Potter Fire Alarm Systems User Manual

Page 29

Fire Alarm Training • 8700055 • Rev F • 4/10

Notification Voltage Drop

When installing notification devices the installer must be aware of the operating voltage of the devices and ensure that the voltage

supplied is within the listing of the device. Failure to make the appropriate calculations could result in the notification circuits to

not operate. The installer must know the voltage of the system, the total current available per circuit, the number of devices that

need to be connected, the current draw of each device, the minimum device operating voltage, the length of the wire run and the

wire size. Commercial fire alarm control panels are generally required to operate at 20.4 volts (85% of 24 volts) on battery back

up. Failing to consider the minimum operating voltage or the characteristics of the devices connected to the system may cause the

system not to operate as intended.
Notification voltage drop calculations are used to determine if the power at the last notification device is sufficient to power

the last device. The voltage drop is a result of the added resistance from the wire as the length of the wire run increases. The

most simplistic way of calculating the voltage drop is to use a computer program. Numerous sources are available with these

calculators, refer to the Automatic Fire Alarm Associations web site at

www.afaa.org

for an example.

The most accurate way to manually calculate the voltage drop is to start with a panel voltage of 20.4 volts (85% of the nominal 24

volts). That is the minimum voltage at which the fire panel is required to operate. This voltage would be worse case after the panel

is operating on battery power for an extended period of time. Calculate the wire resistance from the fire panel to the first appliance

and multiply that by the current draw of all appliances. Subtract that number from 20.4. That will be the available voltage at

that the first appliance. Make sure that number is larger than the lowest operating voltage of the appliance. Calculate the wire

resistance to the next appliance, multiply that by the current draw of the remaining appliances and subtract that number from the

voltage at the previous appliance. Continue for all appliances on the NAC circuit. Make sure the voltage at the last appliance is

within the operating range of the appliance.
The alternative to calculating voltage drop from device to device is to use the lump sum method. The calculations are performed

as though all the appliances are installed at the end of the wire run. This method produces a rather large safety factor. Although

this is a more conservative method, it may result in unnecessary, extra power supplies that will drive up the cost of the installation.
As a general rule it is advisable to keep the voltage drop on a NAC to 2.5 volts or less.

Ohm’s Law = E Voltage drop can be calculated by E

= I

x R1

(I)(R)

Where: E

= Voltage drop, I

= Total Current (of the Notification Appliance), R1 = Resistance (of the wire)

Wire Resistance

AWG#

Ohm’s per 1000 feet

1.6

2.5

4.0

6.4

10.0

16.0

Battery Standby Calculations

The fire alarm system is required to have a secondary power source. Most often this is accomplished through the use of battery

back up. In order for the batteries to power the panel for a given time and still have enough capacity to power the system in alarm

requires the batteries to be properly sized for the given standby and alarm power loads. The battery calculations need to consider

all power requirements of the system. In addition, the standard requires that the batteries be derated to provide a safety margin.
Most fire alarm equipment has a stand-by (Non-alarm State) and alarm current draw. Generally, the alarm condition of the

device is higher than the standby current draw. The total current draw is calculated in a number of steps and the final current

draw is in amp hours. First, the totals of each initiating device types are totaled and multiplied by the standby current. Then all

of the standby currents including the panel, all of the initiating devices, remote annunciators and any other auxiliary currents are

added together in amps. Then, all of the alarm currents are added together in amps. The standby current is then multiplied by the

number of standby hours required. The alarm current is then multiplied by the number of minutes in alarm expressed in hours (for

example 5 minutes divided by 60 minutes per hour equals 0.084). The amp hour of the standby current (usually much larger) and

the amp hour of the alarm current are added together and multiplied by 1.2. This final number is the minimum required amp hour

rating that must be used to achieve the amount of standby and alarm current necessary.
[(Standby Amps) * (# of hours of Standby)] +[ (Alarm Amps) * ( % of hours in Alarm)] = Total Current

Total Current (in Amp Hours) * 1.2 (safety factor) = Minimum Battery Size Required (See example on following page)
Fire alarm systems are usually either 12 volt or 24 volt. The batteries are rated at 12 volts and then have an amp hour rating. The

standard amp hour rating is 4, 7, 8, 12, 18, 26, 28, 33 and 55. The 24-volt DC systems use two 12 VDC batteries wired in series

to provide the needed voltage and maintain the same amp rating. The 12 VDC systems use a single battery, but can wire two

batteries in parallel to double the amp hour rating.