Engineering Documentation

Chapter 2 – Network Electrical Systems
Power Trunk Guidelines
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NOTE:
Rerouting the power trunk so that controllers with the lowest minimum voltage
requirements are at the end of the run, and controllers with the highest minimum
voltage requirements are closest to the transformer can help correct voltage drop
problems.
If this is not possible, or still does not provide the necessary voltage at the last
device, try using a T-shaped power trunk (such as Trunks A or B) rather than a
straight line (such as Trunk C) to reduce the voltage drop even further. In other
words, a T-shaped power trunk allows you to obtain a higher voltage at the last
controller. Using larger gauge wire for the power trunk will also help reduce the
voltage drop.
To complete this example, the results for the last controllers on the remaining runs are
found to be:
Voltage Minimum Status
V(C4) 21.16 19.2 OK
V(C6) 20.63 19.2 OK
V(C10) 21.36 19.2 OK
V(C7) 20.89 19.2 OK
Since there are different types of equipment controllers (various DXR2s, PTEC/TECs,
etc.) with different minimum power requirements mixed on the same trunk, you must
identify the last type of each controller on each trunk. Determine if any of these
controllers has a higher minimum voltage requirement than the controller at the end of
the run. In this example, calculations are also necessary to determine the following:
Voltage Minimum Status
V(C5) 20.94 20.4 OK
V(C8) 21.42 20.4 OK
Since the voltage at each controller was found to be greater than the minimum
requirement, this layout is correct.
Step 4 - Select and Locate the Transformers
1. Using the trunk configuration that was defined and verified in Step 3 above, there
are a number of options available:
– Two 100 VA transformers and one 75 VA transformer can be chosen from
Table Physical Network Limitations and provided with 4A fuses.
– Two 100 VA transformers can be chosen from Table Physical Network
Limitations, each provided with 4A fuses, and one 55 VA or larger inherently
limited transformer from a local source could be used.
– If local codes permit, one 250 VA transformer can be chosen from Table
Physical Network Limitations and provided with three fuses.
2. Locate the transformers and fuse block, with three fuses, if required, at the
breaker panel. Not all transformers require fuses; however, those that do should
be connected as follows:
– Connect the line side of fuses to the secondaries of the transformers.
– One power trunk will be connected to the load side of each fuse where
required.