The First Alert SA521CN-3ST interconnected smoke alarm with hardwire adapter included allows you to create an interconnected fire alarm system without the hassle of wiring. Use this smoke detector to replace an existing hardwired smoke alarm and then install additional First Alert wireless alarms for an interconnected safety network (additional alarms sold separately). This hardwired smoke and fire detector can connect and communicate with up to 18 compatible alarms, so when one alarm is triggered, the entire system will sound. Equip with battery backup, this smoke and fire alarm allows for protection during power outages.