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Proper Fire Alarm Design & Why Expertise Prevails

There is a major consideration when you choose a design company. Expertise can outweigh costs and bring tangible value in the long run. Avoid costly errors that can delay a project or worse cause be the cause of a tragic event for failure to detect or alert occupants.

Understadng Requirments

Proper Fire Alarm System Design
Silently Standing By For Early Notification


Based on NFPA (National Fire Protection Association) Applied Research published in September 2021 from 1980 to 2020 civilian injuries and deaths from structure fires have been reduced significantly.

Due to the stringent code requirements, buildings are safer for occupants and minimize intellectual property loss by today's standards. This is the result of decades worth of research. Thanks to fire protection engineers, and cutting-edge passive and active fire protection, today's buildings are protected.

Fire alarms play a major role in early detection to notify occupants of a possible threat while simultaneously signaling emergency forces. Not only do fire alarms notify occupants, but complex fire alarm systems can also systematically control safety functions. Examples would include control of elevator recall systems, closing magnetically held doors, and fire dampers, and shutting down HVAC air handling units.


There is a major consideration when you choose a design company. Expertise can outweigh and bring tangible value in the long run.  

Design With Intent To Alert & Protect ALL Occupants 

A new R-2 building per IBC section 907.2.9 does not require smoke detectors in the corridors.

Let's assume that the building will be geared towards occupants aged 65 and older and the building is in the South.

Though most occupants are capable of self-preservation they may be slower to respond during an emergency event. The occupants would benefit from voluntary early warning detection in addition to the prescribed code minimum requirements giving people with a medical or physical disadvantage more time to react.


An experienced designer will communicate these factors during Phase II. 

Senior with Mask

Fire Alarm Design

Code Minimum May Not Be Enough

Construction Site

Fire Alarm Design
Understanding The Importance of The Correct Code Models

The other aspect is using the correct code models for the application. For example, the International Building Code IBC is utilized for new buildings classified as new construction where existing buildings (structures) are subject to the NFPA-101 life safety code.

The NFPA-101 requires increased protection due to the inherent risk of fire from building age. The NFPA-101 further divides occupancies into New & Existing. You may be thinking for New the IBC is applied. This is incorrect, the key factor is an existing building. The building may be undergoing a change of use or commodity such as a Use Group B Business Occupancy office building that will be renovated into an R-2 Apartment Building. The NFPA-101 would be applied unless the entire building is demolished and is re-built from the foundation up adhering to the latest International Building Code requirements.

Fire Alarm Design

A Common Error of Applying The Incorrect Code Model

A common mistake where the incorrect code model is applied is within Assembly occupancies. The international building code section 907.2.1.1 states in Group A Assemblies with an occupant load of 1000 or more shall initiate a signal using a voice/alarm communications system. In the NFPA-101 for New Assemblies and for Existing Assemblies, the requirement for a voice/alarm system is triggered at an occupant load of just 300. If a designer inadvertently designs a fire alarm system under the IBC on an existing building the consequences can be extremely costly. The difference between a standard fire alarm system and a fire alarm system with a voice evacuation can be upwards of 80,000 dollars. A movie theater (pictured) is an Assembly Occupancy.

Unfortunately, we have seen this very mistake happen to a building owner and created an unnecessary complex situation that involved legal measures to be taken.

Movie Fun

Fire Alarm Design

Correct Candela Selection

The intensity of the strobe known as the candela is measured in a special luminous intensity and it is required to set the candela on the notification appliance to 177 candela (cd) if it is within 24” of the ceiling. If the device such the entire lens is further than 24” from the ceiling it is permissible to set the device to 110 candela (cd). The reason for the higher intensity requirement, when closer to the ceiling, is due to the reduced effectiveness of the device when smoke is present. A good example is thinking about a lighthouse beacon in the fog. The less-dense the fog the easier it will be to identify. If you installed a stronger brighter beacon, it will be able to penetrate the fog further. This same concept holds true for selecting the proper candela rating. Unfortunately, I have personally had designs come across my desk where the devices were set to 15 candelas. Not only does this not meet the requirements of the NFPA-72, but it can also generate a scenario where not enough power is available on the circuit from limiting factors such as power supply, wire distance, gauge, and other appliances on the same circuit. The higher the intensity of the candela setting the higher the current draw and more resistance.

