Safely wiring a 120v smoke detector externally

Question

I am building a relay box that will cut power to an outlet if an attached 120V smoke alarm detects smoke.

The relay box itself will be a grounded metal electrical box hardwired to a 3 prong plug. It will present a 3-prong socket for the switched device. But I'm not sure what the safest way to connect to the smoke alarm is. Is standard gauge wire fine as long as it's rated for 120VAC? But in consumer devices, wire carrying mains voltage typically has a much beefier jacket, so this gives me pause. How should I splice the connection to the thin wires on the smoke alarm, wire nuts? It seems like a wire could come loose on the smoke alarm side, exposing a live wire.

   male 120VAC 3-prong plug
       |
+----------------+
| junction box   |
|                |
|switched        |
|120V outlet     |
+----------------+
  |            |
  |            | (Line, Neutral, Interconnect) 
device       smoke alarm

Example relay module - Kidde SM120X that would be inside the junction box, using the interconnect to switch the device.

Answer 1

Is standard gauge wire fine as long as it's rated for 120VAC?

You can use any wire with sufficient insulation if properly fused. For this refer to your local electrical code. The factors are wire surface area (awg) and length, those will give you a maximum current.
Typically for device wiring 3x0.75mm2 or 3x1mm2 is used. (17/19 awg)
For smoke alarms a 5x20mm 1A Slow fuse should be sufficient, as found in appliances.

How should I splice the connection to the thin wires on the smoke alarm, wire nuts?

Today you can buy better splices than simple twist on wire nuts. Like Wago 221 terminals that can clip on to anything between 0.14mm2 to 4mm2 stranded or solid.

Obviously above does not provide you with physical strain relief. You'd have to buy smoke alarms that feature this (industrial) or create your own.