Calculation Crunching: Fire alarm math you should know

iStock / AlexMillos
iStock / AlexMillos
Published On
Jul 15, 2021

Knowing how to perform battery and voltage drop calculations is essential to ensuring an installed system will operate properly, and especially important in the plan review process. Battery and voltage drop calculations are included in the list of minimum required documents in Chapter 7 of NFPA 72. They were also required in Section 907.2 of the International Building and Fire Codes up through the 2015 editions. This list was deleted in 2018 with a reference to the list in NFPA 72. In this article, I will review battery calculations, and will discuss voltage drop calculations in the future.

Battery calculations’ purpose is to determine the minimum battery capacity needed to meet the NFPA 72 secondary power requirements of 24 hours of standby followed by 5 minutes (general alarm) or 15 minutes (voice systems for partial initial notification). While this is a good starting point, it does not ensure the system will always last that long. That is why NFPA 72 requires annual battery testing. For a number of years, Table in NFPA 72 stated that batteries had to be replaced within five years of the manufacture date.

Now the language states “Replace batteries in accordance with the recommendations of the alarm equipment manufacturer or when the recharged battery voltage or current falls below the manufacturer’s recommendations.”

DIY calculations

Most, if not all, fire alarm control unit manufacturers provide battery calculation forms and equipment instructions. There is also a generic way to perform these calculations.

Since batteries are rated in amp-hours, current draws must be converted to amps, and time must be converted to hours. All you need to do is add up all nonalarm current draw, such as from the control unit, annunciators, smoke detectors, addressable control modules, etc. Make sure they are all in amps (a milliamp is 0.001 amp) and multiply by 24.

Next, add all alarm current, such as from the control unit, annunciators, horns, strobes, addressable control modules, relays energized in alarm, etc. Multiply by 0.083 hours (60 minutes divided by 5 minutes).

Add the nonalarm and alarm totals together and add a 20% safety margin. This 20% is to help compensate for battery aging. Once this is done, you have the minimum battery size allowed for your system.

A great question that always comes up in my seminars is “What percentage of the system needs to be in alarm for the purposes of performing battery calculations?” Unfortunately, NFPA 72 does not address this. I am a big believer in calculating for a worst-case scenario. I recommend determining the alarm current draw with all current-drawing equipment in alarm. In reality, if you only used 10% or 20% of the devices in alarm, it wouldn’t change the battery size by much anyway. Remember, we are only talking about 5 minutes of operation. I would still recommend using 100%. If you ever had to defend this in court, it would be better to use worst-case rather than a percentage that is not in the codes.

How many times have you added to an existing fire alarm system and the fire department plan reviewer wanted you to perform new battery calculations? I never understood the value of this, since most additions barely change the battery size.

There is a way to perform actual battery calculations on-site so you know exactly what the system needs. You need an ammeter to measure the system current draw. Disconnect a lead from the battery and connect the ammeter in series between the battery and the panel. Set your meter at a high enough setting to prevent damage. Turn off the primary power, so the system is only operating on battery, and then read the current draw. That is the actual amount of current being used in a nonalarm condition for this particular system. Now, once again, turn the meter settings up to the highest setting. Put the system into general alarm and adjust the meter to get a reading. This is your full general alarm current draw.

Add the two readings together and add 20%. That answer will tell you what size batteries should be installed for that system. This is a very useful way to ensure batteries are sized properly. Now, if you need to add more equipment to this system, you have a starting point. Simply add the current draw (nonalarm and alarm) to the appropriate figures and you will know if you will have to increase the battery size for the addition.

About the Author
Tom Hammerberg

Thomas P. Hammerberg

Life Safety Columnist

Thomas P. Hammerberg, SET, CFPS is an independent fire alarm presenter and consultant in The Villages, Fla. He can be reached at

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