Electrical – What type of electrical box do I use for a hard-wired smoke detector

I take it the detector is mounted in the ceiling, and a cut-in 3-O or 4-O (plastic) boxes have a 2 hour fire rating. If this is not enough then you can use metal ceiling octagon box (2 hour fire rating) and use something like a 3M Fire Barrier Moldable Putty Pads. Most companies that deal with fire related caulks and putty make these. A lot of states require these in fire walls and ceilings, especially in multi-family units.

These pads are intumescent, which means that it swells as a result of heat exposure, thus increasing in volume, and decreasing in density. Wikipedia.

If I were you I would take it a step further and install a 120V with a battery back-up smoke/carbon monoxide combo units. You can also pay more for the detector and get either optical or ionization styles. Ionization detectors are more sensitive to flaming fires than optical detectors, while optical detectors are more sensitive to smouldering fires. Wikipedia again. Electrical supply stores should have all this or Amazon. I suggested 3M only because that is what I am used to.

Also, since the electrical boxes are fire rated, you might just use a fire caulk around the box. Caulks are intumescent also.

Ionization smoke detectors use a radioisotope to generate a very small electrical current, which when interrupted causes an alarm to sound. These types of alarms use a metal–oxide–semiconductor field-effect transistor (MOSFET) to prevent the alarm from sounding, as long as the tiny current is flowing from the detector.

A simple dumbed down version of the circuit, might look something like this.

Notice the 9V battery is connected to the Source contact of the MOSFET, the detector is connected to the Gate, and the Alarm is connected to the Drain. This means that as long as there is a current on the gate, current will not flow from the source to the drain. If something (smoke) interrupts the current flow to the gate, current will flow from the source to the drain and the alarm will sound.

If you add a normally closed switch to the circuit, like so.

Opening the switch will prevent current from flowing to the gate, simulating a loss of current flow from the detector. Which in turn, will cause the alarm to trigger.

So when you press the test button on an ionization type smoke detector, you're not actually testing the circuits ability to detect smoke. Rather, you're testing the circuits ability to react to a loss of current flow to the gate contact of the MOSFET. You're also making sure the battery is not dead, and has enough juice left to sound the alarm if need be.