I am installing a new light switch and changing the fan switch next to it for a Lutron timer for the fan. The old switch had no ground, but the ground wires are available in the enclosure, so I will run an appropriately sized 14 gauge pigtail from the new switch for the 15 amp circuit. But, the Lutron timer has an 18 gauge wire for the ground. Clearly this must be okay, but can anyone tell me why? There is an old an often asked question here and in other sites where people have the old 16 gauge Romex ground wire of whether it is acceptable to use the 16 gauge ground on a new device. I just wonder what is special about the Lutron timer that lets it get away with an 18 gauge wire when I need a 14 for the new switch.
This happens because UL has different rules for device parts
The timer's ground pigtail wire is the same as any other pigtail that is part of a device, and thus governed by the applicable UL standards instead of falling under NEC wire sizing rules. You'll notice that the current-carrying pigtail wires on your dimmer are likely 18AWG or 16AWG, and this is true for devices such as light fixtures and fluorescent ballasts as well. All of this uses the same logic in that UL-governed stuff can be tested for temperature rise explicitly at the device's rated nameplate load, and also can use different wire insulation temperature limits due to UL's adult supervision being involved.