Tankless Water Heater Breaker Calculator
This free tankless breaker calculator exists because most people buy the unit before checking the panel. Whole-home electric tankless units need two to four dedicated 40–60 A double-pole circuits — 54 to 150 amps of total connected load — and that surprise usually arrives with the box. Model data comes straight from the manufacturer spec sheets: Rheem RTEX, EcoSmart (a Rheem brand), and Stiebel Eltron's Tempra line.
The panel is the real gatekeeper. A 24 kW unit draws about 100 A at full output — most of a 150 A service, two-thirds of a 200 A — and it enters a load calculation at nameplate, because no special demand factor exists for tankless in NEC 220.82. The calculator checks your service against the manufacturer's recommended minimum and tells you when a load calc is likely to fail. It also debunks the "83% rule" cleanly: that figure belongs to service-conductor sizing (Table 310.12), not water heaters.
One sourced conflict worth knowing: retail listings say "8 AWG" for the Stiebel Tempra 24; the manufacturer's own sheet requires 6 AWG for its 2 × 50 A circuits — the sheet governs. Cold-climate reality check included: incoming water temperature caps flow, so the same unit that runs a Southern home serves one or two Northern fixtures. Reference only; multi-circuit installs and service upgrades are licensed work. Free, no signup.
Tankless Water Heater Breaker Calculator
How many breakers does an electric tankless need — and can your panel take it? Pick your model for the circuit count, wire per circuit, total amps, and an honest service-size check. Free, no signup.
Unit & service
Whole-home electric tankless is the largest load most panels will ever see — the panel, not the plumbing, is usually what decides this purchase.
Calculation Formulas
Whole-home electric tankless is the largest single load most panels ever see.
Example:
24 kW → 100 A at full output.
Each element bank gets its own circuit; sheets specify count, breaker, and wire (75°C copper).
Example:
RTEX-24: 3 × 40 A with 8 AWG each; Tempra 24: 2 × 50 A with 6 AWG.
No special demand factor exists for tankless — 100% of the first 10 kVA plus 40% of the remainder applies to the whole dwelling load.
Example:
Stiebel recommends a 150 A minimum service for the Tempra 24.
The dwelling-service conductor allowance is frequently misapplied to tankless load calcs — two different rules.
Example:
A 200 A service may use 83%-sized service conductors; the heater still enters the load calc at nameplate.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Rheem RTEX-24 / ECO 24 | 100 A · 3 × 40 A · 8 AWG | Current (06/24) manufacturer sheet. |
| Stiebel Tempra 24 | ~100 A · 2 × 50 A · 6 AWG | Mfr sheet says 6 AWG — retail listings saying "8 AWG" are wrong; the sheet governs. |
| RTEX-36 / ECO 36 | 150 A · 4 × 40 A · 8 AWG | The whole panel of many homes. |
| Service minimums (Stiebel) | 150 A (T24) · 200 A (T29) · 300 A (T36) | Manufacturer-recommended minimum services. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
Manufacturer specification sheets(Stiebel Eltron / Rheem / EcoSmart)
View StandardCircuit counts, breaker sizes, and wire gauges come from the published sheets — including the sourced Tempra-24 6-AWG-vs-retail-8-AWG conflict (sheet governs).
Key Requirements:
- •Per-model circuit tables
- •75°C copper
NEC Article 220 — Load calculations(NFPA 70 (2023) 220.82 / 220.83)
View StandardThe optional dwelling method (100% first 10 kVA + 40% remainder) and the existing-dwelling method used to test whether a service can absorb the heater.
Key Requirements:
- •Heater at nameplate; no special factor
NEC Table 310.12 — Dwelling services(NFPA 70 (2023) 310.12)
View StandardThe real home of the "83%" figure: single-phase 100–400 A dwelling SERVICE conductors — not a water-heater rule.
Key Requirements:
- •Applies to whole-dwelling-load conductors only
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Incoming water temperature
The flow-rate reality
Flow (GPM) is capped by temperature rise — the same unit is whole-home in the South and 1–2 fixtures in the North.
Regional Examples:
Housing-stock panels
100 A services
Older homes with 100 A services rarely absorb a whole-home unit; the load calc fails before the plumbing starts.
Regional Examples:
DIY boundary
Multi-circuit + service work
Two to four high-amp circuits and any service upgrade are licensed-electrician territory everywhere.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
Plan disposal before you start
Smaller jobs still produce more debris than a few trash bags can hold. Check what's allowed in a dumpster and which disposal option fits the scope.
See disposal options →
Related Calculators
Breaker Size Calculator
What size breaker do you need? Free NEC calculator from amps or watts — 125% continuous rule, standard 240.6(A) ratings, and the wire it must pair with.
