What Size Wire for a Tankless Water Heater?

It depends on the kilowatt rating, and most whole-house electric tankless heaters are large enough to need more than one circuit. The unit is a continuous load, so each circuit is sized at 125% of its share of the current. A common 18 kW / 240V unit draws about 75A and is typically wired as three 40A circuits of ${ON_40}, or two 50A circuits of ${ON_50} (each circuit at least 125% of its share); a 24 kW unit draws about 100A and usually needs three circuits of ${S24.wire}. Small point-of-use units (3-6 kW) fit a single 30A / #10 circuit. The install manual specifies the exact number of circuits and the breaker size for each, and it governs.

Divide the kilowatts by the voltage: an 18 kW unit at 240V draws 75A (18,000 / 240), a 24 kW unit draws 100A, and a 27 kW unit draws about 112A. Because it is a continuous load, the total circuit ampacity must be 125% of that. Whole-house electric tankless heaters commonly land between 80A and 150A total, which is why they are split across multiple breakers and why many homes need a service-capacity check or upgrade before adding one.

Because the total current exceeds what a single practical residential circuit and conductor can carry. Rather than run one very large feeder, manufacturers split the load into two, three, or four equal double-pole circuits, each with its own breaker and conductor sized at 125% of its share. This keeps the wire at a manageable gauge (often #8 or #6 per circuit) and matches the heater's internal element banks. Wire each circuit exactly as the manual's chart specifies.

Often not without a check. An electric tankless can add 80-150A of load, so on a 100A or 150A service it frequently forces a load calculation and sometimes a service upgrade. Run a dwelling load calculation (NEC 220.83) including the tankless draw before committing. A gas tankless avoids this entirely, needing only a small 120V circuit for the controls and igniter.

Small point-of-use electric tankless heaters (for a single sink, typically 3-6 kW) draw 12-25A and usually fit one dedicated circuit: a 3.5 kW / 240V unit is about 15A on a 20A circuit with #12, and a 6 kW unit is 25A on a 30A circuit with #10. A single-fixture point-of-use heater cycles briefly and is generally treated as a non-continuous load, so the 125% continuous sizing that governs a whole-house unit does not force the circuit up a size; still, confirm against the nameplate, which gives the minimum circuit ampacity. These small units are the exception; whole-house units are the multi-circuit case above.