and missing, incomplete or incorrect secondary power calculations are
among the most common causes for rejection of a submittal to an engineer
or to the authority having jurisdiction (AHJ). A previous article
addressed the requirements for the features and performance of both
primary and secondary power supplies.1 The article showed how
to determine the required demand and durations for the secondary power
supply. This article shows how to use the demands and durations to
calculate the net required capacity for batteries that are used as part
or all of a secondary power supply.
1 shows that batteries will always be a part of a signaling system
power supply. The most common configuration is where batteries are
incorporated to provide separate, switched secondary power to the
system. In that configuration, the batteries are connected in a way that
allows the control unit power supply to switch from the primary source
to the secondary source when the primary is lost or disconnected.
Figure 1. Power Supply Requirements
The figure shows two arrangements for the use of batteries as a switched secondary power supply. The first is where the batteries supply the entire secondary power supply. The second is where the batteries back up a primary power supply that also includes a backup generator.
the previous article, it was shown that the code permits a reduced
duration for the operation of the batteries where the primary power
supply includes a backup generator.2 NFPA 72 does not refer
to the UPS option as "secondary power”. Still, the batteries of the UPS
provide that function. As noted in the previous article, the batteries
on the UPS require the same duration, hence capacity, as those connected
directly to the control unit. Operationally, the UPS is a Type 0 (per
NFPA 1113) where the batteries always provide power to the
system and are recharged by the primary power supply. Thus, there is no
switchover that must take place when primary power is lost.
For each of the three battery configurations permitted by NFPA 72, the code has specified the required duration (time, t) for battery operation. The load, or demand is the amount of current (I) supplied by the batteries at a particular time and is a function of the system design and configuration.
Most errors in calculating the required battery (and generator) capacity (stored energy, E)
occur in determining the required load. The code specifies two types of
loads (demands) and associated durations to be used for determining the
required secondary supply capacity. The first is the normal, quiescent
load. This is the amount of current that the system demands during its
normal, non-alarm state. Depending on the type of system, the code
requires that the batteries be capable of providing that amount of
current for a specified period (see the first article). The code
requires that at the end of the specified quiescent period, the system
must be capable of supplying the alarm load for a specified period.
general alarm systems, the demand current is based on the entire system
operating in the alarm mode. This means that all notification
appliances and emergency control function interfaces are operating. The
demand for emergency voice alarm indication systems (EVACS) and mass
notification systems (MNS) will actually vary over the required
duration. Therefore, the code permits the capacity to be calculated
using the full alarm load, but over a reduced duration in order to
simulate the intermittent operation over a longer period. The total
required capacity is determined by summing the capacity required to
serve the quiescent load and the capacity required to serve the alarm
ETotal = ENormal + EAlarm
ET = INtN +IAtA
Where I is electrical current in amperes, t is the time in hours and E is energy in units of amp-hours.
As a minimum, the code requires that the batteries be sized to supply the actual (design) quiescent load and alarm loads for the specified durations. However, how often is the final installed quantity of devices and appliances the same as the original design? While calculations based on a design are a useful starting point, the code requires that the secondary power system be adequate for the final installed load. Therefore, engineers should do one of two things to assure compliance: 1) require recalculation after the final system configuration; or 2) require the capacity to be calculated using the full load capability of the system. If the first option is used, it is only fair that the contractor be compensated for any change orders that add load that must be accommodated for the completed installation.
second option is the best practice, but is not required by code. For
that option, if a circuit is rated for 2.0 amps by the manufacturer, the
calculation would assume it is fully loaded even if only 0.75 amps of
load is initially being installed. This would ensure that all future
changes would not require a change in batteries. The same argument for
the second option can be made for determining the required wire size.
calculate the battery size using the minimum code approach, the
quiescent and alarm loads must be tabulated and summed for all
equipment, devices, and appliances connected to the power supply. If the
second approach is used, the quantities of initiating devices and
notification appliances is important only for determining the number of
modules and circuits that will be provided to accommodate them. It is
usually best to allow the installer and manufacturer to determine the
number of modules and circuits because it is sometimes less expensive to
install additional circuits than it would be to use fewer, but longer
circuits. Also, even if the load will be calculated using the full
circuit capacity, it is best for the engineer to specify that the
circuits not be loaded more than a certain percentage. This will further
ensure that additional devices and appliances can be added without the
need to add modules and circuits.
battery calculations should be done by the installing contractor,
system distributor or equipment manufacturer and then checked by the
responsible designer and by the authority having jurisdiction (AHJ). The
first step is to gather the manufacturers’ published specification
sheets for all devices, appliances, and equipment. It is necessary to
list all components and the quantities used. For each item, the
specifications will list the quiescent (normal, non-alarm) current
(load) and the alarm current (load) in amps.
