How Many Amps Does a Tankless Electric Water Heater Use?

Tankless Electric Water Heater amperage at 120 V and 240 V

Same 18,000 W load: the current halves when the voltage doubles. Tankless Electric Water Heater is typically a 240 V appliance in US homes.

Breaker and circuit for a tankless electric water heater

Tankless Electric Water Heater runs on its own dedicated 240 V circuit: plan on two or three 40 A double-pole circuits (8 AWG copper each), per the manufacturer. Large tankless heaters are continuous loads split across multiple circuits: 18 kW needs roughly 94 A of 240 V capacity at 125%, never a single branch circuit.

Guidance only: actual circuit sizing depends on your unit's nameplate, wire run length, and local code. Confirm with a licensed electrician. See the wire & breaker size chart for the full NEC ampacity table.

Measure what a tankless electric water heater actually draws

The figures above are typical ratings, and this load is usually hardwired or on a 240 V circuit, so a plug-in meter can't reach it. A clamp meter reads the current on one conductor at the disconnect or panel without breaking the circuit; opening a panel is work for someone qualified, and the voltage pen confirms a circuit is dead first.

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Clamp meter

Fluke 323

Best for: Measuring what a hardwired circuit actually draws, even in a crowded panel

Check price on Amazon: Fluke 323 (opens in a new tab)(paid link)

Non-contact voltage tester

Klein Tools NCVT-3P

Best for: Confirming a circuit is dead before you open a device or box

Check price on Amazon: Klein Tools NCVT-3P (opens in a new tab)(paid link)

Frequently asked questions

Amp draw of other appliances

• Electric Water Heater17 A
• Well Pump8.3 A
• Sump Pump6.7 A
• Heat-Pump Water Heater2.3 A