Kidde VM-1 Technical Reference Manual

Appendix A: System calculations
P/N 3101890-EN • REV 006 • ISS 21JUN18 181
3. Calculate the allowable voltage drop (Vd) between the NAC circuit source 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 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. Assume that the maximum operating current for
each appliance is 100 mA for [DC or 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)
= 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).

Kidde VM-1 Specifications

General

Brand	Kidde
Model	VM-1
Category	Fire Alarms
Language	English