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Installation
Guide
SG Series
225 & 300 kVA
3.7.2 Input/output over current protection and wire sizing
The cabling of the UPS system has to be sized according to the UPS power rating.
Sizing of circuit breakers, fuses and cables for
Input Utility
,
Output Load
and
Battery
must
meet the requirements of local and national electrical codes.
Before connecting the UPS, verify that the
Utility Voltage and Frequency
, the
Output Load
Voltage and Frequency
and
Battery Data
(cells number, floating voltage, autonomy) are
according to the required specifications.
Output Load
configuration may be such that one phase may carry a
Load
current at 100% of
that phases maximum current rating while the other two phases run at 0% or any combination
in between.
Ensure that the
Load
does not consist of any equipment that may require high starting currents
such as electric motors, laser printers, etc.
This may cause the UPS to occasionally go into
Bypass
due to overload conditions
To choose the correct input fuses or circuit breaker, consider the available
short-circuit
current
of the system up-stream.
Choose the correct fuse or breaker using current data shown in the chart and the appropriate
NEC code.
The ratings indicated in the following chart do not consider any
line voltage drop
.
In case of optional input transformer the input protective devices should be sized to allow the
transformer magnetization inrush current.
Caution when using
four-pole circuit breakers
as protection.
A potential problem exists for situations with
Non-linear Loads
:
The neutral current could be greater than the phase currents.
The three-phase
Utility
power supply must be symmetrical with respect to ground, due to the
existence of voltage surge protection devices inside the UPS.
If you use
ELCB breaker (Earth Limiting Circuit Breaker)
to protect the input
connections, consider the
high leakage current
generated by the noise
suppression capacitors.
If these ELCB breakers are strictly necessary, we suggest using the largest type
suitable for
non-linear current and for delayed operations.
To ensure coordination when the UPS is configured for separate
Bypass
and
Rectifier Inputs
,
special care must be taken in choosing the
fuse or circuit breaker ratings
installed in the
output distribution circuits.
Protective devices on the output of the UPS should be coordinated with the Bypass Input
circuit protection.
Due to the relatively low short circuit capability of the UPS inverter, a short circuit in the
Load
will cause an immediate transfer to
Bypass
.
The largest fuse or circuit breaker in the output distribution should be rated at no more than
60% of the rating of the protective device supplying the
Bypass
line.
If circuit selectivity is required while the
Load
is fed from the inverter (
Bypass Utility
not
available), the largest fuse or circuit breaker should be rated at no more than 20% of the UPS
output current rating.
