Electric tankless water heater electrical requirements: breakers, wire and service size
Electric tankless heaters are cheap to buy and expensive to feed. The cost of the appliance is rarely the problem; the cost of getting 112 amps to it frequently is.
The numbers, straight
Divide watts by volts. A 27 kW unit at 240 V draws 112.5 A, split across three 40 A double-pole breakers on three runs of 8 AWG copper. An 18 kW unit draws 75 A on two breakers; a 36 kW unit draws 150 A on four. Any of these needs a 200 A service minimum, and a load calculation under NEC Article 220 to confirm there is capacity spare.
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Electric tankless heaters have a genuine advantage: no venting, no gas line, no combustion, and a unit price that undercuts gas by a wide margin. Then you look at what it takes to feed one.
The arithmetic
Ohm's law, essentially. Amps = watts ÷ volts.
A 27,000 watt heater at 240 volts draws 112.5 amps. That is not spread over time or averaged — it is the instantaneous draw whenever the unit is running at full output, which is whenever someone is taking a shower in winter.
| Unit rating | Amps at 240 V | Breakers | Wire (Cu, 75 °C) | Panel spaces |
|---|---|---|---|---|
| 11 kW | 45.8 A | 1 × 60 A | 6 AWG | 2 |
| 18 kW | 75 A | 2 × 40 A | 8 AWG | 4 |
| 24 kW | 100 A | 3 × 40 A | 8 AWG | 6 |
| 27 kW | 112.5 A | 3 × 40 A | 8 AWG | 6 |
| 36 kW | 150 A | 4 × 40 A | 8 AWG | 8 |
Two things fall out of that table immediately. The panel-space column is a real constraint — six spaces is a lot on a panel that is already full, and finding them often means a subpanel. And the amp column, on a typical 200 A residential service, is over half your capacity going to one appliance.
The load calculation is the gate
This is the step that stops projects, and it happens after the heater has been delivered more often than it should.
NEC Article 220 sets out how to calculate a dwelling's demand load. In simplified terms you total the general lighting and receptacle load, add the fixed appliances, apply the permitted demand factors, and compare the result against the service rating. A 27 kW water heater enters that calculation at its full nameplate value — there is no demand factor that discounts it, because it genuinely can draw 112.5 A at any moment.
A worked sketch for a 2,000 sq ft house on a 200 A service:
| Load | Demand |
|---|---|
| General lighting and receptacles | ~10,000 W after demand factors |
| Electric range | 8,000 W |
| Electric dryer | 5,000 W |
| Air conditioning | 6,000 W |
| Subtotal | 29,000 W ≈ 121 A |
| 27 kW tankless heater | 27,000 W ≈ 112.5 A |
| Total | ≈ 233 A |
That house does not have capacity, and an inspector will say so. The options are a service upgrade to 400 A — commonly $4,000 to $8,000 including the utility's work — or a smaller heater, or gas.
This is why the honest comparison between an electric tankless heater and a gas one is not $600 against $1,400. It is $600 plus possibly $5,000 of electrical work, against $1,400 plus $800 of venting.
Wire sizing, and the two things that get it wrong
8 AWG copper on a 40 A circuit is the default answer, taken from the NEC ampacity tables at the 75 °C terminal rating. It is correct for most installations and it is what the manufacturer's manual will say.
Two situations change it.
Long runs. Voltage drop matters on high-current circuits. The usual target is under 3%, and on a 40 A circuit in 8 AWG copper you reach that at roughly 50 feet. Beyond that, upsize to 6 AWG. A heater at the far end of a long ranch house is exactly this case, and undersized conductors on a long run are the reason a lot of "my breaker keeps tripping" complaints exist.
Aluminium conductors. Aluminium has lower conductivity, so a 40 A circuit in aluminium needs 6 AWG rather than 8. Aluminium also requires antioxidant compound at terminations and connectors rated for it. This is not a place to improvise.
What actually goes wrong after installation
Loose terminations. The single most common fault. A connection carrying 37.5 amps that is not torqued to specification develops resistance, resistance develops heat, and heat degrades the connection further. It is a runaway process and it is the mechanism behind most panel fires attributable to a specific appliance. Terminations should be torqued to the manufacturer's figure with a torque screwdriver, and re-checked after a few weeks of use.
Element failure. Electric tankless heaters have multiple resistance elements staged by a flow sensor. They are consumables in hard water — scale forms on the element, insulates it, and it overheats. Annual descaling is not optional in water above about 10 grains per gallon.
Undersized supply. A unit that works fine in September and struggles in February was never undersized electrically; it was undersized thermally. Colder incoming water means a larger temperature rise and less flow. That is a sizing problem rather than a wiring one, and how to size a tankless water heater covers it.
When electric tankless is actually the right call
We have been fairly negative, so it is worth being clear about where these units make sense.
Point-of-use applications. A 6 to 11 kW unit serving one remote bathroom or a workshop sink is genuinely excellent — one breaker, no venting, instant hot water at the fixture, and no long pipe run wasting water while it warms up.
Homes with no gas supply where propane delivery is impractical. Electric resistance is expensive per BTU, but it is available everywhere.
New builds, where the service can be specified at 400 A from the outset and the incremental cost is a few hundred dollars rather than a retrofit.
Households with substantial solar generation. The running-cost objection is a price objection, and it weakens considerably when the electricity is coming off your own roof.
For whole-home service in an existing house on a 200 A panel, gas is usually the practical answer — and if that is the direction, our APUS tankless water heater review covers a unit that skips the venting cost by mounting outdoors. The direct gas-versus-electric comparison is in APUS against EcoSmart.
Reference: the NFPA National Electrical Code is the governing document, and your local jurisdiction may amend it. Nothing here substitutes for a licensed electrician and a permit.
Questions people actually ask
What size breaker for a 27 kW tankless water heater?
Three 40-amp double-pole breakers, one per heating element bank. The unit's total draw is 112.5 A at 240 V, divided across three internal circuits of roughly 37.5 A each, and 40 A is the standard breaker for that. Confirm against the specific unit's data plate — element staging varies between manufacturers.
What wire gauge for an electric tankless water heater?
8 AWG copper is the standard for 40 A circuits under the NEC 75 °C column, and that covers most 18 to 36 kW units on runs under about 50 feet. Longer runs need upsizing for voltage drop, and aluminium conductors need to go up roughly two sizes. Your inspector will want to see the manufacturer's installation manual, which specifies this.
Can I run an electric tankless water heater on a 100 amp service?
Realistically, no, not for whole-home use. A 100 A service cannot support 112.5 A of water heating plus a range, a dryer, air conditioning and lighting. A small point-of-use unit under 6 kW is feasible. Anything sized for a shower needs a service upgrade first, and that is typically $2,500 to $5,000.
Do electric tankless water heaters need a dedicated circuit?
Multiple dedicated circuits — one per element bank. There is no sharing, no subpanel shortcuts and no tapping an existing range circuit. A 27 kW unit occupies six panel spaces, which on a full panel means a subpanel before you have run a single wire.
Why does my electric tankless heater trip the breaker?
Three common causes. Undersized conductors heating up and the breaker doing its job. A loose termination at the breaker or the unit, which creates resistance and heat — this is the most common and it is why terminations should be re-torqued after the first few weeks. Or a failed heating element drawing more than its rated current.
Is an electric tankless heater cheaper to run than gas?
Almost never in the United States, on current energy prices. Electric resistance heating is 100% efficient at the appliance but electricity costs roughly three to four times as much per delivered BTU as natural gas. The exception is a household with substantial solar generation and net metering, where the calculation changes entirely.