Harrington Signal Tracker T8000 - Install Manual - Manual (Page 42)

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HSI #780-0867 FireSpy Tracker Installation Manual
38
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.