cedar Barrel Saunas citing UL/CSA).
- Nameplate room range, as a format example — Harvia KIP 4.5–8 kW, 100–425 cu ft, Manufacturer. Named to show what a spec-sheet room range looks like; not a recommendation, and to be re-verified against current manufacturer documentation.
- Practical single-phase ceiling — about 8 kW, Convention. The 125% continuous-load rule referenced in that advisory is Code (NEC 210.19/210.20); breaker and conductor sizing belong to the circuit planner, and to your AHJ.
- Wood-fired sizing — about 1.5× the electric equivalent, Convention. Infrared is specified by panel count and occupancy, not volume.
- Standard heater sizes — Convention. A stocking pattern, not a standards body's list, and editable per store.
Constants current as of 2026-08-26 and drawn from Hub B's outline research. Every one of them is a retailer convention unless this page says otherwise; where our sources disagree, this page shows the disagreement instead of resolving it.
This calculator is arithmetic over published conventions, plus whatever you tell it about your room. It does not know your climate, your insulation detailing, how you use the room, or which heater you are looking at. Where the trade's own sources disagree — and on the glass constant they disagree by a factor of nearly five — you get every published position with a name attached, not ours. Confirm the result against the specific heater's own room range before you buy.
No prices anywhere in this tool, and nothing here recommends a product. This is a sizing calculator; what to buy, and who earns what when you do, belongs in the guides and is disclosed there. Across the site we band prices rather than print them: sauna and heater brands do not publish full minimum-advertised-price terms, retailer pricing moves constantly on promotion, and a figure we copied once would be wrong within weeks. Nothing on this page is ordered by what it would pay us.
Sauna circuit, breaker & wire planner
Work out the circuit a 240V sauna heater needs — and see exactly where each figure's authority comes from. Every output is labelled Code, Convention, Manufacturer, Jurisdictional or House, because those are five different kinds of thing and only one of them your inspector cannot overrule.
| Nameplate rating | 9 kW at 240 V | Manufactureryour nameplate |
| Full-load amps — watts ÷ volts | 9000 ÷ 240 = 37.5 A | Conventionvalid because the kW figure is rated at this voltage |
| × 1.25 — continuous-load minimum | 46.9 A | CodeNEC 210.19 / 210.20 — a sauna is a continuous load |
| Next standard breaker at or above that | 50 A double-pole | CodeConventionstandard sizes NEC 240.6(A); the step-up itself is ordinary practice |
| Minimum conductor, 75 °C copper | 6 AWG | Conventionfinal gauge is per run and per inspector — see the two notes below |
| Circuit type | Dedicated — nothing else on it | CodeNEC 210.23 |
| Disconnect | Within sight of the heater | CodeNEC 422.31 (appliances) / 424.19 (fixed space heating); “within sight” is defined in NEC Article 100 as visible and not more than 50 ft away |
| Voltage drop over the run | 40 ft — rarely governs here | Houseour prompt to ask, not a calculation — the NEC's own 3% figure sits in an informational note and is advisory |
| GFCI protection | This tool does not answer this | Jurisdictionalgenuinely unsettled — four defensible positions, resting on sources that conflict; all four are mapped below |
This tool starts from a kW figure because that is usually all a reader has while planning. A heater's own documentation is a better input, and where it specifies a breaker and a conductor, it wins — installing listed equipment against its instructions is itself a code problem (NEC 110.3(B)).
They really do differ. A 6 kW heater draws 25 A; × 1.25 is 31.3 A, which the next-standard-size step takes to a 35 A breaker. HUUM's own DROP 6 documentation specifies a 30 A breaker and 10 AWG. Neither is wrong: a generic 125% calculation is what you do when a kW rating is all you have, while a manufacturer publishes a device size out of the unit's own listing. Treat this tool's output as a sanity check on the nameplate — useful for spotting a spec that looks off, and for planning before you have bought anything — never as a replacement for it.
Manufacturer beats Convention here, and Code NEC 110.3(B) is why.
