Fire Alarm Voltage Drop questions

[olc]

Two questions:

1. Is there s standard de-rated voltage to start from? For example on a nominal 24V system that has devices that work from 16V-33V, should we start at 24V or at some lower voltage? (or do you just say 10% and we are good as long as the starting voltage is 18Vish?) (manufacturer dependent?)


2. I was trying out the AFAA voltage drop calculator. Perhaps I am using it wrong. I assumed the first device is the one closest to the panel. However the voltage drop calculation sums the current of the devices from the bottom of the list up instead of from the first device down. ??????

 

[gadfly56]

Two questions:

1. Is there s standard de-rated voltage to start from? For example on a nominal 24V system that has devices that work from 16V-33V, should we start at 24V or at some lower voltage? (or do you just say 10% and we are good as long as the starting voltage is 18Vish?) (manufacturer dependent?)


2. I was trying out the AFAA voltage drop calculator. Perhaps I am using it wrong. I assumed the first device is the one closest to the panel. However the voltage drop calculation sums the current of the devices from the bottom of the list up instead of from the first device down. ??????

1. This is dealer's choice. I've seen both 10% and even 25% used to de-rate batteries for age.

2. You'll notice that even though it fills out the entire column that the current draw at the end is the same as the last physical device you enter. The current draw is a calculated quantity and it's going to populate the column whether there's a device installed or not. I wouldn't worry about it.

 

[olc]

2. I found the problem. (I did my own spreadsheet starting with the furthest device so I confused the order in the formula on the AFAA spreadsheet.) I didn't change the wire size for the "balance of the circuit". Who changes wire size after the first device? (Obviously someone does. I guess it make sense for devices where the first device is a good distance from the panel. I have done it myself on light circuits which start a good distance from the panelboard.)

1. I saw one from Simplex that started at 19.5V.

 

[lowvoltageeng]

You should always start at 20.4. Never 24.

You should always start at 20.4. Never 24.

Hi there.

You should actually start your calculation at 20.4 volts according to UL 864. This is at a 15% drop.

 

[DM2-Inc]

Here's a good post on the subject matter: Sample Fire Alarm System Calculations, by Robert Schifiliti (former NFPA 72 Member), that does a good job of explaining the subject.

The article walks you though NFPA 72 (albeit pre 2000 edition) and explains that 20.4 volts is based on 85% of the power supplies "Name Plate Rated Voltage", and the need to provide sufficient voltage at the end of the standby period (i.e. system running on batteries for 24 hours).

I too wrote a spreadsheet for this, rather than use AFAA's version as they're version is more suited for commercial fire alarm system and not the industrial systems we're involved with (much higher loads). There are essentially three (3) approaches:

1. Calculations based on the entire load at the end of the circuit (conservative, but leads to larger wire size)
2. Calculations based on the load in the middle of the circuit.
3. Calculations based on a "Point-to-Point" method, which is the most accurate assuming you use the correct cable lengths