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Power and Control Wiring
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Figure 3 Typical static transfer switch one-line diagram
The input and output power wire size should be based on the upstream overcurrent protection device,
observing the NEC and local codes.
The molded case switches contained in the Liebert STS2 are typically non-automatic circuit breakers
that rely on the upstream and/or load overcurrent protection. Upstream overcurrent protection
should be rated equal to or less than the rating of the Liebert STS2 molded case switches.
The Liebert STS2 input and output power and ground and neutral bus bars accommodate a wide
range of wire sizes. The Liebert STS2 bus bars accommodates standard 2 hole lugs.
5.2 Input Junction Box Installation—Optional
The input junction box option is available to simplify input connections to the Liebert STS2. Two
input junction boxes and the associated flexible 10 foot long (3 meter) input cables are provided with
this option.
This option is available only with bottom cable entrance, typically when the unit is located on a raised
floor.
If the input junction boxes will be used, they may be shipped either with the unit or in advance to be
installed during the rough-in stage of construction.
Locate the junction boxes a maximum of 8 feet (2.4 meters) from the planned location of the Liebert
Static Transfer Switch.
Due to a lack of clearance for the cables below the unit, the cables are also shipped loose from the unit
for installation at the site. The input cables connect from the input junction boxes to the input power
bus bars inside the Liebert STS2.
The standard configuration for the input junction boxes and cables is for 3-wire-plus-ground feeds. If
the input neutral is required, special 4-wire-plus-ground input cables and junction boxes are required.
See Figure 29 for wiring details.
Table 2 Input/output conduit plate specifications
Rating Max. number and size
100-250A 6 — 3" conduit
400-600A 9 — 4" conduit
800-1000A 12 — 4" conduits
K2
K1
K1 K3K2 K3 CB4
CB5
CB2
CB3
STS2STS1
OUTPUT
CB1
SOURCE 1
SOURCE 2