Electric Water Heater: Complete Wiring Guide
Bill of materials
Tank-style electric water heaters typically draw 4,500W on a 30A/240V circuit using 10 AWG wire. Tankless units draw far more: a whole-house 24kW unit requires 100A total, often split across two or three 40A circuits. NEC 2023 added GFCI requirements for outlet-connected water heaters.
Tank water heaters: 30A circuit with 10 AWG copper, hardwired with a disconnect within sight per 422.31. Tankless: check manufacturer specs carefully. A 24kW unit draws 100A at 240V, typically requiring three 40A double-pole breakers with 8 AWG to each element bank. NEC 2023 Section 210.8(D) requires GFCI for cord-and-plug connected water heaters. Hardwired units are exempt but some jurisdictions interpret the requirement more broadly.
Tank water heater (4500W standard)
| Component | Specification | NEC basis |
|---|---|---|
| Wire (copper) | 10 AWG THHN | Table 310.16, 110.14(C) |
| Breaker | 30A double-pole | 240.4 |
| Voltage | 240V single-phase | |
| Conduit (EMT) | 1/2" EMT | Chapter 9 Table 4 |
| GFCI | Required (GFCI breaker or device) | 210.8(D) |
| Max distance (3% VD) | 153 ft at 240V copper | 210.19(A) Note 4 |
Tankless water heater (24kW)
| Component | Specification | NEC basis |
|---|---|---|
| Wire (copper) | 8 AWG THHN | Table 310.16, 110.14(C) |
| Wire (aluminum alt.) | 6 AWG THHN | Table 310.16, 110.14(C) |
| Breaker | 40A double-pole | 240.4 |
| Voltage | 240V single-phase | |
| Conduit (EMT) | 3/4" EMT | Chapter 9 Table 4 |
| GFCI | Required (GFCI breaker or device) | 210.8(D) |
| Max distance (3% VD) | 46 ft at 240V copper | 210.19(A) Note 4 |
Installation notes
This project requires GFCI protection per NEC 210.8(D). The simplest approach is a GFCI breaker at the panel, which protects the entire circuit. For 240V circuits, use a 2-pole GFCI breaker sized to match the circuit. GFCI breakers cost $40-80 depending on amperage.
For the wire run, you have two options: NM-B cable (Romex) for interior runs through framing, or individual THHN conductors in conduit for exposed runs, outdoor installations, or runs through unfinished spaces. NM-B is faster to install but cannot be used outdoors or in conduit. For conduit, use EMT (metallic) or PVC Schedule 40.
Run length matters: every foot of wire adds voltage drop, reducing power to your equipment. The table below shows the exact voltage drop at common distances. If your run exceeds the 3% threshold, upsize one gauge.
Voltage drop by distance
Percentage voltage drop at common run lengths. NEC recommends 3% maximum for branch circuits and 5% for combined feeder plus branch.
| Distance | 10 AWG Co | 8 AWG Co |
|---|---|---|
| 25 ft | 0.5% | 0.3% |
| 50 ft | 1.0% | 0.6% |
| 75 ft | 1.5% | 0.9% |
| 100 ft | 2.0% | 1.2% |
| 150 ft | 2.9% | 1.9% |
| 200 ft | 3.9% | 2.5% |
Common products for 10 AWG Copper circuits
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Related wire data
Detailed ampacity: 10 AWG copper. Wire size lookup: 30A circuit. Calculators: voltage drop · panel load calculator.
Frequently asked questions
What wire size do I need for electric water heater?
Use 10 AWG copper or 10 AWG aluminum on a 30A breaker at 240V. This is based on NEC 2023 Table 310.16 ampacity with 110.14(C) termination limits and 125% continuous load sizing per 210.19(A)(1).
Does electric water heater require GFCI?
Yes. GFCI protection is required per NEC 210.8(D).
NEC 2023 references verified April 2026