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) |
| Wire (aluminum alt.) | 10 AWG THHN | Table 310.16, 110.14(C) |
| Breaker | 20A single-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 Al | 10 AWG Co |
|---|---|---|---|
| 25 ft | 1.0% | 1.1% | 0.7% |
| 50 ft | 2.1% | 2.1% | 1.3% |
| 75 ft | 3.1% | 3.2% | 1.9% |
| 100 ft | 4.1% | 4.2% | 2.6% |
| 150 ft | 6.2% | 6.4% | 3.9% |
| 200 ft | 8.2% | 8.5% | 5.2% |
Common products for 12 AWG Copper circuits
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Related wire data
Detailed ampacity: 12 AWG copper, 10 AWG aluminum. 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 10 AWG aluminum on a 20A breaker at 120V. This is based on NEC 2023 Table 310.16 ampacity with 110.14(C) termination limits. The equipment nameplate or installation manual sets the minimum circuit ampacity and maximum breaker; follow it where it differs.
Does bathroom fan/heater combo require GFCI?
Yes. GFCI protection is required per NEC 210.8(A)(1).
NEC 2023 references verified February 2026