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HSI #780-0867 FireSpy Tracker Installation Manual

Eq. C-2 Worst case wire length, general form (for SLC circuits)

Vdrop

Length_w.c. = -------------------------------------------------------

Idraw x { Rpanel + (Rcable x DF2) + (Rdevices x DF2) }

Length_w.c. is the worst case length of wire (distance from panel) in feet

Vdrop is the maximum voltage drop in volts. Use 2.4 to assume 10% voltage drop on a 24V circuit.

Idraw is the total current draw of the circuit in Amps

Rpanel is the resistance of the circuit internal to the panel, in ohms. Use 0 if unknown.

Rdevices is the total internal resistance of the devices (usually isolators), in ohms. Use 0 if unknown.

Rcable is the total resistance of the cable in ohms

Equation Eq. C-3 calculates the voltage drop of a cable run having devices evenly distributed on the

cable, following a run with no devices. The voltage drop must not exceed the allowable voltage drop of

devices installed on the circuit, i.e. the voltage supplied to a device must be above the minimum input

voltage for the device.

Eq. C-3

Vdrop = Idraw x { Rpanel + (Rcable x DF2) + (Rdevices x DF2) }

Eq. C-4

Rcable = 2 x Rwire x 1000 x Ltotal

Eq. C-5

Lfirst ( Ltotal – Lfirst ) x DF1

DF2 = -------- + ---------------------------

Ltotal Ltotal

Eq. C-6

Number of devices + 1

DF1 = -------------------------

Number of devices x 2

Vdrop is the maximum voltage drop in volts

Rwire is the resistance of the wire in ohms per 1000 ft (see Table C-1)

Idraw is the total current draw of the circuit in Amps

Ltotal is the length of wire (distance from panel) in feet

Lfirst is the length of wire to the first device (distance from panel) in feet

Table C-1: Resistance of Solid Copper Wire

Wire gauge (AWG) Resistance (ohms per 1000 ft)

18 6.39

16 4.02

14 2.52

12 1.59

* Values are for uncoated copper at 20°C (68°F) and are only applicable within certain parameters, especially temperature. Not all

parameters are shown here for simplicity. Refer to NEC Chapter 9, Table 8 for additional information.

C.3 Wire selection tables

NOTE: All wire lengths below assume the use of solid copper wire and worst case

loading (all load at the end of the wire run). The length shown is based on

resistance, capacitance and/or voltage drop only. Other parameters may

need to be considered, as well.