08/16/2026
This wiring shortcut looks finished and tests fine on power. It is not fine.
A hardwired interconnect cable has three conductors: hot (black), neutral (white), and the interconnect signal (red, sometimes orange). The red wire is the only one that carries the alarm trigger between units. People in a hurry tap it to a neutral, or jump it across to a second circuit so they can reach a far room without pulling new cable. The units power up, the green light glows, and it looks done.
What actually happened: the alarm signal now has nowhere to go. NEC and every manufacturer manual require all interconnected alarms to be on the same branch circuit with one continuous interconnect conductor, isolated from neutral. Cross it and the link silently fails. You will not know until a test, and many people never test a fresh install.
This is wiring work behind a junction box. If you are not comfortable identifying the traveler conductor and confirming a common circuit, this is the job to hand to a licensed electrician. A single-room interconnect tie-in is usually a small bill, far less than a missed-alarm fire.
If you want to avoid wall-fishing entirely, wireless-interconnect models skip the traveler wire. Each unit gets power locally and links by radio. No splice to get wrong.
How to confirm the link is real: with power on, press test on the unit farthest from the panel. Every other unit must sound. If only the one you pressed sounds, the interconnect conductor is mis-tied and needs to be corrected before you close the box.
A detail that trips people up: the interconnect conductor is not always red. Some cables and some older installs use orange, and a few use a wire that has been re-marked. Never assume by color alone. Confirm which conductor is the traveler against the alarm's harness diagram, because tying the hot or neutral where the signal wire belongs can damage the unit, not just break the link.
There is a related shortcut to also avoid: feeding the interconnect line from a switched circuit, like one tied into a lighting loop. If that switch is ever flipped off, your alarms lose power and the backup batteries quietly drain over the following weeks. Linked alarms belong on a constant, unswitched circuit. If you are not certain your run is unswitched, that is one more reason to confirm it with a licensed electrician before you trust the system.
