If you are wiring a new smoke detector on your own, and you are working with a 120 volt hard wired detector, be aware that the white and black wires coming out of the electrical box are for power and will be ‘hot’. The red wire is the inter-connected wire that will trigger other hard wired detectors in the residence in case of smoke detection or fire. The red wire does not carry power and you can not get shocked by it.
The red wire must be connected to all detectors that are intended to sound at the same time in case of smoke. This would also apply for a carbon monoxide incident but only if all the detectors are also ‘combination type’ ‘detectors. The interconnected red wire does not apply to battery only type detectors, only hard wired type detectors.
Note: In some older detectors the red wire will be yellow.
The red wire travels alongside the white and black power wires, inside of the electrical cable in the ceiling and walls, and is installed at the time of construction of the residence. If one detector has the red wired hooked up, and the other detectors do not, each detector will sound on their own and not in unison.
Older homes in Southern California which do not have the red wire combined with the black and white power wires usually can not be retrofitted to ‘interconnect’ due to the cost of rewiring. A much smarter option is Smart detectors like Google Nest Protect detectors and the like.
Why have an interconnected red wire with a smoke detector in the first place?
Especially in larger homes, if there is a triggered smoke detector at one end of the residence, occupants will be alerted even if they do not smell smoke at the other end with interconnected detectors. Every detector in a house will sound. The interconnected red wire requirement is also a code requirement in the state of California for all new construction, and has been for a while. Retrofitting older hard wired smoke detectors is not a requirement due to cost.
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