Engineering Documentation

Chapter 2 – Network Electrical Systems
Power Trunk Guidelines
125
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Step 2 - Determine the Number of Power Trunks
Required
Use the following steps to select and locate the transformers.
1. Based on the total VA, determine the transformer(s) you will use to supply power
to the trunks.
 For example, if the total VA required for all controllers is 129.3 VA, you could
use any combination of inherently limited Class 2 transformers that supply the
required power. Or, based on the transformers listed in Table Physical
Network Limitations, you could use two 75 VA not inherently limited
transformers, each with a field installed 4A fuse. Or, if acceptable to the
authority having jurisdiction, use one 150 VA transformer with two 4A fuses.
See Figures Power Trunk Layout, Class 2 Circuits [➙ 65] and Power Trunk
Layout, Class 1 Circuit [➙ 65] and Table Power Trunk Transformer
Specification Data [➙ 65] for details.
NOTE:
All transformers listed in Table Power Trunk Transformer Specification Data [➙ 65]
are the “not inherently limited” type. Therefore, you must adhere to the following
guidelines to comply with NEC Class 2 requirements.
- Each transformer must be limited to 100 VA or less.
- Unloaded (open circuit) voltage on any circuit must not exceed 30 Vac.
- Each trunk must be limited to 100 VA or less.
- For 24V power trunks, each circuit must be protected by fuses rated 4 amps or less.
A fuse block for the trunk fuses can be required by local code.
2. Determine the minimum number of power trunks needed:
 Minimum number of trunks = Total VA/100 VA.
 Depending on your loads and how they are positioned, it may be necessary to
use more than the minimum number of trunks.
3. Locate the transformers and fuse blocks, with one fuse for each of the trunks at
the breaker panel. Connect the line side of all fuses to the secondary of the
transformer. One power trunk will be connected to the load side of each fuse.
Example
1.
Determine the total VA ratings for all controllers. In this example, the VA
required is:
1 ×
5.7 VA
=
5.7 VA
1 X
15.0 VA
=
15.0
3 ×
13.6 VA
=
40.8 VA
1 ×
32.7
5
VA
=
32.75 VA
2 ×
27.9 VA
=
55.8 VA