For any electrical device to operate properly, it must have sufficient
voltage to operate as designed. This is especially true for fire alarm
audible/visible notification appliances (A/V devices). Therefore, every
security professional must know how to calculate the voltage drop of
the notification appliance circuit (NAC circuit).
Occupant notification is a critical function of a fire alarm system:
The primary function of a fire alarm system is for the protection of life
and property. For a fire alarm system to meet this objective, it must
perform the following four basic operations:
1. Early detection of a fire
2. Notification of the building’s occupants that a fire condition
exists
3. Operation of other building emergency control functions such
as elevator recall, HVAC control and release of hold open devices
for smoke doors
4. Fire department notification
Why calculate voltage drop?
Needless to say, it is critical that every NAC circuit be properly designed
before the installation. This will insure that every A/V device, including
the last one on a NAC circuit, has sufficient voltage to operate properly,
thereby allowing the building’s occupants to safely exit the premises.
It is important to note, that by taking the time before the installation to
calculate the voltage drop of each NAC circuit, it will save time, and the
financial loss of re-wiring the circuit or having to install NAC power
boosters.
Furthermore, many consulting engineers and Authority Having
Jurisdiction (AHJs) require NAC circuit voltage drop calculations as
part of the job submittals. This will verify that there is sufficient
voltage at the end of the NAC circuit to operate the last A/V device.
The first step to a well-designed NAC circuit:
Changes to the resistance of any electrical circuit, including NAC
circuits, can take place over time. Several reasons for increased circuit
resistance are oxidation and the connection of additional A/V devices
after the initial installation.
Therefore, it is highly recommended by industry experts to add a safety
factor (tolerance) of 10% when designing NAC circuits. By adding a
safety factor of 10% to a circuit, the designer will determine the
minimum voltage at the end of the circuit. Adding a safety factor will
future proof the installation by assuring the circuit will continue to
operate as originally designed. To determine the minimum allowable
end-of-line voltage (ELV) of a NAC circuit, a 24VDC source voltage (SV)
and using a 10% safety factor (SF) the formula would be as follows:
ELV = SV - (SV x SF)
ELV = 24 - (24 x .10)
ELV = 24 - 2.4
ELV = 21.6VDC
In the above scenario the maximum allowable voltage drop (VD) using
a 10% safety factor is 2.4VDC. Even though most all 24VDC A/V
devices and control panels will operate at 21.6VDC it is imperative to
check the manufacturers installation instructions.
The basics of a well-designed NAC circuit:
When designing a NAC circuit, the designer must know the resistance
per foot on the cable used, the total distance of the NAC circuit, how
many A/V devices will be connected to the NAC circuit and the current
draw of each A/V device. The conductor resistance can be found in
Chapter 9 (Tables), Table 8 of the 1999 edition of the National Electrical
Code (NEC). The resistance of the most commonly used copper conductors used for NAC circuits are:
Ohms Per Foot Wire Gauge Chart
American Wire Gauge (AWG)
Ohms per foot (OPF)
#18 AWG 0.00845
#14 AWG 0.00326
#16 AWG 0.00529
#12 AWG 0.00205
The total circuit distance includes both the outgoing conductors and the
return conductors. For example, if the circuit distance from the control
panel or NAC power booster, to the last A/V device is 250’, then the
total circuit distance is 500’. Always check the manufacturers specifications of the A/V device for the total currant draw of the unit.
Determining what wire gauge to use:
Lets determine what conductor size to use for a hypothetical new
installation. Remember, that it is best to do these calculations prior to
the installation to avoid the grief, and the financial loss of re-wiring the
circuit or having to install NAC power boosters.
• NAC circuit source voltage (SV):
• 24VDC*
• Collective Load (CL):
20 horn/strobes with a current of 0.075 amps = 1.5 amps
• Circuit distance (CD): 300’ x 2 = 600’
If a maximum allowable voltage drop of 2.4VDC is used, the maximum
allowable ohms-per-feet (OPF) is calculated by using the following
formula:
OPF = VD/CD
OPF = 2.4/600
OPF = 0.004
Now go to the wire gauge chart and locate the best wire gauge for the
hypothetical installation. In this case the design calls for a #16 AWG
conductor.
Verifying the wire gauge:
It is also a good idea to verify the existing NAC circuit conductor size
when adding new or additional A/V devices of a retrofit installation.
Lets use this hypothetical situation for upgrading an existing installation to meet current ADA requirements:
* Existing wire gauge size: #18 AWG
* NAC circuit source voltage (SV): 24VDC
* Collective Load (CL):
15 new horn/strobes with a current of 0.075 amps = 0.125 amps
* Total circuit distance (CD): 150’
If a maximum allowable voltage drop of 2.4VDC is used, the maximum
allowable circuit distance is calculated by using the following formula:
CD=(SV-ELV)/(2 x CL x OPF)
CD=(24 - 21.6)/(1.125 x 2 x 0.00845)
CD=(2.4)/(0.019)
CD=126’
In this case using the existing #18 AWG conductors would not support
the 15 new horn/strobes. This means the designer will have to
decrease the number of A/V devices or replace the #18 AWG wire with
a number #16 AWG conductor.
Wired for safety:
By calculating the voltage drop for new and retrofit installations, the
designer can rest assured that their fire alarm system will operate
without trouble in the future. Remember, the fire alarm designer has
the ultimate responsibility to insure that the system operates according
to the manufacturers specifications.
Thanks, Richard Roberts,
Product Manager/Fire Products
Check fire codes with local jurisdiction.
For Technical Systems Support call:1-800-ADI-SYS1