At Functional Devices, we’re no strangers to creative relay solutions, and some applications come up more than others. One common challenge our technical support team has encountered involves controlling a 120 Vac exhaust fan using a 277 Vac light circuit and wall switch. In this blog, we’ll walk through two proven wiring scenarios using our RIB® products—specifically the RIB2402B—and explain how to meet this need, whether you have both voltages available or only 277 Vac.
At a Glance
Q: Can I use RIB2402B to control a 120 Vac fan from a 277 Vac light circuit?
A: Yes. RIB2402B can be used in applications where a 277 Vac light fixture controls a 120 Vac fan. The relay’s coil is rated for 277 Vac, allowing it to be energized by the light circuit and switch the 120 Vac fan load using its Normally Open (N/O) contact.
Q: What if I only have 277 Vac power available?
A: You can still use the RIB2402B by adding a 277 to 120 Vac step-down transformer. The relay coil will be powered by the 277 Vac circuit, and the transformer will provide the 120 Vac needed for the fan.
Q: What model should I use if my fan is less than 1/3 HP?
A: For fan loads of 1/3 HP or less at 120 Vac, the RIBH1C is a suitable alternative to the RIB2402B and follows the same wiring method.
Q: How do I choose the right transformer size?
A: The VA rating of your step-down transformer should match or exceed the power requirements of your 120 Vac fan. Functional Devices offers multiple transformer options—contact Technical Support for help selecting the right one.
Q: Where can I get wiring diagrams for these setups?
A: Wiring diagrams for both scenarios—when both voltages are available and when only 277 Vac is present—are shown in Figures 1 and 2.
Q: Can I contact someone for help with a custom application?
A: Absolutely. Our Technical Support Team is available Monday through Friday, 8 AM to 5 PM EST. You can also email us for support with your specific setup.
Are Both 120 Vac and 277 Vac Power Available?
The first scenario of having access to both 120 Vac and 277 Vac power is diagramed in Fig. 1 and requires a single Relay in a Box® (RIB®). When the wall switch is in the ON position, the light is powered, and the coil of model RIB2402B is energized. This pulls in the Normally Open (N/O) contact of the relay, allowing 120 Vac power to turn on the fan. When the wall switch is in the OFF position, the light and fan will both turn off. The RIB2402B can switch motor loads up to 1 HP at 120 Vac. For motor loads of 1/3 HP and less, model RIBH1C will meet your needs, and all wire colors and connections will be the same.
Is Only 277 Vac Power Available?
The second scenario of having access to only 277 Vac power is shown in Fig. 2. This application requires a single RIB® relay and a 277 to 120 Vac step down transformer. The diagram is essentially the same, but the fan is now being powered by a transformer. In this scenario, the ratings of the fan will dictate which RIB® to use, as well as the size of the transformer needed. Functional Devices offers a variety of 277 to 120 Vac step down transformers with different VA ratings.
This blog only covers some of the many possible variations this application may have. While most scenarios can be realized in a similar manner, there is no one-size-fits-all solution. If you have any questions, call our Technical Support Team. We are available Monday through Friday, 8AM - 5PM EST. Alternatively, feel free to send us an email.
We're Here to Help!
Using the right RIB® relay from Functional Devices makes coordinating devices operating at different voltages simple, such as pairing a 120 Vac fan with a 277 Vac lighting system. Whether your setup includes both voltage sources or requires a step-down transformer, our RIB2402B and related products offer flexible, reliable control. For help choosing the right relay or transformer, contact our Technical Support Team—we’re here to ensure your application works as smoothly and safely as possible.
About Functional Devices, Inc.
Functional Devices, Inc., located in the United States of America, has been designing and manufacturing quality electronic devices since 1969. Our mission is to enhance lives in buildings and beyond. We do so by designing and manufacturing reliable, high-quality products for the building automation industry. Our suite of product offerings include RIB relays, current sensors, power controls, power supplies, transformers, lighting controls, and more.
We test 100% of our products, which leads to less than 1 out of every 16,000 products experiencing a failure in the field.
Simply put, we provide users of our various products confidence and peace of mind in every box.