Engineering Documentation

Chapter 2 – Network Electrical Systems
Power Trunk Guidelines
125
-3002 65 | 231
● For 24V power trunks, each transformer circuit must be protected by a single fuse
or integral circuit breaker rated 4 amps or less. This protection is required even if
the transformer is rated at 100 VA or less.
● A fuse block for the trunk fuses may be required by local code.
CAUTION
Always check local codes to determine whether there are differences from the
NEC. Specifically, you should determine whether fused circuits are acceptable
as Class 2 in your area.
Class 2 Power Trunks
The following information will help you lay out power trunks for supplying power to
multiple controllers.
Each power trunk will be supplied by a step-down transformer located near a
convenient source of line voltage. In general, over-current protection will be required
between the step-down transformer and the controllers. 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.
Use Class 2 power trunks where possible because they can often be run without
conduit. Where conduit is required, Class 2 power trunks can be run in the same
conduit with FLN trunks and AI or DI wiring.
Grounding
Earth ground point for Class 2 power trunk transformer secondary neutral must be
connected back to earth ground for Service using a dedicated ground wire. Service
must be same as used for FLN Controller and all other FLN devices.
Restrictions
● When using power trunks, any relays, EPs, or contactors must be protected with
MOVs at their connection to the trunk.
● The fused side of each power trunk must only be connected to terminals labeled +,
24 Vac, or HOT.
● Where different services are used, they must be banded per NEC Article 250, or
Communication Isolation devices must be used.
● Multiple power trunks from the same transformer must be kept in phase. Avoid
using different transformers to power the loads and the controllers. If unavoidable,
use relay modules to provide isolation for loads connected to different
transformers.
● If power trunks are connected to UCs, the unfused side of the transformer must be
grounded at the transformer and can only be connected to device terminals
labeled COMMON or NEUTRAL.
CAUTION
Failure to adhere to these polarity conventions can result in equipment damage.