What Size Breaker for a Water Heater?
A standard 4,500-watt electric water heater draws 18.75 A at 240 V; NEC 422.13 treats it as a continuous load requiring 125% sizing (23.4 A), so a 25- or 30-amp double-pole breaker with 10 AWG copper is standard.
Water heater wattage to breaker size
Electric storage (tank) water heaters run on a dedicated 240-volt circuit with a double-pole breaker sized at 125% of the element current. The table below covers the common residential element wattages. Each row applies the same NEC math: amps = watts ÷ 240 V, then multiply by 1.25 and round up to the next standard breaker size in NEC 240.6(A).
| Element wattage | Current draw | ×125% (NEC 422.13) | Breaker | Copper wire |
|---|---|---|---|---|
| 3,000 W | 12.5 A | 15.6 A | 20 A double-pole | 12 AWG |
| 3,800 W | 15.8 A | 19.8 A | 20 A double-pole | 12 AWG |
| 4,500 W | 18.75 A | 23.4 A | 25 or 30 A double-pole | 10 AWG |
| 5,500 W | 22.9 A | 28.6 A | 30 A double-pole | 10 AWG |
A 3,000-watt water heater needs a 20-amp double-pole breaker on 12 AWG copper. A 3,800-watt water heater also fits a 20-amp double-pole breaker on 12 AWG copper (19.8 A after the 125% factor). A 4,500-watt water heater — the most common residential size — needs a 25- or 30-amp double-pole breaker on 10 AWG copper. A 5,500-watt water heater needs a 30-amp double-pole breaker on 10 AWG copper. Dual-element tanks are wired non-simultaneous by default, so only one element heats at a time and the larger element rating sets the circuit size.
The 125% rule, worked out
NEC 422.13 requires the branch circuit for a fixed storage water heater of 120 gallons or less to be sized at not less than 125% of the appliance's nameplate rating — the code treats it as a continuous load. Here is the full calculation for the standard 4,500-watt tank:
- Current draw: 4,500 W ÷ 240 V = 18.75 A.
- Continuous-load factor: 18.75 A × 1.25 = 23.4 A minimum circuit capacity.
- Breaker: the next standard size at or above 23.4 A per NEC 240.6(A) is 25 A; in practice a 30 A breaker is most common because 25 A breakers are rarely stocked. Both are code-compliant on 10 AWG copper.
- Wire: 10 AWG copper is rated 30 A, matching the breaker.
Note that a 20-amp breaker is not compliant for a 4,500 W element: 23.4 A exceeds 20 A, even though the raw draw of 18.75 A does not. This is the single most common water heater sizing mistake.
Tankless water heaters need far bigger circuits
Whole-house electric tankless water heaters draw 18,000 to 36,000 watts — four to eight times a tank heater — and typically require two to four separate 40- or 50-amp double-pole breakers feeding independent heating banks. The table shows typical manufacturer requirements.
| Tankless rating | Total draw | Typical breakers | Typical copper wire |
|---|---|---|---|
| 18 kW | 75 A | 2 × 40 A double-pole | 2 runs of 8 AWG |
| 24 kW | 100 A | 2 × 50 A double-pole | 2 runs of 6 AWG |
| 27 kW | 113 A | 3 × 40 A double-pole | 3 runs of 8 AWG |
| 36 kW | 150 A | 4 × 40 A double-pole | 4 runs of 8 AWG |
Always follow the manufacturer's installation sheet for a tankless unit — breaker count and size are model-specific. Many 100- and 125-amp panels cannot support a whole-house tankless heater at all without a service upgrade, so run a load calculation before buying one.
Heat pump water heaters
A hybrid heat pump water heater typically installs on the same 30-amp, 240-volt circuit as the tank heater it replaces, because its backup resistance elements are still 4,500 W even though the heat pump itself draws under 5 amps. Some 120-volt plug-in heat pump models need only a 15-amp shared circuit — check the nameplate; the compressor-only models are dramatically lighter loads than resistance tanks.
Safety first
This page is educational reference material. Water heater circuits combine 240 volts, panel work, and bonding requirements, and most jurisdictions require a permit for water heater replacement. Fixed electrical installation work must be carried out by a licensed electrician in accordance with the applicable local wiring code (e.g. NEC/NFPA 70, BS 7671, AS/NZS 3000, IEC 60364).
Frequently asked questions
What size breaker do I need for a 4500-watt water heater?
A 25- or 30-amp double-pole breaker on 10 AWG copper wire. The heater draws 18.75 A at 240 V, and NEC 422.13 requires sizing at 125% (23.4 A), which rules out a 20-amp breaker. A 30-amp breaker is the most common choice in practice.
Can I use a 20-amp breaker for a water heater?
Only for elements of 3,800 watts or less. A 3,800 W element works out to 19.8 A after the NEC 125% factor, which just fits a 20-amp double-pole breaker on 12 AWG copper. The common 4,500 W element requires 23.4 A of circuit capacity and needs a 25- or 30-amp breaker.
Why does a water heater need a double-pole breaker?
Because it runs on 240 volts, which uses both hot legs of a residential panel. A double-pole breaker connects to both legs and disconnects both simultaneously, which is required for 240 V appliance circuits.
What wire size do I need for a hot water heater?
10 AWG copper for the standard 4,500-5,500 W heater on a 25- or 30-amp breaker, or 12 AWG copper for 3,000-3,800 W elements on a 20-amp breaker. Tankless heaters need multiple runs of 8 or 6 AWG — follow the manufacturer sheet.
Does a water heater need its own dedicated circuit?
Yes. An electric water heater is a fixed appliance served by a dedicated 240-volt branch circuit sized per NEC 422.13 at 125% of nameplate. Nothing else should share the circuit.
What size breaker does an electric tankless water heater need?
Far more than a tank: typical whole-house units from 18 kW to 36 kW draw 75 to 150 amps total and require two to four separate 40- or 50-amp double-pole breakers per the manufacturer instructions. Many older panels need a service upgrade first.
Related tools & references
- Breaker size calculator
- Wire size calculator
- Electrical load calculator
- Wire color codes reference
- Electric water heater wiring diagram
- Breaker box wiring guide
Last verified: 2026-07-10