Sound the Alarm: Smoke alarms and detectors in homes

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photo credit: istock / svengine
Published On
Oct 15, 2019

Hardwired and battery-operated fire and smoke alarms for dwelling units have become more sophisticated in recent years, which vastly affects their compatibility with both wired and wireless smoke alarms. Many of these devices now have built-in batteries that last a minimum of 10 years without replacement and combine smoke detection with carbon monoxide detection. Photoelectric smoke alarms have built-in smoke sensors that reduce false alarms from cooking and shower steam, the most common forms of false alarms when layout requires a location near a bathroom or kitchen. With the amount of vapor released, vaping can also be a trigger for detectors or smoke alarms.

Wireless smoke alarms can be monitored using your computer or cell phone, Ring Home Security, Nexia Home Intelligence Systems and other Z-Wave systems.

According to the Smart Home website, a Z-Wave system is “a wireless protocol harnessing low-energy radio waves to help smart devices communicate with each other. It was developed by Zensys in Denmark in 2001, released to the public in 2004 and introduced to the mass market in 2005 with the formation of the Z-Wave Alliance. The Z-Wave system operates at a frequency of 908.42 megahertz in the United States, and with the proper system and components, it can support up to 232 devices. An optimum limit, however, is 40–50 devices resulting in little or no interference from each other.”

This major breakthrough enables wireless devices can interconnect and communicate with each other.

Now, back to our hardwired and wireless smoke alarms and smoke detectors. The difference between a smoke alarm and a smoke detector is that a smoke alarm is self-contained with the detection and notification (alarm) in one unit, and a smoke detector requires the connection to a fire alarm panel with the alarm as a separate device.

To connect smoke detectors to a fire alarm panel, Article 760 of the National Electrical Code provides the wiring requirements, NFPA 72 provides the installation requirements of the fire alarm panel and devices, and the fire alarm panel provides power-limited sources of energy for the alarm devices. Many residential fire alarm panels can also provide burglar alarm protection by using Class 2 and Class 3 circuits complying with Article 725. Large homes often require many smoke detectors for fire protection, so a fire alarm panel is the appropriate answer to the installation. Separate zones at the fire alarm panel will provide individual circuits to the smoke detectors with addressable devices providing sophisticated coverage for large homes and those with valuable personal collections.

Hardwired smoke alarms are either single station or combination (interconnected). As mentioned previously, the single station is self-contained and does not have the capability of a wired interconnection. An interconnected smoke alarm system has a hot and a neutral conductor for the power -supply connection to the 120-volt (V) branch circuit, a battery backup in case of loss of power and a separate wire that can connect to other smoke alarms, so an alarm in one device causes all other devices to also go into alarm. The maximum number of smoke alarms that can be connected is 12. This interconnection function helps notify all occupants of the home that there is a fire and evacuation is necessary. Where more than 12 smoke alarms must be installed in a large single-family home, a fire alarm panel may be the better answer.

There are also hardwired units that can communicate wirelessly with other compatible alarms where the power supply to the smoke alarm is 120V, and the compatible smoke alarms will receive a wireless signal and sound the alarm. Where using a hardwired smoke alarm with the separate wire from detector to detector, a three-wire NM cable with an equipment grounding conductor can be used from one box to the next. The black and white conductors are the 120V power connection, and the red wire would be the interconnection conductor.

Where possible, locate any smoke alarm or detector at least 3—4 feet from a ceiling fan or an air conditioning or heating vent, since it will affect the detector’s reaction time. Most smoke alarms and detectors will cover an area of about 900 square feet or a 30-ft. area around the detector. Locate a detector in each bedroom and hallway outside of the bedrooms, so these devices will provide an alarm if a fire starts in the house and the smoke migrates into the hallway. Do not locate the detectors in the dead air zone within 4 to 12 inches of the ceiling or the same dimension on the ceiling from the wall. Attention in the placement and servicing of smoke detectors and alarms will help ensure safety from fires.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and

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