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Breaker and Circuit Size for Wall-Mount Tankless Heaters

What electrical circuit and breaker size do I need for an instant wall-mounted electric water heater?

The correct circuit and breaker size for an instant (tankless) wall-mounted electric water heater depends on the unit’s rated power (kW) and required voltage (most whole-home and higher-output models are 240V). Because these heaters draw a large load while heating, they’re typically installed on one or more dedicated 240V circuits—each protected by its own correctly sized breaker.

Step 1: Find the heater’s electrical rating

Check the nameplate or manual for voltage and amperage, or for kW. If you only have kW, estimate amps with: Amps = (Watts ÷ Volts). For example, a 9 kW heater at 240V is about 9,000 ÷ 240 = 37.5 amps.

Step 2: Size the breaker for a continuous load

Instant electric water heaters are commonly treated as continuous loads for sizing, so the circuit is often sized at 125% of the heating current (follow the manufacturer instructions and local code). Using the 9 kW example: 37.5A × 1.25 ≈ 46.9A, which typically rounds up to a 50A breaker.

Common breaker sizes you’ll see

Many compact point-of-use units land in the 30A–50A range, while higher-flow models can require 60A, 80A, or multiple breakers (for example, two 40A or two 50A circuits). Some units are designed for 120V with smaller breakers, but those are usually for low-flow, single-fixture use.

Don’t overlook wire size and installation requirements

The breaker must match the conductor size and temperature rating. As a quick reference, copper conductors are often paired like this in typical residential conditions: 30A with 10 AWG, 40A with 8 AWG, 50A with 6 AWG, and 60A with 6 or 4 AWG depending on insulation rating and run conditions. Your heater may also require a local disconnect switch, proper grounding/bonding, and GFCI protection depending on location and code.

For a deeper walkthrough on selecting and installing an instant wall-mount electric heater, see the full guide here: https://hovira.com/blog/guide-instant-wall-mount-electric-water-heater-lcd-control-guide/.

FAQ

Do instant electric water heaters need a dedicated circuit?

Yes, most instant electric water heaters require one or more dedicated circuits because their heating elements draw high current. Sharing the circuit can cause nuisance tripping, overheating, or code violations.