Wire Fill for Electric Water Heater
Electric water heater wire fill: standard 4500W tank uses 10 AWG copper on 30A breaker in 3/4 EMT. Tankless 24kW needs 4 AWG on 100A. NEC 422 governs.
Wire fill for electric water heater circuits is governed by NEC 422.13 (continuous load sizing) and NEC 422.31 (disconnect requirements). A standard 4500W 240V tank water heater uses a 30A 2-pole breaker, 10 AWG copper THHN, and 3/4-inch EMT or PVC for the home-run. Tankless electric water heaters demand much larger circuits — 24 kW units pull 100A and require either two 50A circuits or one 100A circuit, depending on the model. Conduit sizing scales accordingly.
Load characteristics
Tank water heaters are intermittent but treated as continuous per NEC 422.13:
| Heater type | Wattage | Voltage | FLA | × 1.25 | Breaker |
|---|---|---|---|---|---|
| Small tank (30 gal) | 3500W | 240V | 14.6A | 18.2A | 20A |
| Standard tank (40-50 gal) | 4500W | 240V | 18.75A | 23.4A | 30A |
| High-recovery tank | 5500W | 240V | 22.9A | 28.6A | 30A |
| Heat pump water heater | 4500W backup + compressor | 240V | 20A typical | 25A | 30A |
| Tankless 18 kW | 18,000W | 240V | 75A | 94A | 100A or split |
| Tankless 24 kW | 24,000W | 240V | 100A | 125A | 125A or 2× 60A |
| Tankless 36 kW | 36,000W | 240V | 150A | 188A | 3× 60A or 200A |
Heat pump water heaters (Rheem ProTerra, Stiebel Eltron Accelera) typically need only a 30A 240V circuit because the compressor + resistance backup never run together.
NEC code references
- NEC 422.13 — continuous load (water heaters ≤ 120 gal)
- NEC 422.31 — disconnect means
- NEC 422.10 — branch circuit sizing
- NEC 250.122 — EGC sizing
- NEC 110.14(C) — termination temperature rating
Conductor sizing
Use 75°C column of NEC 310.16:
| Breaker | Copper THHN | Aluminum |
|---|---|---|
| 20A | 12 AWG | 10 AWG |
| 30A | 10 AWG | 8 AWG |
| 50A | 6 AWG | 4 AWG |
| 60A | 6 AWG | 4 AWG |
| 100A | 3 AWG | 1 AWG |
Most water heaters are 240V-only — no neutral required, just 2 hots + EGC.
Conduit sizing
| Heater type | Conductors | Area | 1/2" EMT | 3/4" EMT | 1" EMT |
|---|---|---|---|---|---|
| 30A tank (10 AWG) | 2 × 10 + 1 × 10 EGC | 0.0780 in² | 39.6% (tight) | 16.5% | 9.7% |
| 30A tank with neutral (some HPWH) | 3 × 10 + 1 × 10 EGC | 0.1040 in² | over 40% — fails | 22.0% | 12.9% |
| 50A tankless (per leg, 6 AWG) | 2 × 6 + 1 × 10 EGC | 0.1225 in² | over 40% — fails | 25.9% | 15.2% |
| 60A tankless (per leg, 6 AWG) | 2 × 6 + 1 × 10 EGC | 0.1225 in² | over 40% — fails | 25.9% | 15.2% |
| 100A tankless single feed | 2 × 3 + 1 × 8 EGC | 0.2502 in² | over 40% — fails | over 40% — fails | 32.8% |
3/4-inch EMT is the universal answer for standard tank water heaters. Tankless installations with multiple 50-60A circuits each get their own 3/4-inch raceway, or all home-runs share a 1-1/4 inch trunk back to a sub-panel.
Special requirements
- Disconnect within sight: NEC 422.31(B) — either a lockable breaker within sight of the heater or a local disconnect switch.
- Junction box at heater: Most tank water heaters have a top junction box for connection. Bring conductors in via cable connector or conduit fitting.
- Bonding to hot water piping: NEC 250.104(B) — bond hot and cold water piping to the EGC system. Modern PEX systems may not require traditional pipe bonding.
- Tankless dedicated circuits: Tankless units almost always require dedicated circuits not shared with any other load.
- Expansion tank: Code-required in closed plumbing systems but not an electrical concern beyond providing the receptacle if the tank has a circulator.
- GFCI: NEC 210.8(F) extends GFCI to outdoor and crawl-space water heater receptacles in the 2020 NEC.
Worked example — 50 gallon tank water heater, 4500W, 30 ft from panel
- Heater load: 4500W ÷ 240V = 18.75A
- × 1.25 continuous: 23.4A
- Breaker: 30A 2-pole
- Conductors: 10 AWG copper THHN × 2 (L1, L2) + 10 AWG copper EGC
- Raceway: 3/4" EMT from main panel to water heater closet, 30 ft.
- Fill check: 3 × 0.0260 = 0.0780 in² in 3/4" EMT = 16.5%, pass.
- Disconnect: Lock-out tag on 30A breaker in panel (in sight via open closet door) — satisfies NEC 422.31(B).
For a 24 kW tankless installation with two 50A circuits:
- Two 50A 2-pole breakers in sub-panel
- Two pairs of 6 AWG copper THHN + one 10 AWG EGC each
- Two 3/4-inch EMT home-runs or one 1-1/4 inch EMT carrying all 6 conductors:
- Fill: 4 × 6 AWG (0.2028 in²) + 2 × 10 AWG (0.0520 in²) = 0.2548 in². 1-1/4" EMT 40% limit = 0.598 in². Fill = 42.6% — fails. Need 1-1/2" EMT.
- Best practice: Run two separate 3/4" raceways for cleaner identification and easier troubleshooting.
Quick reference
| Water heater | Breaker | Copper wire | Conduit |
|---|---|---|---|
| 30 gal 3500W | 20A | 12 AWG × 2 | 1/2" EMT |
| 40-50 gal 4500W | 30A | 10 AWG × 2 | 3/4" EMT |
| 80 gal 5500W | 30A | 10 AWG × 2 | 3/4" EMT |
| HPWH 4500W | 30A | 10 AWG × 2 | 3/4" EMT |
| Tankless 18 kW (1× 100A) | 100A | 3 AWG × 2 | 1" EMT |
| Tankless 24 kW (2× 50A) | 2× 50A | 6 AWG × 2 each | 3/4" EMT each |
| Tankless 36 kW (3× 60A) | 3× 60A | 6 AWG × 2 each | 3/4" EMT each |