Edwards Signaling FSC Series - Manual (Page 87)

Appendix A: System calculations
FSC Series Technical Reference Manual 79
See the appliance installation sheets for Ia. Remember to use the maximum
operating current specified for FWR power.
2. Calculate the minimum voltage (Vm).
Vm = Vr − (Itot × K)
Where:
Vs = source voltage
Itot = total current (from above)
K = load factor
For the power supply, Vs is 20.4 V and K is 0.25 A/V for three-zone panels,
0.40A/V for five-zone panels, and 0.20 A/V for ten-zone panels
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 unregulated notification
appliances, Va is the lowest operating voltage specified on the appliance
installation sheet.
4. Calculate the maximum resistance (Rmax) the wire can have.
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’re using regulated notification appliances with a ten-zone control
panel. 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.