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2.2
Power Cables
The UPS requires both power and control cabling. All control cables, whether shielded or not, should
be run separate from the power cables in metal conduits or metal ducts which are electrically bonded
to the metalwork of the cabinets to which they are connected
The cable design must comply with the voltages and currents in Table 7, follow local wiring practices
and take into consideration the environmental conditions (temperature and physical support media),
room temperature and conditions of installation of the cable and system's overload capacity (see 5.0 -
Specifications).
For cable entry terminal, refer to Figure 15.
WARNING
!
Risk of electrical shock. Can cause injury or death.
Before cabling the UPS, ensure that you are aware of the location and operation of the
external isolators that connect the UPS input/bypass supply to the power distribution panel.
Check that these supplies are electrically isolated, and post any necessary warning signs to
prevent their inadvertent operation.
NOTICE
For dual input operation ensure that any busbars between the bypass and the rectifier input
are removed.
When sizing battery cables, a maximum volt drop of 2VDC is permissible at the current ratings given
in Table 11.
The following are guidelines only and superseded by local regulations and codes of practice where
applicable:
• Take special care when determining the size of the neutral cable (grounded conductor), because
current circulating on the neutral cable may be greater than nominal current in the case of non-
linear loads. Refer to the values in Table 7.
• The grounding conductor should be sized according to the fault rating, cable lengths, type of
protection, etc. The grounding cable connecting the UPS to the main ground system must follow
the most direct route possible.
• Consideration should be given to the use of paralleled smaller cables for heavy currents, as this
can ease installation considerably.
• AC and DC cables must be run in conduits according to local codes, national codes and standard
best practices. This will prevent creation of excess EMI fields.
2.2.1
Sizing the Input Breaker Feeding a Liebert NXL UPS
The rectifier nominal input current value (Table 8) is based on providing enough power to operate
the inverter at full rated load when the input voltage is nominal. The published rectifier maximum
input current value is based on the maximum input current limit setting of 125%.
The recommended input breaker ampacity is based on the NEC calculation for a branch circuit with
the current limit set at the maximum value. The calculation for an 80% rated breaker is 125% of the
nominal current (continuous) plus 100% of the difference between the nominal and the maximum
currents (non-continuous). The calculation for a 100% rated breaker is 100% of the nominal current
(continuous) plus 100% of the difference between the nominal and maximum currents
(non-continuous).
The rectifier input current limit can be adjusted from 25% to 125% of the nominal value. This setting
is accessible only to a factory-trained service engineer. The effect of changing this setting is similar to
that of changing the setting of the trip unit on a solid-state circuit breaker. With a lower current limit
setting, the calculation for the branch circuit will result in a lower value.
The input current limit should not be set less than 105% of the current needed to support the inverter
at full load for normal operation. This results in sufficient power to recharge the battery in a
reasonable time and to operate over the published input voltage range.
8
UPS Electrical Installation
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