Appendix A: System calculations
P/N 3101890-EN • REV 006 • ISS 21JUN18 177
Notification appliance circuit calculations
Introduction
This topic shows you how to determine the maximum cable length of a notification appliance circuit for a given
number of appliances.
Two methods are presented: worksheet and equation. The worksheet method is simpler, but your installation
must meet the criteria listed on the worksheet. If your installation does not meet these criteria, you need to use
the equation method.
The methods given here determine cable lengths that work under all operating conditions. The calculations
ensure that the required operating voltage and current will be supplied to all notification appliances. To do this, we
assume these two worst-case conditions:
• The voltage at the NAC terminals is the minimum provided by the power supply
• The notification appliances are clustered at the end of the NAC cable
Other, more detailed methods that distribute the appliance load along the NAC cable may indicate that longer
cable runs are possible.
What you will need
Appliance and cable values
Whether you use the worksheet method or the equation method, you will need to know the following:
• The minimum operating voltage required for the appliances
• The maximum operating current drawn by each appliance
• The resistance per unit length of the wire used (Ω/ft)
This information can be found on the appliance installation sheets and on the cable specification sheet.
Power supply values
For either method, you will need some fixed or calculated operating values for your specific power supply. The
fixed values are:
• Maximum voltage = 26.4 V
• Source voltage = 22.4 V
• Load factor = 0.0 V/A
• Power type = DC
Note: The PS10-4B has an onboard boost circuit to keep battery voltage at a required value. If the battery voltage
drops below the required value, the boost circuit activates.
The maximum voltage is the highest voltage measured at the NAC terminals. This value is not used in the
calculations, but is given so you can ensure appliance compatibility.
The source voltage is the theoretical operating minimum for the NAC source, and is calculated as 85% of 24 volts
[minus the diode drop].
The load factor is a measure of how the power supply voltage reacts when a load is applied. The load factor
measures the voltage drop per ampere of current drawn by the load.
The power type reflects the type of power supplied to the NAC terminals at minimum voltage. The current draw of
notification appliances can vary substantially with the type of power supplied: full-wave rectified (VFWR) or direct
current (VDC). It is important to know the power type at minimum terminal voltage.
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