What Size Breaker for Tankless Water Heater?
A tankless water heater requires a 40A breaker with 8 AWG wire on a 240V multiple dedicated circuit. This is a continuous load — breaker rated at 125% of actual draw.
🔌 Wire Length & Voltage Drop Check
Tankless Water Heater Wiring Guide — NEC Requirements
Whole-house electric tankless water heaters typically require 2-4 separate 40A/240V circuits (total 80-160A). This is a massive electrical load — verify your panel has sufficient capacity.
Pro Tip
💡 Most tankless units need a 200A panel minimum. A single-point unit for one sink may only need one 30A circuit.
Why a 40A Breaker?
The tankless water heater draws approximately 120A at 240V (28800W). As a continuous load (3+ hours), NEC 210.20 requires the breaker at 125%: 120A × 1.25 = 150.0A → 40A breaker.
Wire Sizing per NEC 310.16
For a 40A/240V circuit, 8 AWG copper is required per NEC Table 310.16 at 75°C. For runs over 75 feet, upsize to 6 AWG. Use our Wire Size Calculator and Voltage Drop Calculator.
Complete Parts List
To wire a tankless water heater circuit: a 40A double-pole breaker, 8 AWG NM-B cable (Romex) for interior runs or THHN/THWN-2 in conduit for exposed runs, a NEMA 14-50 receptacle (or hardwired), cable clamps, and appropriate junction boxes.
Disclaimer: This guide is for reference only. All electrical work must be performed by or verified by a licensed electrician. Local codes may differ from NEC.