Appendix A: System calculations
EST3X Technical Reference Manual 247
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: The NAC source is the PS10-4B. You are using regulated
notification appliances. Assume that the maximum operating current for each
appliance is 100 mA for DC 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)
= 22.4 V − (2 A × 0.0 V/A)
= 22.4 V − 0.0 V
= 22.4 V
Vd = Vm − Va
= 22.4 V − 16.0 V
= 6.4 V
Rmax = Vd / Itot
= 6.4 V / 2.0 A
= 3.2 Ω
Lc = (Rmax / Rw) / 2
= (3.2 Ω / 0.002 Ω/ft) / 2
= 1600.0 ft / 2
= 800.0 ft
So the maximum wire run for this NAC would be 800 ft (rounding down for
safety).
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