Greetings Douglas,

Do you know what the Multiplier Factor for 15 Minutes is if the Multiplier Factor For 5 Minutes is 0.084?

Thank you,**TM**

Do you know what the Multiplier Factor for 15 Minutes is if the Multiplier Factor For 5 Minutes is 0.084?

Thank you,

I assume from your question that you are calculating the size of needed power backup of the fire alarm system during a 24-hour power blackout, and at the end of 24 hours, either 5 minutes or 15 minutes of sounding the alarm.
**Battery Capability**

The battery is rated in Amps of Current available for 1 Hour. An 8 Amp Hour battery can produce **8 amps**, for **1 hour**. Another way of looking at the 8 Amp Hour battery is that it's good for **1 amp** in a period of **8 hours**. The same battery is good for **1/3 of an amp (0.333 amps)** for **24 hours**.
**Standby Battery Usage**

During an electrical blackout, the battery has to keep the fire alarm system working for 24 hours. To figure out the amount of Amp Hours needed to keep the system working for 24 hours, multiply the standby current for the fire alarm system by 24 (Hours). 24 is your standby multiplier factor. This is how many Amp Hours needed just to keep the fire alarm system standing-by for 24 hours.

**Alarm Battery Usage**

At the end of 24 hours, the alarms have to sound for either 5 minutes or 15 minutes.
**5 minutes is 1/12 of an hour, or about 0.084 hours. The multiplication factor for 5 minutes is 0.084.**
**15 minutes is 1/4 of an hour, or about 0.25 hours. The multiplication factor for 15 minutes is 0.25**.
To figure out the amount of Amp Hours needed to keep the system working for 5 minutes or 15 minutes, multiply the standby current for the fire alarm system by either 0.084 (Hours) or 0.25 (hours), which is your standby multiplier factor. This is how many Amp Hours that are needed just to keep the fire alarm system in alarm for 15 minutes.
**Headroom**

Add the standby battery Amp Hours to the alarm battery Amp Hours to get a rough total number of Amp Hours that are needed for the standby batteries.
This is an electronic system, and it is also a life-safety system. There should be a minimum of 20% headroom in case something changes over time.
Take the rough total Amp Hours that are needed, and multiply it by 1.20 to get the minimum number of Amp Hours needed for the standby batteries.
**Actual Batteries**

The number of Amp Hours you calculated are never the number shown on the size of the batteries. You can't use lower Amp Hour batteries than you calculated, but you can always install larger batteries.
Use the next bigger battery size available for the real batteries that are installed.

**Douglas Krantz**

Get the Book **Make It Work - Adressable Signaling Line Circuits**

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