KMC Controls VEP-43 User Manual

Digital Designer’s Guide

Application Note–AN0604D

Revision B

Tips for connecting 24-volt power

This application note covers choosing a transformer and connecting
24 volt AC power to a KMC Controls DDC controller.

Multiple factors must be considered before choosing and connecting a
transformer to power a KMC controller. This application note provides
information to help system designers and installers make decisions about
powering controllers with transformers and then installing them correctly.

Choosing a
transformer

Danger

You can be killed or seriously injured if you contact live, high voltage circuits.
Disconnect AC mains and follow all local lock-out and tag-out procedures
when connecting or working on transformers.

Most KMC controllers require a 24-volt, AC power source for operation. The
recommend source for power is a UL listed, Class 2 transformer. When choosing a
transformer, consider the following:

◆ Mounting

◆ Primary voltage

◆ Secondary voltage

◆ Load capacity

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Tips for connecting 24-volt power

Choosing a transformer

Mounting Transformers supplied by KMC Controls may be either foot or

knock-out mounted. Foot mounting is usually used inside of an enclosure.
Knock-out mount attaches the transformer directly to a conduit fitting such as a
cover plate for a handy-box.

Primary voltage The primary voltage rating of the transformer must match the
voltage of the AC mains to which it will connect. Transformers are manufactured
for either a single primary voltage such 120 or 240 volts or multiple voltages. The
two XEE–6000 series transformers in
primaries.

Circuit breaker
(optional)

Bracket for foot
mounting

Illustration 1 KMC XEE–6000 series transformer

KMC Controls

Threads for knock
out mounting

Illustration 2 are examples of both types of

Circuit Breaker

120 VAC

Common

Single voltage primary transformer

Secondary voltage The secondary voltage for KMC transformers is 24 volts AC.
Some transformers include an optional circuit breaker which protects the
transformer and wiring from current that exceeds the capacity of the circuit
components.

Load capacity The load capacity is a measure of how much power the
transformer can deliver to attached devices. This is usually expressed as VA, an
abbreviation for Volt-Amperes. See
additional information on transformer load capacity.

480 VAC
277 VAC

24 VAC



240 VAC

120 VAC
Common

Multiple tap primary transformer

Illustration 2 Transformer schematics

Calculating transformer size on page 1-21 for

Circuit Breaker

24 VAC



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AN0604D Revision B

Digital Designer’s Guide

Connecting to
controllers

Tips for connecting 24-volt power

Connecting to controllers

KMC Controls recommends supplying power to each controller from a
transformer dedicated to the controller.

Illustration 3 is a typical connection of a
KMC XEE–6000 series transformer connected to a KMC controller. The usual
practice is to connect the phase (usually blue) wire of the transformer secondary
to the phase or ~

terminal and the common (usually brown) wire connected to the
– or ground terminal. Other transformers may use a different color scheme.
Unless required by an electrical code requirement, do not ground either lead of
the secondary.

24 VAC120 VAC



Illustration 3 Connecting to a single controller

Connecting several controllers to a single transformer

When more than one controller must be powered from a common transformer,
connect as shown in

Illustration 3. In addition, observe the following

requirements:

◆ Connect the phase (~) terminals in parallel with other phase terminals and

the commons (–) in parallel with the other common terminals. See
Illustration 4.

◆ If the secondary must be grounded, ground it at only one location. See

Grounded secondary connections

◆ Connect only controllers with the same type of power supply. Controllers

on page 1-18.

with half-wave power supplies and full-wave power supplies cannot be
powered directly by the same transformer. See Grounded secondary

connections on page 1-18.

◆ Choose a transformer with the correct VA rating. See Calculating transformer

size on page 1-21.

◆ Choose a wire size that limits the voltage drop to no more than 5%. See

Calculating voltage drop

on page 1-22.

AN0604D Revision B

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