Most electric tank water heaters (30–80 gallons, up to 5,500W) require 10 AWG copper wire on a 30-amp, 240V circuit. Tankless electric water heaters are different — they're power-hungry units that require multiple circuits, typically 2–4 separate runs of 6 AWG or 8 AWG wire, depending on the kW rating.
Below you'll find complete wire sizing charts for every common water heater type, from compact 20-gallon tanks to whole-house tankless units.
Quick Answer: Water Heater Wire Size Chart
Tank Water Heaters (Standard Electric)
| Tank Size | Wattage | Amps at 240V | Wire Gauge | Breaker | Cable Type |
|---|---|---|---|---|---|
| 20–30 gallon | 3,000–3,500W | 12.5–14.6A | 12 AWG | 20A DP | 12/2 NM-B |
| 30–40 gallon | 4,000–4,500W | 16.7–18.75A | 10 AWG | 30A DP | 10/2 NM-B |
| 40–50 gallon | 4,500W | 18.75A | 10 AWG | 30A DP | 10/2 NM-B |
| 50–65 gallon | 4,500–5,500W | 18.75–22.9A | 10 AWG | 30A DP | 10/2 NM-B |
| 65–80 gallon | 5,500W | 22.9A | 10 AWG | 30A DP | 10/2 NM-B |
| 80+ gallon (commercial) | 5,500–6,000W | 22.9–25A | 10 AWG | 30A DP | 10/2 NM-B |
The 4,500W / 30A combination covers 90%+ of residential tank water heaters. Even large 80-gallon tanks typically use 4,500W or 5,500W elements, which stay comfortably under the 30A breaker limit. 10 AWG wire on a 30A circuit is the near-universal standard for tank water heaters.
Tankless Water Heaters (Electric)
| Model kW Rating | Amps at 240V | Number of Circuits | Wire Gauge (each) | Breaker (each) |
|---|---|---|---|---|
| 8–11 kW | 33–46A | 1 | 6 AWG | 50–60A DP |
| 13–14 kW | 54–58A | 2 | 8 AWG | 40A DP |
| 18 kW | 75A | 2 | 8 AWG | 40A DP |
| 21–24 kW | 88–100A | 2–3 | 8 AWG | 40A DP |
| 27 kW | 113A | 3 | 8 AWG | 40A DP |
| 29–36 kW | 121–150A | 3–4 | 6–8 AWG | 40–50A DP |
Heat Pump Water Heaters
| Type | Wattage | Amps at 240V | Wire Gauge | Breaker | Notes |
|---|---|---|---|---|---|
| Hybrid (heat pump + backup) | 500–4,500W | 2.1–18.75A | 10 AWG | 30A DP | Backup element same as tank |
| Heat pump only mode | 500–600W | 2.1–2.5A | 10 AWG | 30A DP | Very low draw |
Why Most Tank Water Heaters Use 10 AWG / 30A
The Math Behind the Standard
Standard residential water heater elements are either 4,500W or 5,500W. Here's the load calculation:
4,500W element:
- Amps = 4,500W ÷ 240V = 18.75A
- Continuous load (water heaters run 3+ hours): 18.75A × 1.25 = 23.4A
- Required breaker: minimum 25A, standard 30A
- Required wire: 10 AWG (rated 30A at 60°C)
5,500W element:
- Amps = 5,500W ÷ 240V = 22.9A
- Continuous load: 22.9A × 1.25 = 28.6A
- Required breaker: minimum 30A, standard 30A
- Required wire: 10 AWG (rated 30A at 60°C) — marginal but acceptable
Why not larger elements? Residential electrical panels and typical wiring can't efficiently support elements much larger than 5,500W without requiring 40A+ circuits. Since water heaters heat in cycles (not continuously), 4,500W provides adequate recovery for most households.
Upper vs. Lower Elements
Most tank water heaters have two heating elements — upper and lower. But they don't run simultaneously. A thermostat-controlled switch energizes only one element at a time. This means:
- 4,500W upper element OR 4,500W lower element = 4,500W total draw
- Never 4,500W + 4,500W = 9,000W
The wire size is based on the single-element load, not combined. That's why a water heater with two 4,500W elements still only needs 10 AWG wire and a 30A breaker.
Exception: Simultaneous-operation water heaters. Some commercial and high-recovery residential units can run both elements at once (requiring 9,000W circuits). These are rare and require 6 AWG wire on a 50A breaker — or two separate circuits. Always check the nameplate.
Tankless Electric Water Heater Wiring
Tankless water heaters are electrical monsters. Unlike tank heaters that maintain stored hot water, tankless units heat water on demand — requiring massive instantaneous power.
