In a multifamily bathroom, an exhaust fan is often set to run whenever the light is on, whenever humidity rises above a setpoint, or both. A multifamily bathroom fan interlock does this with a relay: the light circuit or a humidity control energizes the relay coil, and the relay contact switches the fan. Because the coil and the contact are separate circuits, a 277 Vac lighting circuit can run a 120 Vac fan, and neither the wall switch nor the humidity control carries the fan load. The relay, its wiring, and the sequence are detailed once per unit type and repeated across the building.
Functional Devices, Inc. manufactures RIB® (Relay-In-a-Box®) relays used for fan and lighting interlocks, along with the transformers that supply them. This guide covers the light interlock, the step-down arrangement for buildings with only 277 Vac available, adding a humidity sensor, and the resulting sequence of operations.
How a Relay Interlocks the Bathroom Fan With the Light
The relay coil is connected to the light’s switched circuit, so the coil is energized whenever the wall switch turns the light on. The relay’s N/O contact is wired in series with the fan. When the light turns on, the contact closes and the fan runs; when the light turns off, the contact opens and the fan stops. The coil operates at the light’s voltage and the contact switches the fan at its own voltage, so the two can differ.
The relay is selected by the light circuit’s voltage and the fan’s rating:
- For a 208–277 Vac light circuit, the RIB2402B is a 20 A SPDT relay with a 24 Vac/dc or 208–277 Vac coil. Its contact is rated 1 HP at 120 Vac and 2 HP at 277 Vac.
- For fans of 1/3 HP or less, the RIBH1C provides a 10 A SPDT contact rated 1/3 HP at 120–240 Vac on the N/O contact, with a 10–30 Vac/dc or 208–277 Vac coil. Wiring is the same as the RIB2402B.
- Where the light circuit is 120 Vac, the RIBU1C has the same contact ratings as the RIBH1C with a 10–30 Vac/dc or 120 Vac coil.
When Both 120 Vac and 277 Vac Are Available
With a 277 Vac lighting circuit and a separate 120 Vac circuit for the fan, a single relay completes the interlock. The 277 Vac switched leg feeds both the light and the relay coil. The 120 Vac fan circuit runs through the relay’s N/O contact to the fan. The wall switch turns on the light, the coil pulls in, and the fan starts; the switch turns off the light, the coil drops out, and the fan stops.
When Only 277 Vac Is Available
Where the bathroom has only 277 Vac, the fan is powered through a 277 to 120 Vac step-down transformer, and the relay contact switches the transformer’s 120 Vac output to the fan. The relay coil is still energized from the 277 Vac switched leg. The fan’s rating determines both the relay and the transformer size.
The transformer’s VA rating is equal to or greater than the fan’s. The fan’s VA is its rated voltage multiplied by its full-load current from the nameplate. The TR100VA008, for example, is a 100 VA transformer with a 480/277/240/208 Vac primary, a 120 Vac secondary, and a circuit breaker, which covers fans drawing up to about 0.8 A at 120 Vac.
Adding a Humidity Sensor With OR Logic
A humidity control turns the fan on when humidity rises above its setpoint, independent of the light. To keep the light interlock and add humidity control, the fan runs when either condition is true: light on OR humidity high.
The humidity control’s output energizes a second relay, and that relay’s N/O contact is wired in parallel with the light relay’s N/O contact. Either contact closing completes the fan circuit. The fan stops only when both contacts are open: the light is off and humidity is below the setpoint.
The second relay is selected by the humidity control’s output:
- For a humidity control with a 24 Vac output, or a dry contact switching a 24 Vac source, the RIBU1C coil accepts 10–30 Vac/dc.
- For a 24 Vac signal where the low-voltage and line-voltage wiring must enter the relay housing separately, the RIBT24P is a 20 A DPDT relay with a 24 Vac/dc coil and high/low voltage separation. Its second pole can carry a separate circuit, such as a status signal to a building automation system (BAS).
Where a humidity control’s own contact is rated for the fan at the fan’s voltage, it can be wired directly in parallel with the light relay’s contact instead of driving a second relay.
Sequence of Operations
| Light switch | Humidity | Light relay contact | Humidity relay contact | Fan |
|---|---|---|---|---|
| Off | Below setpoint | Open | Open | Off |
| On | Below setpoint | Closed | Open | On |
| Off | Above setpoint | Open | Closed | On |
| On | Above setpoint | Closed | Closed | On |
Neither relay provides a delay. Where the fan should continue running for a set time after the light turns off, a separate off-delay timer is added to the light relay’s coil circuit or contact circuit.
Frequently Asked Questions About Multifamily Bathroom Fan Interlocks
Can a 277 Vac light circuit run a 120 Vac bathroom fan?
Yes. The RIB2402B’s coil is energized by the 277 Vac light circuit, and its N/O contact switches a separate 120 Vac circuit to the fan. Where no 120 Vac circuit is available, a 277 to 120 Vac step-down transformer supplies the fan through the relay contact.
Which relay is used for a fan under 1/3 HP?
The RIBH1C, for a 208–277 Vac light circuit, or the RIBU1C, for a 120 Vac light circuit. Both are rated 1/3 HP at 120–240 Vac on the N/O contact and wire the same way as the RIB2402B.
How is a humidity sensor added to a light-interlocked fan?
The humidity control energizes a second relay, and the two relays’ N/O contacts are wired in parallel to the fan. The fan runs when the light is on or humidity is above the setpoint.
Does the relay keep the fan running after the light turns off?
No. The relay contact opens when the light turns off, and the fan stops unless the humidity relay is closed. A separate off-delay timer provides run-on after the light turns off.
How is the step-down transformer sized?
The transformer’s VA rating is equal to or greater than the fan’s VA, which is the fan’s rated voltage multiplied by its full-load current.
Sourcing Fan Interlock Relays for Multifamily Projects
For help with a sequence or parts list, contact Functional Devices technical support. For project quantities, find an authorized Functional Devices distributor near you.