9 kW at 240 volts draws 37.5 amps; the 125 percent continuous-load rule makes that 46.9 amps, which takes a 50 amp double-pole breaker and 6 AWG copper. Your inspector confirms all of it.
This tool carries exactly three breaker→conductor pairings it can trace to a source: 30 A → 10 AWG, 40 A → 8 AWG, 50 A → 6 AWG, all 75 °C copper. Your result landed outside that set, so it prints nothing rather than interpolating one. A guessed conductor size is the one output this page will never produce.
This happens often — most heater ratings land on a breaker size we have no sourced pairing for, and we would rather show you that than quietly make one up. Two ways forward, both better than a number from us: read the conductor off the heater's own documentation, which is the answer that actually governs; or have your electrician size it from the ampacity table against your real conductor, insulation and termination ratings, which is what they would do with our figure anyway.
Watts ÷ volts is only the right sum when the kW figure on your nameplate is the rating at that voltage. Feed a heater whose element is rated 240 V from a 208 V supply and it does not draw watts ÷ 208 — it draws less current, and it produces roughly 75% of its rated output, because power in a resistive element varies with the square of the voltage. That is a heater-sizing problem before it is a circuit problem: the room may never reach temperature. Check whether your heater carries a 208 V rating at all, and size from that line of the nameplate.
Convention Electrical fundamentals, not a code rule.
This is not a prohibition. Plenty of 9 kW heaters run single-phase at 240 V in ordinary houses. ~8 kW is the point at which a widely-repeated practical ceiling starts to bite: the breaker, the feeder, the panel's spare capacity and the AHJ conversation all get real at once, and the alternatives — 208 V/three-phase, a subpanel, or a dual-feed heater wired as two circuits rather than one larger one — become worth pricing out with an electrician before you commit to a heater.
Convention A practical ceiling, not a code limit.
The continuous-load figure for that rating exceeds 100 A, and this planner stops there deliberately. Above it the question is about your service and your feeder, not about picking a breaker off a list, and a tool that kept printing numbers would be pretending to a competence it does not have. Take the nameplate to an electrician.
The run: 40 ft
At this length voltage drop rarely governs the conductor size at these breaker ratings — the ampacity table usually wins. Still your electrician's check to make, not this page's.
House Alder & Rime's own planning prompt — three length bands, no external source, and no voltage-drop arithmetic. The NEC's familiar 3% figure appears in an informational note, which is advisory rather than enforceable; the real calculation needs the conductor, its temperature rating and the termination ratings at both ends, none of which this tool knows.
Panel gut check
A 50 A breaker against a 200 A main is 25% of the main's rating — before a single other thing in the house is counted.
Breaker amps do not add up to panel capacity. A real NEC Article 220 load calculation works from the dwelling's actual connected load with demand factors applied, and the sum of the breakers in a healthy panel routinely exceeds the main's rating by design — that is normal, not a fault. The percentage above is a conversation-starter for the electrician, nothing more. Whether your panel can take this circuit depends on the whole house's load and on physical space for a double-pole breaker, and answering that is their job.
House A ratio of two numbers you supplied. No demand factors, no connected load, no verdict.
The pairings this tool uses are for copper. Aluminium is a different ampacity table and different terminations, and this tool does not cover it. The 75 °C column is normally the one that governs even when the wire itself is rated 90 °C, because the equipment's terminations are commonly listed for 75 °C and NEC 110.14(C) makes the lowest-rated part of the connection decide which column applies. Sizing off the 90 °C column because the wire says 90 °C is one of the more common ways a DIY circuit fails inspection.
Code NEC 110.14(C) governs which temperature column applies; the pairings themselves are Convention.