Tankless Wiring Requirements by Model
| Model | kW | Circuits | Wire per Circuit | Breaker per Circuit | Total Amps |
|---|---|---|---|---|---|
| Rheem RTEX-08 | 8 kW | 1 | 8 AWG | 40A | 33A |
| Rheem RTEX-11 | 11 kW | 1 | 6 AWG | 60A | 46A |
| Rheem RTEX-13 | 13 kW | 2 | 8 AWG | 40A ea | 54A |
| Rheem RTEX-18 | 18 kW | 2 | 8 AWG | 40A ea | 75A |
| Stiebel Eltron Tempra 12 | 12 kW | 2 | 8 AWG | 40A ea | 50A |
| Stiebel Eltron Tempra 20 | 20 kW | 2 | 8 AWG | 50A ea | 83A |
| Stiebel Eltron Tempra 24 | 24 kW | 2 | 8 AWG | 50A ea | 100A |
| Stiebel Eltron Tempra 29 | 29 kW | 3 | 8 AWG | 40A ea | 121A |
| Stiebel Eltron Tempra 36 | 36 kW | 4 | 8 AWG | 40A ea | 150A |
| EcoSmart ECO 11 | 13 kW | 2 | 8 AWG | 40A ea | 54A |
| EcoSmart ECO 18 | 18 kW | 2 | 8 AWG | 40A ea | 75A |
| EcoSmart ECO 27 | 27 kW | 3 | 8 AWG | 40A ea | 113A |
| EcoSmart ECO 36 | 36 kW | 4 | 8 AWG | 40A ea | 150A |
Tankless electric water heaters can overload residential panels. A 27 kW unit draws 113 amps — more than half of a 200A service. Before installing a tankless electric unit, verify your panel has sufficient capacity. In many cases, a panel upgrade is required. For whole-house hot water, a heat pump water heater (30A single circuit) is often a better choice than tankless electric (multiple circuits, 80–150A total).
Why Tankless Units Need Multiple Circuits
Tankless manufacturers split the load across multiple circuits for several reasons:
-
Breaker availability: Residential panels don't have 150A breaker slots. Splitting a 36 kW load into four 40A circuits uses standard breakers.
-
Wire cost: Running one 150A circuit would require expensive 1/0 AWG or larger wire. Four runs of 8 AWG is cheaper.
-
Staged heating: Some units energize heating elements progressively based on flow rate, reducing peak demand.
Wire Run Length and Voltage Drop
Tank Water Heater Voltage Drop (10 AWG at 18.75A)
| Distance | Voltage Drop | % Drop | Status |
|---|---|---|---|
| 25 ft | 1.2V | 0.5% | ✅ Excellent |
| 50 ft | 2.3V | 1.0% | ✅ Good |
| 75 ft | 3.5V | 1.5% | ✅ Good |
| 100 ft | 4.6V | 1.9% | ✅ Acceptable |
| 125 ft | 5.8V | 2.4% | ✅ Acceptable |
| 150 ft | 7.0V | 2.9% | ⚠️ Marginal |
| 200 ft | 9.3V | 3.9% | ❌ Upsize to 8 AWG |
Water heaters are typically located near the panel (basement, utility room, garage), so long runs are uncommon. Most installations are under 50 feet.
When to Upsize Wire
| Scenario | Standard Wire | Upsized Wire |
|---|---|---|
| Run under 100 ft | 10 AWG | — |
| Run 100–150 ft | 10 AWG (marginal) | 8 AWG recommended |
| Run 150–250 ft | 8 AWG | 6 AWG |
| Tankless, any distance | Per manufacturer | — |
Wiring a Tank Water Heater: Step-by-Step
Materials Needed
| Item | Quantity | Cost |
|---|---|---|
| 10/2 NM-B cable | Length + 20% | $0.65–$1.00/ft |
| 30A double-pole breaker | 1 | $8–$20 |
| 4" × 4" steel junction box | 1 | $3–$8 |
| Cable clamps | 2 | $2–$5 |
| Wire nuts or Wago connectors | 3 | $2–$5 |
| Flexible metal conduit + fittings | 3 ft | $10–$20 |
Wiring Steps
Step 1: Turn off main breaker. Install 30A double-pole breaker in panel.
Step 2: Run 10/2 NM-B from panel to water heater location. Secure with staples every 4.5 feet and within 12 inches of boxes.
Step 3: Mount junction box on wall near water heater (typically above or beside).
Step 4: Connect NM-B to junction box. Wire nuts: Black to black, white (re-identified with black tape) to white/red, bare to ground.
Step 5: Run flexible metal conduit from junction box to water heater connection point.
Step 6: At water heater terminals, connect per wiring diagram (typically labeled L1 and L2).
Step 7: Restore power. Test for 240V between L1 and L2 with multimeter before filling tank.
