System calculations
B.16 EST3 Installation and Service Manual
Example:
You’re 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)
= 20.4 V − (2 A × 0.37 V/A)
= 20.4 V − 0.74 V
= 19.66 V
Vd = Vm − Va
= 19.66 V − 16.0 V
= 3.66 V
Rmax = Vd / Itot
= 3.66 V / 2.0 A
= 1.83 Ω
Lc = (Rmax / Rw) / 2
= (1.83 Ω / 0.002 Ω/ft) / 2
= (915 ft) / 2
= 457.5 ft
So the maximum wire run for this NAC would be 457 ft
(rounding down for safety).
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