Engineering Documentation

Chapter 2 – Network Electrical Systems
Power Trunk Guidelines
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Power Trunk Transformer Specification Data
Primary Volts: As Required
Secondary Volts 24 (50/60 Hz)
1
24 Volts Secondary
Volt-Amp Rating Output Amperage Fuse Amps
2
50 2.08 2.5
75 3.12 3.2
100 4.16 4.0
150
3
6.35 4.0
250
3
10.4 4.0
350
3
14.6 4.0
500
3
20.8 4.0
750
3
31.2 4.0
1000
3
41.7 4.0
1)
NEC requires that the Secondary must be grounded if the Primary exceeds 150 volts to ground.
2)
The fuse for each circuit from a transformer rated greater than 100 VA must be 4.0 amps maximum.
The type of fuse required depends on local interpretation of the National Electric Code. Most
frequently, transformers with multiple output circuits and multiple fuses are interpreted as Class 1
circuits.
3)
To comply with NEC Class 2 requirements, each circuit from transformers cannot be greater than
100 VA and transformers cannot exceed 100 VA. Circuits connected to transformers rated over 100
VA must be treated as Class 1 – that is, in conduit, separate from trunk and point wiring. When
power requirements exceed 100 VA, it is recommended that multiple transformers 100 VA or less
be used, rather than a single transformer. Check local codes to determine whether larger
transformers, in combination with fused circuits, can be classified as Class 2 circuits.
Power Trunk Layout
Layout is accomplished by completing the following procedures:
1. Determine the VA rating minimum voltage input for each controller.
2. Determine the number of power trunks required.
3. Determine the wring runs and calculate the voltage at the last controller of each
trunk type.
4. Select and locate the transformers.