Interdisciplinary Studies

During the course of a typical workday, I suspect you design electrical systems. Moreover, when you design those electrical systems, I am sure everything seems to flow easily because you have developed a familiarity with and knowledge of electrical power; NFPA 70, National Electrical Code; and standard installation techniques. Designing electrical installations becomes second nature, and you reach a certain comfort level in which you can provide consistently reliable electrical systems that meet your customer’s needs.

Do you feel the same way when you design a fire alarm system or mass notification system (MNS)? In most, if not all, cases, when you design an electrical system, you know what it takes to ensure reliability. If you only design relatively few fire alarm systems during any given year, you may not know what makes such a system reliable. Understandably, a fire alarm system may constitute only a small portion of your overall electrical project, but it can reduce your profits immensely if you do not pay attention to proper design techniques and understand the requirements of NFPA 72 2010, the National Fire Alarm and Signaling Code.

When you perform fire alarm system designs, you assume additional responsibility for the life safety of the building occupants. Let’s face it. As a professional, when you design and install typical electrical systems in ordinary commercial or industrial occupancies—unless some outside force attacks the installation—there is little chance that loss of power to the building will cause a loss of life to the occupants. But, when a fire alarm system fails, it can jeopardize the lives of the occupants. If the system does not promptly detect a fire, occupants may not make it out. In addition, if the system causes continual false alarms, first responders are put in danger while responding unnecessarily. Therefore, your level of responsibility increases every time you design and install a fire alarm system.

Obviously, designing fire alarm systems represents serious work where you need to take care to ensure that you meet the design goals of reliability and credibility. When I mention “reliability,” I am speaking to the issue of ensuring the fire alarm system will operate properly when called on. When I mention “credibility,” I am speaking to the issue of having a fire alarm system where its design, installation and maintenance ensures no false alarms will occur.

NFPA 72 2010 contains the application guidance and installation requirements for fire alarm systems and for voice communication systems, such as emergency voice/alarm evacuation systems (EVACs) and MNSs. The code also provides design guidance for detection devices, alarm notification appliances and special applications.

Typically, the design will start by determining the owner’s needs and then applying your knowledge of detection and notification principles to meet them. The code will provide the spacing requirements for detection devices, but it will not tell you which detector to choose. You will need to reference other sources, such as the NFPA Fire Protection Handbook, to gain detector application knowledge. Or, you need to have experience in the applications of various types of detectors.
Generally speaking, heat detection, in its various forms, offers a relatively slow form of detection of fires from ordinary fuels. Heat detectors most often provide property protection or detection in areas when the use of smoke detection would prove inappropriate due to difficult environmental issues.

Smoke detectors generally offer life-safety-related detection, but in certain special applications, such as high-value electronic equipment, smoke detectors may provide property protection. NFPA 72 2010 describes the requirements for the application and spacing of both heat detectors and smoke detectors. Because improperly located or applied smoke detectors cause most false alarms, you need to take special care when designing fire alarm systems that use these detectors.
The code allows “normal” spacing of smoke detectors to consist of a nominal 30 feet on center. In corridors less than or equal to 10 feet wide, you may extend the smoke detector spacing to 41 feet on center.

The code requires you to account for numerous factors that will affect smoke detection, including ceiling shape, surface and height; configuration of contents in the protected area; combustion characteristics and probable equivalence ratio of the anticipated fires involving the fuel loads within the protected area; compartment ventilation; and ambient temperature, pressure, altitude, humidity and atmosphere. Some of these factors affect smoke detection, and others affect smoke detector operation.

Clearly, your design must intend to detect a fire before it will have an adverse affect on the occupants—or in the special cases mentioned above, on the sensitive electronic equipment. At the same time, you want the occupants to believe the system when it signals an alarm, so an alarm will warrant their action. Therefore, you need to consider where you place smoke detectors and whether the chosen location will cause the smoke detector to initiate a false alarm. This constitutes the credibility issue referred to earlier.

The code also offers guidance to reduce false alarms. NFPA 72 2010 requires you to select and place smoke detectors to account for both the performance characteristics of the smoke detectors and the environment of the areas where you intend to install them to prevent nuisance alarms. Keep in mind that electrical and mechanical influences, aerosols such as steam, or other substances such as dust can all adversely affect smoke detectors.

Where the code does not allow the installation of smoke detectors, you must review specific environmental conditions before installing smoke detectors; these conditions include temperatures below 32°F, temperatures above 100°F, relative humidity above 93 percent, and air velocity greater than 300 feet per minute.

If your design intends to replace an existing fire alarm system, you might become tempted to simply replace existing components one-for-one with new devices and appliances. Bad idea! The existing system may not meet the code requirements. If you do not properly analyze and create a competent design, you will fall into the trap of becoming liable for a new system that, itself, does not comply with the requirements of the code. Your analysis may also disclose the fact that the existing system’s improper smoke detector placement may have made that system susceptible to frequent false alarms. Why repeat that same mistake?

The code also requires all fire alarm systems to ensure the occupants will clearly hear or see and understand the alarm signals throughout the occupied portion of the building by meeting the sound pressure measurements required by the code. You will also meet the visible requirements for compliance with the Americans with Disabilities Act (ADA) by following the code’s placement requirements.

To ensure a cost-effective design for audible and visible notification appliance locations, I recommend you calculate the visible signal requirements first, locating them on the drawings as you move through the building. Then calculate the audible notification appliance needs to ensure you will comply with the audibility requirements of the code (15 dBA above average ambient sound level in the building or area or 5 dBA above the maximum sound level having a duration of at least 60 seconds, whichever is greater).

Once you have separately placed the visible and audible notification appliances on your design drawings, you may then use combination appliances wherever the two types of appliances fall in proximity to the same position. In other words, to ensure an economical notification appliance design, you do not want to use combination audible/visible notification appliances throughout the building.

If you design a voice communication system, you also must ensure the message meets the code’s audibility and intelligibility requirements. Annex A of the code offers guidance to assist you in the design of the speaker layout for most “normal” buildings. For example, in a standard building configuration with a normal ceiling height (8–12 feet), acoustical tile ceiling construction, standard wall configurations, and carpeted floors, you should ceiling-mount speakers in all normally occupiable spaces and in corridors installed at a maximum spacing of twice the ceiling height. This guidance means that, in a 10-foot-wide corridor, you would need to install twice as many speakers as smoke detectors.

You may need to include other devices and appliances to meet the building code or owner’s requirements. Obviously, the information supplied here cannot possibly provide an all-inclusive list of what you should do, but my goal is to raise awareness of the importance of thinking through a design to ensure the installed fire alarm system will work reliably when a fire occurs and not cause false alarms that will destroy its credibility.

MOORE, a licensed fire protection engineer, frequent speaker and an expert in the life safety field, is a past chair of the NFPA 72 Technical Correlating Committee. Moore is a principal with Hughes Associates Inc. at the Warwick, R.I., office. He can be reached at

About the Author

Wayne D. Moore

Fire/Life Safety Columnist
Wayne D. Moore, a licensed fire protection engineer, frequent speaker and an expert in the life safety field, is a principal member and past chair of NFPA 72, Chapter 24. Moore is a vice president with JENSEN HUGHES at the Warwick, R.I., office. He c...

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