Aprilaire 24v humidifier transformer. Input 120 vac, output 24v, 10 watt (10 va). Mounts in a utility box.
Same as Skuttle 000-0814-008.
The humidifier transformers mount to the side of a utility box.
The white and black wires connect to a 120 vac. And the 24 vac is supplied at the two screws. Use thermostat wire for the 24 vac humidifier wiring.