What Size Wire for a Tankless (Electric) Water Heater?
Wire and breaker sizing for a Tankless (Electric) Water Heater — the typical 240V circuit, plus the code details (NEC 310.16) that change the answer.
Calculations are built on the 2023 National Electrical Code (NEC) and cross-checked against the published code tables. How we verify. Last reviewed June 2026.
On a 50A 240V circuit at 75°C terminations. 60°C terminations: 6.
Protected by a 50A breaker (NEC 240.6).
Tankless Water Heater current varies by model. The size above is the common case — always size the conductor to the unit’s nameplate rating and protect it at the labeled maximum breaker/fuse.
Tankless Water Heater Circuit Details
- Whole-house electric tankless heaters draw 80–150A total (18–36 kW) and usually need two or three dedicated circuits, not one.
- A common 27 kW unit needs three 40A circuits (8 AWG copper each) or equivalent.
- Always wire to the manufacturer’s circuit table — this is one of the largest residential loads.
Long run to your tankless water heater?
Over distance, voltage drop can require a larger wire than the table above. See the 50A wire size by distance:
Size it for your exact install
Open the calculator pre-filled with 50A at 240V to add distance, material and temperature.