There are two ways to wire smoke detectors: in parallel and independently. Wiring smoke detectors in parallel will result in an interconnection between all smoke detectors such that if one detector sounds, all connected detectors will go off as well. The National Electric Code now requires hard wired interconnected smoke detectors in every bedroom, outside of every sleeping area, stairways and at least one on each floor. Hard wired smoke detectors must have battery backup in case of electrical failure.
Run No.12/2 electrical wire from the service panel to the location of the first smoke detector. This wire will carry power to all detectors.
Run No.12/3 wire between the first smoke detector and the second one. Do the same for the rest of the smoke detectors. Strip off 8 inches of the sheathing at the end of each wire, revealing the inner wires. The wires are black, white, red and green or bare copper. At the first smoke detector, there is No.12/2 wire coming from the service panel and the No.12/3 wire travelling to the next smoke detector.
Join the three black wires together, the one coming from the service panel, the one going to the next detector, and the one on the smoke detector itself. Twist them together using the pliers, then attach a wire nut on top of the wires. Rotate the wire nut clockwise to tighten it until it is snug. Repeat the steps with the three white wires and the three green wires. The green wires are then connected to the metal back box of the smoke detectors.
Connect the red wire for the interconnection between detectors. The red wire is connected to the third wire on the smoke detector (usually a yellow wire.) This wire carries a special signal to all other detectors when an alarm goes off. The signal will result in all smoke detectors sounding off as well. This is required by the NEC in all new constructions, according to the 2008 and 2011 versions.
Plug the wires into the smoke detector wiring module, if there is one that comes with the smoke detector. Feed the module through the mounting plate, then insert it in the right place on the smoke detector. Wire the rest of the smoke detectors the same way and test the connection to make sure it works.
When testing the connection, do not just test the circuit. Test the smoke detectors' functionality as well using a smoke can. Do this weekly to ensure all detectors are working properly.