How do I wire a bathroom exhaust fan to the light switch or should it be separate?

Wiring a Bathroom Exhaust Fan to the Light Switch vs Separate Switch

This is a practical wiring question that comes up in almost every New Brunswick bathroom renovation. The Canadian Electrical Code (CEC) and New Brunswick building code both require mechanical exhaust ventilation in bathrooms, but how you wire the fan to the switch has real implications for energy use, moisture control, and code compliance.

The Two Main Wiring Options

Option 1: Fan and light on the same switch
The fan turns on and off with the bathroom light. One switch controls both. This is the simplest wiring configuration — a single 14/2 NMD90 cable from the switch to a junction box in the ceiling, then short runs to the fan and light fixture.

Pros:

• Simplest wiring — one switch, one cable run


• Fan always runs when someone is in the bathroom (assuming they turn on the light)


• Lowest installation cost ($50-$100 less than separate switching)

Cons:

• Fan runs even when not needed (quick hand wash, grabbing something from the cabinet)


• Fan stops immediately when the light is turned off — but moisture removal typically needs 15-20 minutes of runtime after a shower to prevent mould


• Higher energy consumption from unnecessary fan operation

Option 2: Fan on a separate switch (recommended)
The fan has its own dedicated switch next to the light switch. This requires running a 14/3 NMD90 cable (instead of 14/2) from the switch box to the ceiling, providing an extra conductor for independent fan control.

Pros:

• Occupant controls the fan independently


• Can leave the fan running after leaving the bathroom


• Pairs well with a timer switch (see below)


• More energy efficient

Cons:

• Requires 14/3 cable run (marginally more expensive — about $15-$20 extra for typical cable length)


• Extra switch in the gang box

The Best Solution: Timer Switch for the Fan

The ideal setup for New Brunswick bathrooms is a separate timer switch for the fan. Timer switches (like the Leviton LTB series or Lutron Maestro) let you press a button for 10, 20, 30, or 60 minutes of fan runtime. The fan runs for the selected period and shuts off automatically.

This is particularly important in New Brunswick where high humidity from Maritime air combined with poor bathroom ventilation leads to persistent mould problems. Mould remediation in a New Brunswick bathroom costs $1,500-$5,000 — a $40 timer switch that ensures the fan runs long enough after every shower is excellent insurance.

Installation: A timer switch fits in a standard single-gang box and requires a neutral wire. Most New Brunswick homes built after 1985 have neutral wires at switch boxes. Older homes may not — in that case, specify a timer switch that works without a neutral (fewer options but they exist, like the Lutron Maestro MA-T51).

What the Code Requires

The Canadian Electrical Code does not specifically mandate how the fan is switched — only that bathrooms have exhaust ventilation. However, the NB Building Code (which adopts the National Building Code) requires:

• Bathrooms must have mechanical ventilation capable of exhausting at least 25 litres per second (50 CFM) for a standard bathroom
• For larger bathrooms (over 100 sq ft), increase to 1 CFM per square foot
• The ventilation must exhaust to the outdoors — not into the attic, soffit, or wall cavity. This is a common code violation found during TSANB inspections in older NB homes

Wiring Details for DIY or Your Electrician

For separate switching with a timer:

Run 14/3 NMD90 cable from the switch box to the ceiling fan/light location

Black wire: light circuit (controlled by standard switch)

Red wire: fan circuit (controlled by timer switch)

White wire: neutral (shared)

Bare copper: ground (shared)

At the ceiling, connect the fan motor leads to the red wire and the light leads to the black wire

The timer switch needs the neutral (white) wire connected at the switch box

For combined switching:

Run 14/2 NMD90 cable from switch to ceiling

Both fan and light connect to the black (hot) wire

Single switch controls both

Fan Sizing for New Brunswick Bathrooms

Do not under-size the fan. For effective moisture removal in New Brunswick's humid Maritime climate:

• Standard bathroom (under 100 sq ft): 50-80 CFM
• Larger bathroom (100-150 sq ft): 80-110 CFM
• Bathroom with separate toilet room or large shower: 110-150 CFM

Choose a fan rated at 1.0 sone or less for quiet operation — nobody runs a loud fan long enough to remove moisture. Panasonic WhisperCeiling (0.3 sone) and Broan InVent series (0.8 sone) are widely available at Kent Building Supplies and Home Depot locations in Moncton, Fredericton, and Saint John. Expect to pay $100-$250 for a quality quiet fan.

Cost Summary

• Fan on same switch (basic): $150-$300 installed (fan + wiring)
• Fan on separate switch: $200-$350 installed
• Fan with timer switch: $240-$400 installed (includes $35-$50 timer switch)
• Adding ventilation to a bathroom with no existing fan: $400-$800 (includes cutting ceiling hole, ductwork to exterior, and exterior vent cap)

All bathroom electrical work in New Brunswick requires a TSANB permit if you are running new wiring or adding new circuits. Simply replacing a fan on existing wiring does not typically require a permit, but adding a new fan where none existed before does.

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