I would like to use a smart relay like Shelly 1 UL to control a couple of hardwired light fixtures. The load is at most 3.5 A and the relay is rated for 15 A.
Question 1: Is it okay to use the 15 A smart relay even though the circuit is protected by a 20 A breaker?
Homework: We use regular 15 A light switches ("snap switches") on 20 A circuits all the time. I traces that back to 404.14(A), which says that it's enough for the switch to be rated for its load. The Shelly is not a snap switch, but I found the following in 404.14(E), so I think the answer to my question is yes:
(E) Dimmer and Electronic Control Switches
General-use dimmer switches shall be used only to control permanently installed incandescent luminaires unless listed for the control of other loads and installed accordingly. Other electronic control switches, such as timing switches and occupancy sensors, shall be used to control permanently connected loads. They shall be marked by their manufacturer with their current and voltage ratings and used for loads that do not exceed their ampere rating at the voltage applied.
Question 2: The terminals on the Shelly 1 UL are too small for 12 AWG wire. Is it okay to use 14 AWG pigtails to connect the Shelly to the 20 A circuit, given that the circuit also has duplex receptacles (not downstream of the Shelly)?
Homework: Section 210.19(A)(2) says that
[c]onductors of branch circuits supplying more than one receptacle for cord-and-plug-connected portable loads shall have an ampacity of not less than the rating of the branch circuit.
Section 210.19(A)(4) has some exceptions. Individual non-receptacle outlets are an exception, but I don't think the Shelly with downstream luminaires qualify as a non-receptacle outlet. Instead, it's part of the branch circuit and needs to be 12 AWG, so I think the answer to my question is no and my only option is to swap out the breaker for a 15 A breaker.