22 • Cresnet® Network
Design Guide — Doc. 9292A
The R (resistance) value correlates with a wire gauge as shown in the following table. Therefore,
using the equation above, the selected wire gauge must correlate with a resistance value that is
less than the value derived from 40,000 / (L x PF).
Resistance Value to Wire Gauge Comparison Table
Resistance (R) Value
Wire Gauge
1.6 Ω
12 AWG (3.31 mm
2
) - Cresnet High-Power
6 Ω
18 AWG (0.82 mm
2
) - Cresnet Standard Power
Example Scenarios
The following example scenarios show how the resistance equation can be used to solve for
different variables within a Cresnet wiring run.
Example Scenario #1: Calculate Wire Gauge
This scenario explains how to calculate the wire gauge needed for powering one GLS-ODT-C-CN
occupancy sensor (requiring 1.5 W) that is 1,500 ft (457 mm) away from its power source.
Use the resistance formula to calculate the resistance value of the wire run, where 1,500 is the
value for L (length of wiring run) and 1.5 is the value for PF (Cresnet power factor):
l
R < 40,000 / (1,500 x 1.5)
l
R < 17.8
Next, use the table above to compare the resistance value of the wiring run with the resistance
value of the Cresnet wires. For this scenario, any wire gauge with a resistance value less than
17.8 can be used to supply power to the GLS-ODT-C-CN.
Example Scenario #2: Calculate Wire Run Length
This scenario explains how to calculate the maximum distance that twelve SSC-102-EL room
occupancy hallway signs (requiring 2.4 W each for a total of 28.8 W) can be placed from a client
control system using 12 AWG (3.31mm
2
) wire.
Rewrite the resistance formula to solve for L, where 1.6 is the value for R (resistance value
correlating with the wire AWG) and 28.8 is the value for PF (Cresnet power factor):
l
L < 40,000 / (1.6 x 28.8)
l
L < 868
For this scenario, the maximum length of wiring that can be run to the SCC-102-EL devices from
the power source is 868 ft (265 m).
NOTE: if using standard power Cresnet wire (18 AWG), the maximum length of wiring would
be 231 ft (70 m) , as derived from L < 40,000 / (6 x 28.8).
