A 27 kW electric tankless water heater needs three 40-amp double-pole breakers on three separate 240V circuits. A 36 kW unit needs four 40-amp double-pole breakers. The exact breaker size depends on the unit's kW rating, the number of heating elements, and the wire gauge you run — all governed by NEC Article 422 and Article 240.
Getting this wrong creates a fire hazard (undersized breaker/wire) or nuisance tripping (wrong breaker rating). Here is the complete calculation method and a reference table for every common unit size.
The Breaker Sizing Formula
The NEC requires that the breaker rating be at least 125% of the continuous load for appliances that run for 3+ hours. Tankless water heaters are classified as continuous loads because they can run indefinitely when a faucet is open.
Step 1: Calculate amperage per circuit
Amps = (kW per element group x 1,000) / Voltage
Most electric tankless units split their heating elements into groups, each on its own circuit. A 27 kW unit with three element groups draws 9 kW per group: 9,000 / 240 = 37.5 amps per circuit.
Step 2: Apply the 125% rule
Breaker rating = Amps x 1.25
37.5 x 1.25 = 46.9 amps. Round up to the next standard breaker size: 50 amps.
However, most manufacturers specify the breaker size in their installation manual, and the manual takes precedence. For many 27 kW units, the manufacturer specifies 40A breakers because the element groups are designed to draw slightly under 32A each (within the 40A breaker's continuous rating of 32A at 80%).
Always follow the manufacturer's installation manual for breaker sizing. The NEC sets minimum requirements, but the manufacturer knows the exact element configuration and may specify a different breaker size. If the manual says 40A, use 40A — even if your calculation suggests 50A.
Breaker Size Reference Table
Double-pole breakers use 2 slots in your electrical panel (one for each hot leg of the 240V circuit). A 36 kW unit needing four double-pole breakers requires 8 open slots in your panel. Many older panels have only 20–30 total slots, and most are already occupied. Check available slots before purchasing a large electric tankless.
Popular Model Breaker Requirements
Here are the manufacturer-specified breaker sizes for the most popular electric tankless units:
Panel Capacity Assessment
Before installing, verify your panel can support the additional load.
Step 1: Check Your Panel Rating
Look at the main breaker at the top of your panel. Common ratings: 100A, 125A, 150A, 200A, or 400A.
Step 2: Calculate Existing Load
Add up the breaker ratings of all existing circuits. This gives a rough maximum load, but actual demand is lower because circuits never all draw maximum simultaneously. An electrician can calculate the actual demand load per NEC Article 220.
Rule of thumb: Existing demand in a typical home is 80–120A for a 200A panel. A 36 kW tankless adds 150A of potential demand. If your existing load plus 150A exceeds 200A, you need a panel upgrade or a demand management strategy.
Step 3: Check Available Slots
Count the empty breaker slots in your panel. A 36 kW unit needs 8 slots. A 27 kW unit needs 6 slots. If your panel is full, you need a sub-panel ($300–$600) or main panel upgrade ($1,500–$3,000).
A 200A panel is the practical minimum for whole-home electric tankless (24+ kW). If you have a 100A or 125A panel, your only options are: (1) a small point-of-use unit (8–13 kW), (2) a panel upgrade to 200A ($1,500–$3,000), or (3) switching to gas tankless instead.
Common Mistakes to Avoid
Mistake 1: Using the Wrong Breaker Type
Tankless water heaters require double-pole breakers, not single-pole. A double-pole breaker connects both hot legs of a 240V circuit. Using a single-pole breaker on a 240V circuit creates a dangerous half-circuit that can damage the unit and create shock hazard.
Mistake 2: Sharing Circuits
Each heating element group needs its own dedicated circuit from the panel to the unit. Never share a tankless circuit with another appliance. The load is too high — a shared circuit trips constantly and can overheat wiring.
Mistake 3: Undersizing Wire for Long Runs
The breaker table above assumes wire runs under 50 feet. For longer runs, voltage drop requires upsizing the wire. At 50–75 feet, go up one wire size (from #8 to #6 AWG). At 75–100 feet, go up two sizes (from #8 to #4 AWG). See the wire size calculator article for detailed tables.
Mistake 4: Not Accounting for Voltage Drop
A 5% voltage drop at 240V means the unit receives only 228V. Power output drops proportionally: a 27 kW unit at 228V delivers only 24.3 kW — 10% less hot water. Keep wire runs short and use adequately sized wire.
Mistake 5: Forgetting the 80% Rule
Circuit breakers are rated for 80% continuous load. A 40A breaker handles 32A continuously. A 50A breaker handles 40A continuously. This is already factored into the manufacturer's specifications, but DIYers sometimes select a breaker based on the full rating rather than the 80% continuous rating.
Real-World Wiring Examples
Example 1: EcoSmart ECO 27 — Standard Install
Panel: 200A, 40 slots (12 available). Existing load: 95A.
Requirements: Three 40A double-pole breakers (6 slots). Three runs of #8 AWG copper (25 feet each from panel to unit in utility closet).
Process: Electrician installs three double-pole 40A breakers, runs three #8 AWG NM-B cables through the wall, connects at the unit's terminal block. Total time: 3 hours. Cost: $650 (labor $400, materials $250).
Example 2: EcoSmart ECO 36 — Long Run
Panel: 200A in basement. Unit location: 2nd floor bathroom (60-foot wire run).
Requirements: Four 40A double-pole breakers (8 slots). Due to 60-foot run, wire upsized from #8 to #6 AWG to limit voltage drop.
Process: Four runs of #6 AWG through floor joists and wall cavities. Required drilling multiple joists and using nail plates. Total time: 6 hours. Cost: $1,200 (labor $700, wire/materials $500).
Example 3: Panel Upgrade Required
Panel: 100A, 20 slots (2 available). Desired unit: EcoSmart ECO 27 (needs 6 slots, 113A).
Solution: Upgraded main panel from 100A to 200A/42-space. New panel provided ample slots and amperage.
Cost: Panel upgrade: $2,200. Tankless wiring: $600. Total electrical: $2,800. This is why checking panel capacity before purchasing is critical.
Key Takeaways
- Most common configuration: Three 40A double-pole breakers for 24–27 kW units
- 36 kW units need four 40A double-pole breakers — 8 panel slots and 150A capacity
- Always follow the manufacturer's installation manual — it specifies exact breaker and wire sizes
- A 200A panel is the minimum for whole-home electric tankless (24+ kW)
- Each circuit must be dedicated — never share a tankless circuit with other appliances
- Upsize wire for runs over 50 feet to prevent voltage drop
- Budget $300–$600 for standard wiring or $1,500–$3,000 if a panel upgrade is needed
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