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CTV-PRB004-EN
Electrical System–Power Wire Sizing
This section provides guidance for sizing the power conductors forTrane CenTraVac water chillers
including wiring from the distribution system to the starter, and wiring from the starter to the
compressor motor. The information provided does not constitute a design manual and does not
supersede local code requirements or National Electric Code (NEC) recommendations. Final wire
size selection must be verified by the Engineer of Record for the installation.
Wire size and count should be coordinated with the starter selection so the starter can be ordered
with the proper terminal lugs. The starter is shipped with standard lugs as shown on the as-built
submittals. Alternate lugs are often available and can be purchased locally. Standard lugs accept
a wide range of wire sizes, but not all possible combinations.
General wiring requirements
•
When more than one lead per phase is used (paralleled conductors), the leads must be of the
same size and length so that they share the load equally. Paralleled conductors are only
permitted for wire sizes of 1/0 and larger.
•
When two or more conduits are used, all three phases must be equally represented in each
conduit to avoid overheating.
Specific requirements for distribution system to starter wiring
•
Wiring to the starter must be copper. A discussion of why aluminum should not be used can
be found in the Glossary (
“Aluminum conductors,” p. 73
).
•
Wiring to all starters must be in multiples of three.
Specific requirements for starter to motor wiring
•
All wiring to the CenTraVac motor must be copper. The UL listing on the CenTraVac requires
copper conductors, and the terminal blocks on the motor studs are only compatible with copper
wire.
•
Wiring to motors used with variable frequency drive, across-the-line, autotransformer, or
primary reactor starters must be in multiples of three. Wire for these starters is sized following
the procedure used for sizing supply wire from the distribution system to the starter.
•
Wiring to motors used with wye-delta (also referred to as star-delta) or inside-the-delta solid-
state starters must be in multiples of six.The solid-state starter wire sizing tables included here
are only applicable to the inside-the-delta solid-state starter designs offered for CenTraVac
chillers shipped since June 1999. All other solid-state starter designs should follow the across-
the-line wire sizing tables.
•
A minimum of three feet of flexible conduit is required at the connection to the motor junction
box to provide vibration isolation.
Wire size guidelines
The following information is based on the 2011 National Electric Code (NEC). Most local codes are
based on the NEC; however, local codes may vary from the NEC on specific details. Final wire size
selection must be verified by the Engineer of Record for the installation.
Various factors are involved in selecting the proper wire including amperage draw of the load, type
of load, voltage, temperature rating of the wire, number of conductors within a conduit, and
ambient temperature. In addition, the wires connecting the starter to the motor may need to be
sized differently than the wires that connect the distribution system to the starter.
Good circuit design also involves checking the voltage drop of the distribution circuit.The NEC does
not require this step, but does suggest that 3 percent voltage drop on a branch circuit feeder and
5 percent overall will provide “reasonable efficiency of operation”.
The NEC requires HVAC manufacturers to calculate and include “minimum supply circuit
conductor ampacity” (also referred to as Minimum Circuit Ampacity [MCA]) on the unit name plate
(section 440.4 [B]). MCA is equal to 1.25 times the RLA of the largest motor plus 100 percent of all
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