1 is a simplified example for how the capacity of secondary batteries
is calculated. For this example, the required duration is 24 hours in
quiescent mode and five minutes (0.083 hours) in alarm mode. The
calculation will be done for a specific system configuration with a
specific quantity of devices and appliances. If the batteries are to be
sized for full circuit loading (option 2), the list of initiating
devices and notification appliances would be replaced with a listing of
the circuits and their full load current capability.
|Standby Current, Amps||Alarm Current, Amps|
|PB Smoke Det.||2||0.0450||0.0900||0.0600||0.1200|
|15 cd strobes||15||0.000||0.0590||0.8850|
|110 cd strobes||4||0.000||0.1450||0.5800|
|Horn/15 cd strobe||10||0.000||0.0830||0.8300|
|Horn/110 cd strobe||5||0.000||0.1930||0.9650|
|Net Standby Load, Amps:||0.9409|
|Net Alarm Load, Amps:||4.6857|
|Enter Required Standby Duration:||24||Hours|
|Enter Required Alarm Duration:||5||Mins||0.083||Hours|
|Total Standby, Amp-Hours:||22.5816||Amp-hours|
|Total Alarm, Amp-Hours:||0.3905||Amp-hours|
|Total Calculated Battery Capacity:||23.0||Amp-hours|
|Required Factor of Safety:||20%|
|Code Required Battery Size/Capacity:||28||Amp-hours|
|Supplied Battery Size/Capacity:||36||Amp-hours|
|Actual Factor of Safety:||57%|
Table 1. Simplified Secondary Power Calculation Example
The required capacity is calculated by multiplying the load by the required duration for both the quiescent condition and the alarm condition. In this example, for the quiescent condition, the total standby (quiescent) load of 0.9409 amps is multiplied by 24 hours to get 22.6 amp-hours of required quiescent capacity. The total alarm load of 4.6857 is multiplied by 0.083 hours to get a required alarm capacity of 0.4 amp-hours. They add together and round to a required capacity of 23 amp-hours. New in the 2010 edition of NFPA 72 is a required 20% factor of safety, bringing the net required capacity to 27.6, or 28 amp-hours after rounding.
have calculation programs to determine the battery capacity. In reality,
most systems will have many more entries for panel components.
are several entries in the above example worth discussing. The alarm
current listed for the power supply is the current that the power supply
uses as it supplies the other loads. The option 2 method could be
modeled by simply assuming that the power supply is at full load. So, a
power supply listed to provide a maximum of 4 amps would list 4 amps as
the alarm load regardless of how many modules, circuits, devices, or
appliances are actually connected to it.
smoke detectors and any initiating devices that draw power, how many
should be considered to be in alarm? This example has all 52 smoke
detectors in alarm, but it is also common to use a number that
represents the largest one or two fire areas in the building. The relays
are shown to be energized in the non-alarm condition and dropping out
(de-energized) upon alarm. Systems might also have relays that are
normally not energized until there is an alarm.
final step is to select a battery that has the required stored capacity
and that can discharge at the required rates. In this example, a
battery is needed that has a capacity of 28 amp hours or more and that
can discharge at a rate of 1 amp (0.9393 rounded) for 24 hours and then
be able to discharge at a rate of 4.7 amps for a duration of 5 minutes.
analogy might help. A gravity water tank has a certain maximum stored
capacity. The flow rate from the tank is a function of the outlet and
distribution piping and the height of water in the tank. When full, the
hydraulic head is actually greater and the system will flow at a higher
rate than when the tank is near empty. As the tank empties, the flow
The system (tank or
batteries) must be designed to provide the required discharge at all
stages of use. Battery manufacturers and suppliers can provide
documentation regarding a battery’s ability to discharge at certain
rates at the end of the discharge cycle.
are required by NFPA 72 to be labeled with the date of manufacture. In
prior editions of the code, there was a five-year replacement
requirement. In the 2013 edition, replacement is required as recommended
by the manufacturer or when the batteries fail during testing. The
five-year requirement was removed in favor of "replacement as
recommended by the manufacturer,” which may be less than five years.
respect to occupancy hazards and risks, engineers should consider that
the secondary supply is only for the fire or signaling system control
unit. Any transmitters or sub-panels used for communications will have
their own power supplies with the same requirements for secondary power.
The public communications infrastructure is outside the jurisdiction of
NFPA 72 recognizes that
the Federal Communications Commission has jurisdiction over the
installation requirements for parts of the communication infrastructure
used to transmit signals from a protected premises to a supervising
station. Traditional telephone central offices and managed facilities
voice network (MFVN) facilities used by Internet service providers will
typically have 24 hours or more of standby battery capacity in addition
to backup generators.
modern communications methods, including telephone and Internet service,
may not be powered entirely from the central office. Instead, they may
have in-building circuits powered from a network interface device at the
property that requires primary power and includes a backup battery.
Those backup batteries are a part of the communications system, not the
fire alarm or signaling system, and are sized for only about eight hours
of standby. Both traditional telephone and Internet services provided
by an MFVN will usually have field located concentrator units along the
path from the protected premises to the central office or MFVN. These
local concentrator units, which can frequently be seen on poles or in
pedestals throughout a community, also have primary power and batteries
for secondary power. So, while a fire alarm or signaling system designed
in accordance with NFPA 72 might continue to operate during an extended
power outage, its ability to communicate off premises might be limited
to eight hours or less. This needs to be factored into emergency
planning for the property.
actual selection of power supplies and calculations of battery capacity
are not difficult, selecting the proper parameters and combinations of
power supplies requires engineering consideration. The designer must
consider the environmental conditions, hazards involved and the
resulting risks when specifying power supply durations for fire alarm
and signaling systems.
- "Power Supply Requirements for Fire Alarm and Signaling Systems,” Fire Protection Engineering, 1st Quarter 2012.
- NFPA 72, National Fire Alarm and Signaling Code, National Fire Protection Association, Quincy, MA, 2013.
- NFPA 111, Standard on Stored Electrical Energy Emergency and Standby Power Systems, National Fire Protection Association, Quincy, MA, 2010.