Safely wiring a 120v smoke detector externally

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.

• Probably this is not an issue, but in principle it would be correct to check the cable not only for the used voltage but also for the current it has to deliver. – Manumerous Oct 14 '20 at 6:19
• Wire nuts are not the only way to connect wires together. – Simon B Oct 14 '20 at 7:26
• Fair, solder and heat shrink, or crimp on connectors are more elegant options. Strain relief is more of a concern as Jeron3 mentions. – PBJ Oct 14 '20 at 8:08