What the labels mean
Most sauna wiring guides print one table of breaker and wire sizes with nothing attached to it, which quietly implies every cell has the same standing. They do not. A figure that comes from NEC text and a figure that comes from what electricians usually do are different kinds of thing, and the difference matters most in exactly the situation where you are about to spend money on copper.
| Label | What it means | Who can overrule it |
|---|---|---|
| Code | NEC text, with the article cited. Adopted, amended and enforced locally. | Your AHJ, via a local amendment or a different adopted edition. Not us, and not your electrician. |
| Convention | What the trade normally does. Sound practice, often conservative, but not a rule. | A competent electrician, for a documented reason — run length, termination ratings, the nameplate. |
| Manufacturer | The nameplate or the manual for one specific unit. Installing listed equipment against its own instructions is itself a code problem (NEC 110.3(B)). | Nobody, for that unit — but manufacturers openly contradict each other, and a manual cannot waive a code rule your AHJ enforces. |
| Jurisdictional | Genuinely varies by state, county or inspector. There is no national answer to give you. | Nobody, in general. Only your specific AHJ, in writing, for your specific install. |
| House | Alder & Rime's own planning prompt. No external source at all — labelled this way precisely so you can discount it. | You. It is a prompt to ask a question, never an answer to one. |
GFCI: four defensible positions, and no answer from us
Every competitor's sauna wiring guide picks a side on this and states it flatly. We are not going to, because the sources genuinely conflict — and they conflict across authority tiers, which is the part that makes it unresolvable from a web page. Two of the positions below rest on code text. One rests on manufacturers' own manuals telling you the opposite. One says the code articles everyone argues about were never the right ones. Read the tier badges down the table: that spread is the answer to "why doesn't anyone just say".
| Position | Tier | What it rests on | Who argues it | What it implies for a 240 V hardwired heater | Its weak point |
|---|---|---|---|---|---|
| A — the pool and spa rules don't reach a sauna | Code…but miscited | NEC 680.44, plus the “150 V to ground / 60 A” Class A GFCI threshold. | Retailer wiring guides, very widely repeated. | A 240 V heater sits above the 150 V-to-ground threshold, so the pool/spa mandate never attaches. | 680.44 governs the outlet supplying a self-contained, packaged or field-assembled spa or hot tub — not a sauna. And the 150 V/60 A language lives in 680.5(B), not 680.44. The argument is repeated far more often than it is checked. |
| B — the 2023 outdoor-outlet expansion catches it | Code | 2023 NEC 210.8(F): outdoor outlets, single-phase, ≤150 V to ground, ≤50 A require GFCI — with the enumerated locations expanded to accessory buildings, garages at grade and boathouses. | Eaton's NEC-2023 explainer; code-update summaries quoting the article. | An outdoor barrel sauna, or one in a detached outbuilding, plausibly falls inside it. | Whether a hardwired heater is an “outlet”, and whether your building counts, are both readings your inspector makes. Exception No. 2 exempts listed HVAC equipment and expires 1 September 2026 — it is the exception people reach for, and a sauna heater almost certainly is not HVAC. The 2026 edition also raised the threshold from 50 A to 60 A. |
| C — the manufacturer says do not | Manufacturer | Manuals, verbatim. Dundalk Sauna Craft's Luna manual: “Do NOT use a G.F.I breaker”. Almost Heaven's Harvia Wall Heater manual: “Do not use GFCI with this heater”. Harvia's own Spirit and PC60E manuals forbid feeding the heater through an RCD/GFI. Homecraft's H-Series notes a GFI usually will not work with their heaters. | The manufacturers themselves, in the documents that ship in the box. | Installing it on a GFCI is installing listed equipment against its own instructions, which NEC 110.3(B) itself makes a problem. | A manual cannot waive a code requirement your AHJ enforces. Where the two genuinely collide you have a real conflict that only the AHJ can resolve — and the manufacturer may need to be in that conversation. |
| D — saunas were never a 680 question | Conventionattributed practitioner argument | The NEC has no “sauna” article. A hardwired heater is a permanently connected appliance (Art. 422) and fixed electric space-heating equipment (Art. 424), with 426 in the neighbourhood. | Jack Marquardt, licensed electrician (Electric Avenue PNW, Oregon CCB# 248553), in a published article; Peak Saunas concurs on 424. Practitioners on Mike Holt and ElectricianTalk corroborate — cited here as evidence the dispute is live among licensed people, not as authority. | 680's GFCI mandates never attach at all, so positions A and B's whole argument is beside the point. | It is a classification argument, not code text saying saunas are exempt. And it does nothing about position B: 210.8(F) applies to outdoor outlets regardless of which appliance article the heater lives under. |
A Class A GFCI must trip at 6 mA of ground-fault current, must not trip below 4 mA, and must act within 25 milliseconds — UL 943, quoted via the Building Code Forum; the 4–6 mA range also appears in an NEC informational note. UL 875 §43.2 separately permits up to 0.75 mA of leakage on a sauna heater's equipment grounding conductor.
