Engineering Documentation

Chapter 2 – Network Electrical Systems
Power Trunk Guidelines
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NOTE:
Different controllers have different power ratings. You may need to calculate the
voltage available at the last controller of each type on each wiring run. To calculate
these voltages, you must know the following:
- The length of each leg of the wiring run.
- The VA rating for each controller on the wiring run.
- Which devices pull power through each leg.
3. Determine how many VA are being drawn through each leg by summing the VA
ratings for all controllers pulling power through each leg.
4. Determine the voltage drop for each leg:
 Voltage drop = (total VA)/24V × 0.005 ohms/ft × distance in feet
 Where:
0.005 is the resistance in ohms/ft for a pair of No. 14 AWG wires.
NOTE:
If a different wire gauge is used, the corresponding resistance must also be used.
The values for all approved wire pairs are as follows:
- AWG 14 = 0.005 ohms/ft
- AWG 16 = 0.008 ohms/ft
- AWG 18 = 0.012 ohms/ft
- AWG 20 = 0.020 ohms/ft
- AWG 22 = 0.033 ohms/ft
- AWG 24 = 0.051 ohms/ft (UTP resistance greater than 2C = 0.048 ohms/ft)
5. Determine the voltage available at the last controllers:
a. Calculate the starting voltage:
Starting voltage = transformer voltage × 0.9
Where:
0.9 is an efficiency factor to account for transformer inefficiencies and lint
voltage variations.
b.
Calculate the voltage drop to the last controllers:
Sum the voltage drops of all legs between the transformer and the
last controller
For example, in Figure Wiring Run
Voltage drop to C5 = (Vdrop L1) + (Vdrop L2) + (Vdrop L5)
c. Calculate the voltage at the last controller
Starting voltage – Voltage drop to the last controller
(Step 5a minus Step 5b)
d.
Check the power source requirements for the DXR2 or PTEC/TEC and
verify that your total (the voltage available at the last controller) is greater
than the minimum required for that controller type. If your total is not
greater than the minimum, the power trunk must be reconfigured.