Appendix B: Worksheets
198 P/N 3102351-EN • REV 005 • ISS 28DEC18
Refer to the appliance installation sheets for Ia. Remember to use the maximum operating
current specified for FWR power.
2. Calculate the minimum voltage (Vm).
Vm = Vs − (Itot × K)
Where:
Vs = source voltage
Itot = total current (from above)
K = load factor
VS1 panel: For the power supply, Vs is 20.4 V and K is 0.20 V/A.
VS4 panel: For the power supply, Vs is 20.4 V and K is 0.24 V/A
3. Calculate the allowable voltage drop (Vd) between the power supply and the appliances.
Vd = Vm − Va
Where:
Vm = minimum voltage (from above)
Va = appliance minimum voltage
For regulated notification appliances, Va is 16 V. For special application notification
appliances, Va is the lowest operating voltage specified on the appliance installation sheet.
4. Calculate the maximum resistance (Rmax) for the wire.
Rmax = Vd / Itot
Where:
Vd = voltage drop
Itot = total current
5. Calculate the maximum length of the cable (Lc), based on the maximum resistance
allowed, the resistance of the wire, and the number of wires in the cable (two).
Lc = (Rmax / Rw) / 2
Where:
Rmax = maximum resistance
Rw = wire resistance factor
Example: You are using regulated notification appliances. Assume that the maximum
operating current for each appliance is 100 mA for FWR power, and that 20 appliances will be
placed on the NAC. The cable is 12 AWG wire, and the manufacturer specifies a wire
resistance factor of 0.002 Ω/ft.
Itot = ΣIa
= 20 × 0.1 A
= 2 A
Vm = Vr − (Itot × K)
= 20.4 V − (2 A × 0.20 V/A)
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