Knowledge & Expertise Is Absolutely Necessary

A new designer with limited experience places 10 horn/strobes set at 15 candela on a circuit that is 500 feet long and specifies the wire size at 18 gauge which carries a resistance of 8.08 ohms per 1000’. Each device draws 0.054 milliamps. The power supply is rated for 3 amps and the circuit is a 24 Volt DC (Direct Current). The designer also did not factor in the safety margin.


Upon performing calculations, the designer sees the total combined load is .540 amps with a 4.36 volt drop from the source which leaves us with an end of line voltage of 21.31 Volts. The last device, at the end of line, requires a minimum of 16 volts to operate. This looks okay to the design and completes the calcs. The submittal is sent in, the fire marshal is overburdened and quickly processes the application. The field installers simply follow the drawing set and install as planned and they pass their rough inspection and the walls are closed. The installers are now installing devices as per the plans. The fire marshal arrives to perform the building CO (Certificate of Occupancy) and the fire alarm is a part of this process. The fire marshal discovers inside two of the ADA units the candela rating is too low. The fire marshal fails the CO and installers change the candela rating on the 2 units classified as ADA rooms.


Fire Scene Investigation

Pictured Above

The aftermath of A Multi Building Fire

A Horn/Strobe Set To 15 Candela

Inadequate For Sleeping Rooms 

Requiring a Candela Setting of 110-177 Based 

On Installation Height From The Ceiling

They set the candela to 185 CD because the device is less than 24” from the ceiling. The installers go to test the devices and everything appears to be operational. Going back to the calculations the new load is now 0.922 amps as opposed to .540 amp due to the 2 devices that were changed and at 185 CD they are drawing 0.245 milliamps as opposed to .054 milliamps at 15 CD. The end of line calculation is 18.32 Volts. We need 16 volts minimum to operate so everything seems correct right? Remember the designer forgot to apply the correction factor? Fire alarms and devices must be able to operate within 85% of their nameplate voltage. 85% of 24 Volts DC is 20.4 Volts. This is required per NFPA-72 as the starting voltage. A windstorm disabled power lines and the building lost power for 23 hours. Near 23 hours the fire alarm has been running on battery backup which is slowly using its ampacity. Think of a car with the headlights left on overnight. It may barely have amp hours to turn the starter over. There is a fire near the end of the building and occupants succumb to smoke inhalation. A fire investigation is launched and other tenants recall not hearing the fire alarm at the end of the building as they were traversing the path of egress. An investigation reveals the fire alarm system devices did not activate and a specialized engineer is called in to analyze the point of failure of the fire alarm system. Through analysis, the engineer finds a correction factor was not applied. With a starting voltage of 20.4 instead of 24.0 Volts DC, the end of line voltage is 14.72 Volts. The weak state of the batteries did not have enough amperage to power the devices with a safety factor across the 18 gauge wire. Had power not been lost to the building, arguably, the system devices would have alerted the occupants.

Had the designer reviewed the specifications carefully the ADA units would have been taken into account and the designer would have required a minimum of 16 gauge cable as opposed to 18 gauge? With 16 gauge wire and a correction factor, the end of line voltage would have been 16.83 Volts. Marginally above the 16 Volt minimum. 

In the above scenario I, personally, would have specified a 14 gauge cable. At 14 gauge we would be at an end of line voltage of 18.16 Volts with a resistance of 3.19 ohms per 1000’ referencing Table 8 of NFPA-70 conductor properties for copper conductors for DC circuits. 


- Joseph Montuori, CEO, SET


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View A Proper FA Drawing

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