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Today I installed some new Kidde PI2010 Hardwired Dual Photoelectric and Ionization Sensor Smoke Alarm with Battery Backup smoke alarms. I am interconnecting 5 of them in my home. When I hooked up the third one the other two went into alarm. I measured 120V on the red interconnect wire. Is this correct? Im reading here that it should be 9V? Any help would be appreciated.