Wire Size Calculator
What gauge wire do you need? Free NEC wire size calculator by amps, wiring method, and run length — with derating, 240.4(D), and appliance presets.
Voltage Drop Calculator
How much voltage do you lose over a wire run? Free NEC K-factor calculator for volts and % drop by wire size, length, and load — with the 3% check.
How to Use This Calculator
- Pick your tankless model — or the closest kW match — from the manufacturer-sheet list.
- Select your electrical service size (read the main breaker at the top of your panel).
- Click Calculate for the circuit count, breaker and wire per circuit, total amps, and the service check.
- If the service check comes back tight, get a real NEC 220.82/220.83 load calculation before buying — not after.
Why the Panel — Not the Plumbing — Decides This Purchase
An electric tankless heats water only while it flows, which sounds efficient until you see the instantaneous demand: heating 2.5 gallons per minute from 55°F to 120°F takes about 24 kW — one hundred amps — delivered the moment a shower starts. That is why manufacturers split the elements across multiple 40–50 A circuits and publish minimum service recommendations (150 A for a Tempra 24, 300 A for a Tempra 36). In a load calculation the heater enters at full nameplate alongside everything else in the house, and on a 100 A service the math almost never closes for whole-home units. The honest decision path: run the calculator here for the screen, then a real 220.82/220.83 load calc; if it fails, the choices are a smaller point-of-use unit, gas, or a service upgrade — licensed work with utility coordination that should be priced into the comparison from the start.
Frequently Asked Questions
How many breakers does an electric tankless water heater need?
Whole-home units need 2–4 double-pole breakers on dedicated circuits: a Rheem RTEX-18 takes 2 × 40 A, the RTEX-24/27 take 3 × 40 A, the RTEX-36 takes 4 × 40 A (each with 8 AWG copper per the spec sheet), and Stiebel's Tempra 24 takes 2 × 50 A with 6 AWG. Only small point-of-use units (RTEX-13 class) run on a single 60 A circuit. Every circuit is dedicated — this is the part that surprises buyers after the box arrives.
Will a 100 amp service run a tankless water heater?
Almost never for a whole-home unit. A 24 kW heater draws ~100 A by itself — the entire service — and it enters a load calculation at nameplate with no special demand factor. Stiebel Eltron recommends minimum services of 150 A for the Tempra 24, 200 A for the 29, and 300 A for the 36. On a 100 A service the realistic options are a point-of-use unit, gas, or a service upgrade. Run a real NEC 220.82/220.83 load calculation before purchasing, not after.
Can a 200 amp panel handle a tankless water heater?
Usually yes for units up to about 27 kW — a 24 kW heater takes roughly half of a 200 A service at full output — but it isn't automatic. The heater enters the NEC 220.82 load calculation at full nameplate, so a house that also runs an electric range, dryer, central AC, and an EV charger can still fail the math. The 36 kW class is the exception: at ~150 A of connected load, Stiebel Eltron recommends a 300 A service for the Tempra 36. Have the load calculation run on your actual panel before ordering, and check the physical spaces too — three or four double-pole breakers need six to eight free slots.
Is there an 83% rule for tankless water heaters?
No — that figure is real but belongs to a different rule entirely. Table 310.12 lets single-phase dwelling SERVICE conductors (100–400 A) be sized at 83% of the service rating because they carry the whole house's diversified load. It is not a demand factor for water heaters: in the 220.82 optional method, a tankless enters at 100% of nameplate within the '100% of the first 10 kVA + 40% of the remainder' framework. Applying 83% to the heater conflates two unrelated rules — a mistake common enough that this calculator calls it out.
What wire does each tankless circuit need?
Per the manufacturer sheets: 8 AWG copper for each 40 A circuit (Rheem RTEX and EcoSmart), 6 AWG for Stiebel's 50 A circuits and for single 60 A point-of-use circuits — all 75°C copper. Note the sourced conflict on the Tempra 24: several retail listings say '8-gauge,' but Stiebel Eltron's own spec sheet requires 6 AWG for its 2 × 50 A configuration, and the manufacturer sheet governs.
Why does my tankless flow drop in winter?
Because flow is capped by temperature rise. A 24 kW unit delivers about 2.5 GPM at a 120°F setpoint when incoming water is 55°F — but Northern groundwater arrives at 40–45°F in winter, demanding more energy per gallon and cutting the deliverable flow. That's why the same unit is 'whole-home' in Georgia and 'one shower' in Minnesota. Check the manufacturer's temperature-rise chart against your local groundwater temperature before sizing — it changes which unit (and how much electrical work) you actually need.