0.75 mA is well under the trip window on its own — but it is the same order of magnitude, and it stacks: long element runs and long cable runs couple capacitively, and element insulation in a hot, humid box carries residual moisture. That is the mechanism practitioners point to when they describe nuisance tripping, and it is why the phenomenon is not simply user error. We have not found a controlled measurement of it, and the UL 875 figure reaches us through an electrician's citation rather than from the standard directly, so treat the mechanism as well-attested and the numbers as pending verification.
Code UL 943 trip window, via a named secondary reproduction. Manufacturer UL 875 leakage allowance, practitioner-cited — verify against UL 875 directly before relying on it.
What to actually do
Ask your AHJ, before rough-in, and get the answer in writing. Bring three things: the heater's manual open at the page that mentions GFCI, whether the sauna is indoors or outdoors, and whether it is hardwired or on a receptacle. Those three facts are what the question actually turns on. If the inspector requires GFCI and the manual forbids it, that conflict is real and it is theirs to resolve — not something to quietly pick a side on at the panel.
Whether you may legally do this yourself depends on where you live
There is no national answer to this either, and guides that give one are describing their own state. Two real examples, at opposite ends — not a survey, and not a substitute for checking your own jurisdiction.
The TDLR licensing requirement carries an exemption for a homeowner doing electrical work in their own owner-occupied dwelling (Occupations Code §1305.003(a)(6)). The exemption is about the licence, and it is narrower than "homeowners can do electrical work in Texas" — read the actual wording against your situation.
JurisdictionalA licensed electrician is required, and homeowners cannot pull permits for most electrical work. The same project that is a legal weekend job in one state is not one here.
JurisdictionalThese are two separate requirements and people routinely collapse them. Being exempt from holding an electrician's licence does not exempt you from pulling a permit, from the inspection, or from the NEC as your jurisdiction has adopted and amended it. Skipping the permit is also the version of this that surfaces years later — at resale, or at a homeowner's insurance claim on an unpermitted 240 V circuit feeding a heater.
Jurisdictional TDLR and a state-by-state summary. Two examples chosen to show the spread; confirm your own state, county and city before you plan around either.
Representative nameplates
Reference only — nothing in the calculator above reads these figures, and none of them is a substitute for the nameplate on the unit in front of you. They are here to show what real manufacturer specs look like, and where they disagree with the general pairing table.
| Unit | Draw | Breaker | Conductor | Tier |
|---|---|---|---|---|
| HUUM DROP 6 | 25 A | 30 A | 10 AWG | Manufacturer |
| HUUM DROP 9 | 37.5 A | 50 A | 8 AWG | Manufacturervia a retailer's reproduction |
| HUUM HIVE 12, at 240 V | — | 30 + 30 A, dual feed | 10 + 10 AWG | Manufacturer |
| HUUM HIVE 15 | 62.5 A | — | — | Manufacturer240 V single-phase |
The manufacturer and the general pairing can disagree, and the manufacturer wins. The DROP 9 lands on a 50 A breaker with 8 AWG, where the calculator's Convention pairing puts 6 AWG on a 50 A breaker. Both are defensible — 8 AWG carries 50 A in the 75 °C copper column, which satisfies the 125% conductor requirement, and the conservative pairing simply leaves more headroom. We ship the conservative one and show you the conflict rather than hiding it. Your nameplate and your inspector settle it.
Some large heaters want two circuits, not one bigger one. The HIVE 12's dual 30 + 30 A feed is not a workaround; it is how the unit is designed to be fed. A calculator that only ever produces one breaker size cannot tell you that — the nameplate can.