Real installation — 50-gallon Rheem tank in garage:
- Distance from panel: 30 feet
- Cable: 10/2 NM-B, 35 feet (extra for connections)
- Breaker: 30A double-pole
- Voltage drop: (2 × 30 × 18.75 × 1.24) / 1000 = 1.4V = 0.6% ✓
- Total materials: ~$60–$90
- Professional installation: $250–$500
Water Heater Types and Wiring Comparison
| Type | Typical Wattage | Wire | Breaker | Annual kWh | Operating Cost (at $0.166/kWh) |
|---|---|---|---|---|---|
| Standard tank (50 gal) | 4,500W | 10/2 | 30A | 4,000–5,000 | $664–$830/yr |
| High-efficiency tank | 4,500W | 10/2 | 30A | 3,500–4,500 | $581–$747/yr |
| Heat pump hybrid | 500–4,500W | 10/2 | 30A | 1,500–2,000 | $249–$332/yr |
| Tankless (18 kW) | 18,000W | 2 × 8/2 | 2 × 40A | 3,000–4,000 | $498–$664/yr |
| Tankless (27 kW) | 27,000W | 3 × 8/2 | 3 × 40A | 3,500–5,000 | $581–$830/yr |
Heat pump water heaters are the efficiency champions. They use the same 10/2 wire and 30A breaker as standard tanks but consume 60–70% less energy. Federal tax credits in 2026 cover up to $2,000 of the purchase price, making them cost-competitive with standard tanks upfront while saving $300–$500/year in operating costs.
Common Water Heater Wiring Mistakes
Mistake 1: Using 12 AWG Wire on a 4,500W Heater
Why it's wrong: 4,500W ÷ 240V = 18.75A. With continuous load factor: 18.75 × 1.25 = 23.4A required capacity. 12 AWG wire is only rated for 20A. This combination will cause the wire to overheat before the breaker trips.
The fix: Always use 10 AWG wire for 4,500W and 5,500W water heaters.
Mistake 2: Connecting to a 120V Circuit
Why it fails: A 4,500W element connected to 120V draws only 1,125W (power is proportional to voltage squared). The water heater will "work" but heat incredibly slowly — 4× slower than designed.
The fix: Water heaters require 240V, which means a double-pole breaker and two hot conductors. Verify 240V with a multimeter between L1 and L2.
Mistake 3: Not Using a Junction Box
Why it's a code violation: NEC 422.16(B)(2) requires accessible connections for water heaters. Running NM-B directly into the water heater's wiring compartment without a junction box makes future service difficult and may not meet code.
The fix: Install a junction box near the water heater. Run NM-B to the box, then flexible conduit from box to water heater.
Mistake 4: Undersizing Tankless Circuits
Why it's catastrophic: A 27 kW tankless water heater requires 113 amps across 3 circuits. Wiring it with a single 30A circuit means the water heater can only produce a fraction of its rated output — like having a trickle of lukewarm water.
The fix: Follow the manufacturer's wiring requirements exactly. Tankless electric units require multiple high-amperage circuits — there are no shortcuts.
Real-World Examples
Example 1: Replacing Gas Water Heater with Electric Tank
Scenario: 40-gallon gas water heater failed. Homeowner wants to switch to electric. Panel has space; water heater is 25 feet from panel.
Wiring needed:
- Cable: 10/2 NM-B, 30 feet
- Breaker: 30A double-pole (takes 2 panel spaces)
- Materials cost: $50–$80
- Professional installation: $250–$500 (plus new water heater)
Example 2: Installing Heat Pump Water Heater
Scenario: Upgrading from standard electric tank to heat pump model for efficiency. Existing circuit: 10/2 NM-B, 30A breaker.
Wiring change needed: None. Heat pump water heaters use the same 10/2 wire and 30A breaker as standard tanks. The existing circuit is compatible — just swap the water heater.
Example 3: Tankless Installation Requiring Panel Upgrade
Scenario: Family of 5 wants a 27 kW tankless for whole-house hot water. Current panel: 150A main with 8 spaces remaining.
Wiring challenge:
- 27 kW unit needs 3 × 40A circuits = 120A at the panel
- 150A panel can't support 120A additional load
- Panel upgrade to 200A required: $1,500–$4,000
- Plus tankless wiring: 3 runs of 8/2 NM-B, 3 × 40A breakers
Alternative: Install a heat pump water heater instead (30A single circuit, no panel upgrade needed). Lower upfront cost, lower operating cost, easier installation.
Example 4: Point-of-Use Tankless at Kitchen Sink
Scenario: Adding a small tankless unit under the kitchen sink for instant hot water. Model: 4 kW point-of-use.
Wiring:
- 4,000W ÷ 240V = 16.7A
- Continuous load: 16.7 × 1.25 = 20.8A → 25A or 30A breaker
- Wire: 10/2 NM-B (can use 12 AWG with 20A breaker if unit draws under 16A)
- Run: 20 feet from panel → negligible voltage drop
Key Takeaways
- 10/2 NM-B wire with a 30A breaker handles 90%+ of residential tank water heaters (4,500W–5,500W)
- Tankless electric units require multiple circuits — typically 2–4 runs of 8 AWG wire with 40A breakers
- Heat pump water heaters use standard 10/2 / 30A wiring — same as tank heaters, but 60–70% less energy
- Upper and lower elements don't run simultaneously — wire size is based on single-element draw (4,500W)
- For runs over 150 feet, upsize to 8 AWG to keep voltage drop under 3%
- Tankless electric can overload residential panels — verify capacity before installing or consider a heat pump unit
- Always check the nameplate — manufacturer specs override generic charts
Frequently Asked Questions
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