Bathroom Fan/Heater Combo: Complete Wiring Guide
Bill of materials
A bathroom fan-heater-light combo unit draws 1300-1500W for the heater plus 50-80W for the fan and light. At 12.5A total, it needs a dedicated 20A circuit - which NEC 210.11(C)(3) already requires for bathrooms. The unit must be connected to a fan-rated electrical box and requires GFCI protection.
Bathroom circuits are dedicated 20A per NEC 210.11(C)(3). Fan-heater combos need a fan-rated box (standard boxes cannot support the weight and vibration). All bathroom equipment requires GFCI.
Fan-heater combo (1500W)
| Component | Specification | NEC basis |
|---|---|---|
| Wire (copper) | 12 AWG THHN | Table 310.16, 110.14(C) |
| Breaker | 20A double-pole | 240.4 |
| Voltage | 120V single-phase | |
| Conduit (EMT) | 1/2" EMT | Chapter 9 Table 4 |
| GFCI | Required (GFCI breaker or device) | 210.8(A)(1) |
| Max distance (3% VD) | 72 ft at 120V copper | 210.19(A) Note 4 |
Installation notes
This project requires GFCI protection per NEC 210.8(A)(1). 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.
Standard 120V circuits use NM-B cable with 12/2 cable (hot, neutral, ground). For conduit runs, pull individual 12 AWG THHN conductors. All connections must be made in accessible junction boxes.
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 | 12 AWG Co | 10 AWG Co |
|---|---|---|
| 25 ft | 1.0% | 0.7% |
| 50 ft | 2.1% | 1.3% |
| 75 ft | 3.1% | 1.9% |
| 100 ft | 4.1% | 2.6% |
| 150 ft | 6.2% | 3.9% |
| 200 ft | 8.2% | 5.2% |
Common products for 12 AWG Copper circuits
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Related wire data
Detailed ampacity: 12 AWG copper. Wire size lookup: 20A circuit. Calculators: voltage drop · panel load calculator.
Frequently asked questions
What wire size do I need for bathroom fan/heater combo?
Use 12 AWG copper or 12 AWG aluminum on a 20A breaker at 120V. 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 bathroom fan/heater combo require GFCI?
Yes. GFCI protection is required per NEC 210.8(A)(1).
NEC 2023 references verified April 2026