A whole-home electric tankless water heater is an enormous electrical load — a 24–36 kW unit needs two to four dedicated 240 V double-pole circuits (typically 40–60 A each) totaling roughly 100–150 A of breaker space. The wire is usually 6 or 8 AWG copper per circuit. The catch isn't the wire — it's the panel: a 27–36 kW unit often needs a 200 A (or larger) service and several open double-pole slots, which many homes don't have. Always size from the specific model's spec sheet — it lists the exact circuits, breakers, and wire.
Going electric tankless? The tankless breaker calculator looks up the circuits, breakers, wire, and recommended service size by model — free, no signup. This guide explains why these units are really a panel question, not a plumbing one. (For the appliance's hot-water sizing, see the tankless sizing calculator — that's a different question.)
🔌 Multiple big circuits, not one
Unlike a tank water heater on a single 30 A circuit, a whole-home electric tankless unit splits its huge draw across several dedicated 240 V circuits. A typical breakdown:
- 13 kW (point-of-use) — one 60 A circuit, 6 AWG copper
- 18 kW — two 40 A circuits, 8 AWG copper
- 24 kW — three 40 A circuits (or two 50 A), 8–6 AWG copper
- 27–36 kW — three to four 40–50 A circuits
Add those up and a 27 kW unit is pulling well over 100 A — as much as a small home's entire service. Each circuit is its own home run to the panel.
🏠 The panel is the real constraint
The wire and breakers are straightforward once you know the model. What stops most electric-tankless projects is the service. A 27–36 kW unit commonly needs a 200 A service — sometimes 300 A — plus three or four free double-pole breaker slots. A 100 or 150 A panel that's already serving a house often can't absorb it.
Run the load calc first. Before buying an electric tankless unit, do an electrical load calculation to confirm your service can carry it. This is the number-one reason these installs stall — the plumbing is easy, the 150 A of new load is not. A gas tankless or a heat-pump water heater is often the practical answer in a smaller panel.
📄 Follow the spec sheet
Every manufacturer publishes the exact electrical requirements — number of circuits, breaker amps, and wire gauge — for each model, and those govern over any general rule. There are real differences: some makers treat the unit as a non-continuous load (overcurrent at 100% of the load rather than 125%), and wire specs vary by model.
Watch for conflicts between retail listings and the manufacturer sheet — a listing might say “8 AWG” where the maker's document specifies 6 AWG for its 50 A circuits. The manufacturer spec sheet always wins. The calculator uses the published per-model data.
✅ The bottom line
An electric tankless water heater needs multiple big dedicated 240 V circuits — often 100–150 A total — and its real gatekeeper is your panel: many units require a 200 A+ service and several open double-pole slots. Size from the model's spec sheet (circuits, breakers, wire), and run a whole-house load calculation before you commit.
This is a planning tool; adopted NEC editions, model specs, and local amendments vary. Look up your model with the tankless breaker calculator, confirm the service with the load calculator, and have a licensed electrician verify the install.
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