Each smoke detector comes with a base plate and a 3-wire pigtail that is wired into the house wiring.

This metal ceiling junction box has three 14-3G cables entering it. When multiple AC smoke detectors are wired together, 3-conductor wire must be used.

The third wire is for interconnecting with other smoke detectors so all detectors will beep in the event one detector senses smoke.

There usually is a threaded hole (red arrow) in the back of a metal J-box that will accept a ground screw.

I tightened the screw and made sure the wire was snug. Note that a good half-circle of wire needs to be beneath the screw head.

The wires need to stick out of the junction box by at least 6 inches. I avoid leaving too much wire sticking out, because then it's difficult to pack all those wires into the J-box.

With three number 14 wires, I find that the wires become twisted when I apply the wire nut. With heavier gauge wires, or more wires, I find it necessary to pre-twist the wires with a pair of lineman's pliers.

This is stranded wire, and I always strip stranded wire slightly longer than the solid wires it's being connected to.

Next I twisted each stranded wire so the individual strands would stay together and not fray.

Note that the yellow pigtail wire gets connected to the red solid wire.

Sometimes the J-box is too deep behind the drywall, and I need to use longer screws. These are #8-32 machine screws, which can be found at any hardware store.

Note: I am not aware of any standard size/style of smoke detector connector. Different brands of smoke detectors use a different pigtail connector, so if a detector needs to be replaced, and it's a different brand, the pigtail will likely also need to be replaced.

The red arrows point to the holes that engage the tabs in the base plate.

Easier said than done.

First a 4-inch hole is cut into the wallboard and the electrical wire is fished through the space behind the drywall. Then this junction box simply clamps to the drywall.

Since this J-box is plastic, the ground wire does not need to be attached to the box. Since smoke detectors do not use a ground connection, the ground wire in this 14-3G cable is not needed.

It's never a good idea to cut off unused ground wire... somebody else might change this box someday and need the ground wire.

When the solid wire is perfectly straight and connected to a stranded wire, I find that the wire nut often falls off because the stranded wire gets all bunched up in the wrong spot. Making this kink in the solid wire seems to help.

The mounting screws (A) had to be used with the slots in the base plate.

The LINE IN cable begins at a junction box that is part of the basement lighting circuit. Note that this wire is NOT SWITCHED, and it is NOT PROTECTED BY A GFI (GROUND-FAULT INTERRUPTER).