Manufacturer Collected 2026-08-22 and pending verification against current manufacturer documentation. European brands publish baseline amperage without the NEC's 125% continuous-load adjustment, so an EU spec sheet is not automatically a US install spec. Verify every row against the current manual before you plan around it.
Sources
- 125% continuous-load sizing — NEC 210.19 / 210.20, attributed to NFPA 70 §§210.19/210.20 by an electrician/contractor source. Code
- Standard overcurrent-device sizes — NEC 240.6(A). The step up to the next standard size is ordinary practice on top of it. Code Convention
- Amps = watts ÷ volts — arithmetic, valid only where the kW figure is the nameplate rating at that voltage. Convention
- Dedicated circuit — NEC 210.23. Code
- Disconnect within sight — NEC 422.31 (appliances) / 424.19 (fixed space heating); “within sight” defined in NEC Article 100 as visible and not more than 50 ft from the equipment. Code
- Breaker → conductor pairings, 75 °C copper — 30 A → 10 AWG, 40 A → 8 AWG, 50 A → 6 AWG, from an electrician/contractor guide plus two retailer wiring guides. Only these three ship; nothing is interpolated. Which temperature column applies is governed by NEC 110.14(C). Convention
- ~8 kW single-phase practical ceiling — widely repeated planning guidance, not a code limit. Convention
- Representative nameplates — HUUM's own published tables, one row reaching us via a retailer's reproduction. Pending manufacturer-document verification. Manufacturer
- GFCI positions A and B — NEC 680.44 and 680.5(B); 2023 NEC 210.8(F) with its listed-HVAC exception (expiring 1 September 2026 per TIA 1653), via a manufacturer NEC explainer and a code-update summary quoting the article. The 2026 edition raised 210.8(F)'s threshold from 50 A to 60 A. Code
- GFCI position C — Dundalk Sauna Craft (Luna), Harvia Spirit and PC60E, Almost Heaven's Harvia Wall Heater, and Homecraft H-Series owner's manuals, quoted verbatim. Installing listed equipment contrary to its instructions engages NEC 110.3(B). Manufacturer
- GFCI position D — a named licensed electrician's published article (Electric Avenue PNW, Oregon CCB# 248553) and a sauna retailer concurring on Art. 424; licensed practitioners on Mike Holt and ElectricianTalk corroborate the classification, cited as evidence the dispute is live rather than as authority. Convention
- GFCI trip window and sauna leakage allowance — UL 943 (4–6 mA within 25 ms) via a forum reproduction and an NEC informational note; UL 875 §43.2 (up to 0.75 mA on the EGC) via an electrician's citation, not from the standard directly.
- Homeowner DIY legality — Texas Department of Licensing and Regulation, Occupations Code §1305.003(a)(6); a state-by-state licensing summary for Massachusetts. Two examples, not a survey. Jurisdictional
- Run-length bands and the panel percentage — no external source. Alder & Rime's own planning prompts. House
Figures current as of 2026-08-26. NEC article numbers on this page were sourced through the named secondary reproductions above, not from the NFPA 70 reader directly, and are pending re-verification — including against the edition your jurisdiction has adopted. Where a figure is retailer-published rather than manufacturer-published, or practitioner-cited rather than taken from the standard, this page says so at every place that figure appears, not only here.
What this tool is. Arithmetic over rules it names and tiers, applied to figures you type in. It does not know your panel, your jurisdiction, your inspector, your heater's manual, or the condition of the wiring already in your walls. Where the sources genuinely disagree — GFCI is the clearest case, and the 50 A conductor pairing is a quieter one — you get every position with its tier attached rather than our preference.
What it is not. It is not a compliance check, a load calculation, a permit, or advice from a licensed electrician, and it makes no claim about what is legal or approvable in your jurisdiction. Take the output to a licensed electrician and to your AHJ. If they disagree with this page, they are right.
Nothing on this page is monetised. No product links, no prices, no figures for anything. This is a planning tool, and pointing it at a heater we earn on would corrupt the only thing it is for.