Engineering Documentation

Chapter 2 – Network Electrical Systems
Power Trunk Guidelines
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Power Trunk Guidelines
A Class 2 circuit, as defined in the National Electrical Code (NEC, operates at less
than 30 volts AC (Vac), and is limited to 100 volt-amps (VA) or less. Class 2 circuits
are granted special exceptions in the NEC for installation wiring, making it
unnecessary to use conduit in most applications.
Class 2 Power Sources
There are two types of Class 2 power sources:
● Inherently limited
● Not inherently limited
Inherently Limited Class 2 Power Source
An inherently limited Class 2 power source has some form of current-limiting
characteristic designed into the product. Sources of this type are often protected by a
current-limiting impedance or embedded fusible link, but other methods are also used.
As long as the current limiting is an integral part of the power supply, it will fall into this
category.
NOTE:
Because of this built-in current-limiting characteristic, a circuit powered by this type of
source needs no further protection to qualify as a Class 2 circuit.
Inherently limited Class 2 transformers are generally available with ratings up to about
60 VA. They will often be direct plug-in type transformers, similar to those used to
power calculators or other small devices. This makes them well suited to applications
using a separate transformer for each controller. They can also be used for small
power trunk applications, up to the VA rating of the transformer.
Not Inherently Limited Class 2 Power Source
A Class 2 source that is not inherently limited does not have built-in current limiting
protection. At the time of installation, a current-limiting device must be installed
between the source and the loads. The most common current limiting device for this
application is a single fuse or integral transformer circuit breaker, which must be sized
so that the power available to the loads does not exceed 100 VA.
Transformers that are not inherently limited are most commonly used for power trunk
applications. Transformers of this type are usually direct wire types, and are available
in sizes that permit power trunks up to the full 100 VA allowed. It should be noted that
with the additional power capabilities come additional requirements and restrictions at
the time of application.
NOTE:
In order to meet NEC Class 2 requirements, using a transformer that is not inherently
limited is subject to the following rules:
● Each transformer must have a nameplate rating of 100 VA or less.
● Unloaded (open circuit) voltage on any circuit cannot exceed 30 Vac.
● Each trunk must be limited to 100 VA or less.