An electric tankless water heater looks compact, but a whole-house model can be one of the largest electrical loads in a home. Depending on the exact heater, installation may require 240V power; one, two, or three dedicated double-pole circuits; several breaker spaces; multiple conductor runs from the panel; roughly 50A to 150A of heater load in common 12–36 kW whole-house examples; a residential load calculation; panel or service work if the existing system cannot support the heater; and permits and inspections. The exact model controls the electrical design.
Before buying the heater, verify:
amps → breaker count → breaker size → wire size → panel space → service capacity → permit path
Electric tankless water heater electrical requirements at a glance
| Requirement | What to verify |
|---|---|
| Voltage | Whole-house models commonly use 240V; some support 208V |
| Amperage | Depends on kW and voltage; common whole-house examples range from about 50A to 150A |
| Breakers | One to several dedicated double-pole breakers |
| Circuits | Dedicated; larger models may require two or three independent circuits |
| Wire size | Exact manufacturer requirement plus applicable code |
| Panel space | Each required double-pole breaker needs panel positions |
| Service capacity | Existing service must support the heater plus the rest of the home |
| Panel upgrade | Sometimes needed, not automatic |
| Permit | Water-heater and new-circuit permits may apply |
| Inspection | Permitted work must be inspected before concealment where required |
These are planning ranges, not installation instructions. Use the exact manufacturer’s installation manual and applicable California/local code for final circuit design.
How many amps does an electric tankless water heater use?
Electric tankless water heaters are rated in kilowatts. At 240V:
amps = watts ÷ volts
Theoretical current at 240V is the starting math, not the circuit design:
| Heater output | Approximate total current |
|---|---|
| 12 kW | 50A |
| 18 kW | 75A |
| 24 kW | 100A |
| 27 kW | 112.5A |
| 36 kW | 150A |
This table shows theoretical heater current only. It is not a breaker-size or wire-size chart. Actual equipment can divide that load across multiple heating sections and multiple branch circuits.
Actual manufacturer example: Stiebel Eltron Tempra. Current specifications show how circuit architecture changes as heater output increases:
| Example model at 240V | Published max amp load | Published circuit arrangement |
|---|---|---|
| Tempra 12 | 50A | 1 circuit |
| Tempra 15 | 2 × 30A | 2 circuits |
| Tempra 20 | 2 × 40A | 2 circuits |
| Tempra 24 | 2 × 50A | 2 circuits |
| Tempra 29 | 3 × 40A | 3 circuits |
| Tempra 36 | 3 × 50A | 3 circuits |
A whole-house electric tankless heater is therefore a model-specific electrical project, not one generic “water heater circuit.”
What size breaker does an electric tankless water heater need?
There is no universal breaker size. The required breaker arrangement comes from the exact heater. Stiebel Eltron’s current Tempra manual, for example, specifies 1 × 50A double-pole breaker for Tempra 12; 2 × 30A double-pole breakers for Tempra 15; 2 × 40A double-pole breakers for Tempra 20; 2 × 50A double-pole breakers for Tempra 24; 3 × 40A double-pole breakers for Tempra 29; and 3 × 50A double-pole breakers for Tempra 36. Smaller units may use one breaker; mid-size whole-house units may use two; large whole-house units may use three. Other manufacturers can use different configurations. Search by manufacturer, exact model, and installation manual — not by kW alone.
Can one large breaker replace several required breakers? No, not when the equipment is designed and listed for multiple independent circuits. If the manual requires two or three supply circuits, follow that circuit architecture.
Does an electric tankless water heater always need a 125% breaker? No universal 125% rule should be applied blindly. Stiebel Eltron’s current Tempra manual states that its recommended overcurrent protection is sized at 100% of load and instructs installers to check local codes for compliance. The correct hierarchy is:
- exact equipment listing;
- manufacturer instructions;
- applicable electrical code;
- local requirements.
A generic internet multiplier should not replace those inputs.
Does an electric tankless water heater need dedicated circuits? Yes. Whole-house electric tankless units generally require dedicated branch circuits according to the equipment design. Stiebel Eltron’s Tempra manual requires one dedicated circuit for Tempra 12; two independent circuits for Tempra 15, 20, and 24; and three independent circuits for Tempra 29 and 36. These circuits should not be shared with general household loads.
How many panel spaces does an electric tankless water heater need? A standard double-pole breaker commonly occupies two panel positions. That means a heater requiring 1 double-pole breaker commonly needs 2 spaces; 2 double-pole breakers commonly need 4 spaces; 3 double-pole breakers commonly need 6 spaces. Panel design matters, so verify the actual panel. This is why a large tankless heater can create a panel-space problem even before service capacity is considered.
What wire size does an electric tankless water heater need?
There is no universal wire size. Required conductor size depends on the exact model, breaker size, number of circuits, conductor material, terminal limitations, wiring method, run length and voltage drop, and applicable code. Stiebel Eltron’s Tempra specifications illustrate the point: as heater output rises, the published installation uses multiple copper conductor sets rather than one oversized cable.
A large tankless heater may need several separate conductor runs, not one larger wire.
Do not size conductors from a generic “tankless water heater wire size” chart.
Does an electric tankless water heater need 240V? Many whole-house residential electric tankless water heaters use 240V. Some whole-house equipment can also operate at 208V, usually at reduced wattage. Point-of-use models can be different. A.O. Smith currently lists a 120V, 2.4 kW point-of-use electric tankless water heater at 20A with a 20A breaker and #12 AWG wire. So “electric tankless = always 240V” is false. Point-of-use units may be 120V, 208V, 240V, or another listed voltage; whole-house residential units commonly use 240V. Always check the exact model.
Do not apply point-of-use electrical requirements to a whole-house heater. Point-of-use units are designed for a limited fixture or application such as one lavatory, a utility sink, or a remote handwashing location, and electrical demand can be much smaller. Whole-house units are designed to support much larger hot-water demand. As required output rises, so do kW, total current, breaker count, conductor count, panel-space requirements, and service-capacity concerns. A heater being physically small does not mean its electrical requirement is small.
What size electrical service does an electric tankless water heater need?
There is no universal rule such as “electric tankless requires 200A.” The heater must be evaluated with the rest of the home. Relevant loads include HVAC, an electric range, an electric dryer, an EV charger, water heating, lighting and receptacle load, and other fixed appliances. The correct service-size question is answered with the exact heater and a residential load calculation.
Can a 100A service handle an electric tankless water heater? Possibly for a small unit. A large whole-house model can be impractical on 100A service because the heater alone may approach or exceed the scale of the entire service rating. That does not mean every 100A home is automatically disqualified. It means do not buy a large whole-house electric tankless heater before the service is evaluated.
Can a 125A service handle an electric tankless water heater? Sometimes. A 125A service provides more capacity than 100A, but a large tankless heater can still consume a substantial share of available service capacity. The answer depends on the heater, HVAC, cooking, dryer, EV charging, and other major loads.
Is 200A service enough for an electric tankless water heater? Sometimes. Not for every heater. In Stiebel Eltron’s current Tempra manual, footnotes to the electrical data state that Tempra 24 requires a 150A main service, Tempra 29 requires a 200A main service, and Tempra 36 requires a 300A main service. Those statements apply to those specific models. They show why 200A service is not automatically enough for every large whole-house electric tankless heater.