Liebert Static Transfer Switch 2, Installation, Operation & Maintenance Manual

Page: 27 / 150

Power and Control Wiring 15
For preparation purposes, the following tables provide the dimensions and wire sizes for the input
junction boxes.
5.3 System Grounding
Equipment grounding — Grounding is primarily for equipment and personnel safety, although
proper grounding also enhances equipment performance.
All input and output power feeds must include an equipment grounding means as required by the
NEC and local codes.
An insulated equipment ground conductor is recommended to run with each input and output
power feed. The equipment ground conductors should be at least the minimum size conductor per
the NEC based on the upstream overcurrent protection device.
4-Wire-Plus-Ground Systems — When 4-wire-plus-ground input feeds are utilized, the input
power sources must be properly grounded. Because the neutral is not switched by the STS2, the
neutrals of the two power sources are solidly interconnected. The NEC prohibits grounding a power
source at more than one point. Connecting the neutrals of two grounded power sources together
effectively grounds each of the sources at more than one point, which allows neutral current to flow
on the ground system, defeats ground fault protection, creates a safety hazard, and violates the
NEC.
Where possible, the two power sources should be located in close proximity and a single
neutral-to-ground bond made (as shown in Figure 4 on page 16 or as is typical with a double-
ended substation).
Table 4 Input Junction Box Dimensions
Switch Rating
Length
Inches (mm)
Width
Inches (mm)
Height
Inches (mm)
100 amp 16 (406) 14 (356) 6 (152)
250 to 600 amp 30 (762) 16 (406) 6 (152)
Table 5 Input Junction Box Terminal Wire Size Range
Switch Rating Power and Ground Lug Wire Range
100 amp #6 to #2/0 AWG
250 to 600 amp (2) #6 AWG to (2) 500 kcmil
!
WARNING
IF CONDUIT IS USED AS A GROUNDING MEANS, ADEQUATE
ELECTRICAL CONTINUITY MUST BE MAINTAINED AT ALL
CONDUIT CONNECTIONS. THE USE OF ISOLATING BUSHINGS
WITH A METAL CONDUIT CAN BE A SAFETY HAZARD AND IS
NOT RECOMMENDED.