Det-Tronics EA2100CG Eagle 2000 Communication Gateway - Manual (Page 22)

Temperature. Resistance increases as operating tem-
perature rises. Calculate resistance at a worst case
temperature of 75˚C. See Table 5.
Type of device. Refer to the “Specifications” section for
current ratings. Always consider the maximum (not
nominal) current draw of each device. This typically
occurs at startup and/or when relays are energized.
Note that combustible gas DCUs draw considerably
more current than other DCU models.
Number of units per wiring run. Add the totals of all
units on the wiring run.
Allowable voltage drop. The DCU needs a minimum
of 18 vdc (measured at the DCU) to ensure proper oper-
ation. Subtract 18 vdc from the power supply output
voltage to determine the maximum allowable voltage
drop.
The maximum wiring limits can be calculated using
Ohm’s law and the information in Table 5.
Example A:
What is the longest wiring distance for one com-
bustible gas DCU using 18 AWG wire?
NOTE
For the following examples, assume that a 24 vdc
nominal power supply is being used. A com-
bustible gas DCU draws approximately 500 ma at
startup.
24 vdc (power supply output) minus 18 vdc (mini-
mum voltage required) = 6 vdc (maximum allow-
able voltage drop).
Voltage ÷ Current = Resistance (Ohm’s law)
Maximum allowable voltage drop ÷ maximum cur-
rent draw = maximum allowable resistance
6 vdc ÷ 500 ma = 12 ohms
12 ohms ÷ 7.765 (resistance for 1000 ft. 18 AWG
wire) = 1.55
1.55 x 1000 = 1550 feet
Example B:
How many combustible gas DCUs can operate at
the end of a 1000 foot length of 14 AWG wire?
Voltage ÷ Resistance = Current (Ohm’s law)
Maximum allowable voltage drop ÷ resistance for
1000 ft. of 14 AWG wire = maximum current draw
6 vdc ÷ 3.071 ohms = 1.95 amps
1.95 amps ÷ 500 ma (current draw per unit) = 4
units
Example C:
What wire size is needed to power 10 combustible
gas DCUs at a distance of 500 feet from the power
supply?
10 units x 500 ma (maximum current draw per
unit) = 5 amps (total current draw at startup)
Voltage ÷ Current = Resistance (Ohm’s law)
Maximum allowable voltage drop ÷ Total current
draw = maximum allowable resistance for 500 feet
of wire
6 vdc ÷ 5 amps = 1.2 ohms
500 feet of 12 AWG wire is 0.9665 ohms
NOTE
The following section provides a general description
of the basic procedure for installing the Eagle sys-
tem. Before wiring any Eagle device, refer to the
instruction manual that was provided with the device
for complete installation and wiring instructions:
Communication Module 95-8425
DCU 95-8426
Network Extender 95-8430
Relay Module 95-8423
Gateway 95-8424
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