It is my understanding that low-voltage wires can share a double-gang box with 120V wires, as long as they are isolated from each other by a permanent divider. I was looking at getting this relay to operate an inline duct booster fan using the 24V thermostat signal wire. However both the low-voltage and the 120V wires would come into the box bundled together through the same knockout. Is there an exception for relays, or something that I am missing? Or can I just punch a hole in a divider and snake the low-voltage wire through and just keep the splices separate? This is in the United States in a state on 2017 NEC if it makes a difference
You're on the right track. The same manufacturer (and others) make similar relays where the low and high voltage wires come out opposite sides of the device, with either the high voltage or low voltage ones coming through the knockout, so you just buy the appropriate one depending on whether you want the relay inside or outside the junction box.