As we stated in the previous article “Stationary UPS Sizing Calculations Part One” That Stationary UPS Sizing Calculations include:
 The UPS sizing calculations,
 Rectifier/Charger sizing calculations,
 Inverter & Static Switch sizing calculations,
 The Battery Sizing Calculations.
We explained the UPS sizing calculations in the above article and we explained in article “Stationary UPS Sizing Calculations Part Two” , the following calculations:
 Rectifier/Charger sizing calculations,
 Inverter & Static Switch sizing
calculations,
 The Battery sizing calculations, which
includes:
First: The Manufacturers’ methods, which
include:
 Method#1: Watts per cell method,
 Method#2: Watts per bank method,
 Method#3: Ampere per cell method.
And in Article “Stationary UPS Sizing Calculations Part Three”, we explained
The IEEE methods of Battery Sizing
Calculations which includes:
 Method#1: The IEEE 485 method,
 Method#2: The IEEE 1184 method.
Also, in Article “Stationary UPS Sizing Calculations Part Four”, we explained:
 UPS Backup time calculation,
 Selection and sizing of UPS protective devices (CBs or Fuses).
Today, we will explain the following:
 Selection and sizing of UPS Cables,
 Sizing a generator set for UPS system
Selection and Sizing of UPS Cables 
Selection of UPS Cables The
cross section of cables depends on two main factors:
For a given load, each of these parameters results in a minimum permissible cross section. The larger of the two must be used. When
routing cables, care must be taken to maintain the required distances between
control circuits and power circuits, to avoid any EMI disturbances caused by
HF currents. 
1 Temperature Rise Permissible
temperature rise in cables is limited by the withstanding capacity of cable
insulation. Temperature rise in cables depends on:

2 Voltage Drops The
maximum permissible voltage drops are: For
AC circuits (50 or 60 Hz): If
the voltage drop exceeds 3% (5060 Hz), increase the cross section of
conductors. For
DC circuit: If
the voltage drop exceeds 1%, increase the cross section of conductors. To
calculate the voltage drop for threephase
circuits, 5060 Hz, 380 V / 400 V / 415 V, cos φ = 0.8, balanced system 3L+N,
use the following table1 for AC circuits & table2 for DC Circuits: Table1 for AC circuits Table2 for DC Circuits Important note:
Example#1:
Consider a 70meter 400 V threephase circuit, with copper
conductors and a rated current of 600 A. Standard IEC 60364 indicates, depending on the installation
method and the load, a minimum cross section. We shall assume that the minimum cross section is 95 mm^{2}.
Calculate the voltage drop? Solution: It is first necessary to check that the voltage drop does not
exceed 3%. table1 for threephase circuits on the previous page indicates,
for a 600A current flowing in a 300 mm^{2} cable, a voltage drop
of 3% for 100 meters of cable, i.e. for 70 meters: Voltage drop = 3×70/100=2.1% Therefore less than 3% and it is acceptable Example#2: For a DC current of 1,000A in a tenmeter cable of 240 mm^{2}.
Calculate the voltage drop? Solution: From table2, The voltage drop for 100 meters of 240 mm^{2} cable
is 5.3%, i.e. for ten meters: Voltage drop = 5.3×10/100=0.53% Therefore less than 1% and it is acceptable 
Sizing of UPS Cables We will explain how to size different cables used for UPS system, which are:

1 UPS Input Cables The cross section of cables required for the input of the UPS can be derived based on the input power in KVA which depend on the:
To
size UPS Input Cables, follow the below (4) steps: Step#1: Arrive at the input power of Inverter Inverter
Input Power= Capacity of UPS in KVA X Output Power Factor X 1000/Inverter
Efficiency Step#2: Calculate the battery charging power in W Battery
Charging Power = 2.2VX No of Cells X Charging Current The
charging current is typically 10% of AH Capacity Step#3: Calculate the Input power of Rectifier in W Rectifier
Input Power = Inverter Input Power + Battery Charging Power / Efficiency of
Rectifier Step#4: Calculate the input current drawn The
rectifier input power calculated in step 3 needs to be converted to KVA by
taking into consideration the input power factor: Input Power in VA = Rectifier Input Power in W /Input Power Factor Rated
Current in (A) = VA / SQRT 3 X Vphph where: Vphph is the minimum operating Voltage of rectifier As thumb rule, we can consider 2A/sq mm to arrive the cross section of the required cables. Cross
Section of cables in sq mm = Rated Current in A(I) / 2 Step#5:
adjust the calculated cross section from table3 in below as
per the rated current. Table3 for Minimum
and maximum crosssections of copper conductors suitable for connection of
one copper cable per terminal. (Extract from IEC 604391) 
2 UPS Output Cables To size the cross section of the UPS Output cable, the output current needs to be calculated using the below formula: Rated
Current in A(I) = KVAX1000 / SQRT 3 X Vphph Using the cable manufacturer’s datasheet and the conditions linked with routing and bunching of cables, the required cable can be selected. Cross Section of Cables in sq mm = Rated Current in A(I)/2 Then, adjust the calculated cross section from table3 as per the rated current. 
3 Neutral Conductors In
threephase systems, the thirdorder harmonics (and their multiples) of
singlephase loads add up in the neutral conductor (sum of the currents on
the three phases). For this reason, to size Neutral Conductors the following rule may be applied: Neutral
cross section = 2 x phase cross section in Sq mm 
4 UPS to Battery Cables The selection of UPS to battery bank cables has to be based on the current at minimum discharge voltage, which can be derived based on the below formula: Battery Current = UPS Power Rating / UPS DC Voltage rating Then,
use Uninyvin cables tables4&5
to get the proper size as per applied conditions. Uninyvin cables are generally preferred for cables between UPS & battery due to high current carrying capacity and smaller cross sectional area. Uninyvin cables tables4 Uninyvin cables tables5 
Example#3: Consider an
installation that involves a Powerware 40kVA 9355(3phase) UPS rated
at 432VDC, 36 batteries, 6 cells/battery. Calculate the cable sizes
required to install the UPS assuming the UPS will be operating at full
load for a backup time of 12 hours. UPS rating;
40,000VA 1 UPS input cable selection: Inverter
Input Power= Capacity of UPS in KVA X Output Power Factor X 1000/Inverter
Efficiency = 40*0.98*1000/0.95 = 41.263 KW Step#2:
Calculate the battery charging power in W Battery
Charging Power = 2.2VX No of Cells X Charging Current = 2.2*216*10=4.75 KW The
charging current is typically 10% of AH Capacity Step#3:
Calculate the Input power of Rectifier in W Rectifier
Input Power = Inverter Input Power + Battery Charging Power / Efficiency of
Rectifier = 41.263 + 4.75 = 46 KW Step#4:
Calculate the input current drawn The
rectifier input power calculated in step 3 needs to be converted to KVA by
taking into consideration the input power factor Input
Power in VA = Rectifier Input Power in W /Input Power Factor = 46000/0.96
=47.916 KVA Rated
Current in (A) = VA / SQRT 3 X Vphph = 47916/(1.73*400) = 69.24 A where
Vphph is the minimum operating Voltage of rectifier Cross
Section of cables in sq mm = Rated Current in A(I) / 2 = 69.24/2 = 34.62 mm2 Step#5:
adjust the calculated cross section from table3 as per the rated current. minimum cable size
required will be 34.62 mm2 (from table3
for 80A, Cable 35 mm2 is the maximum standard) 2 UPS Output Cable selection: Rated Current in A(I) = KVAX1000 / SQRT 3 X Vphph =
40*1000/(1.73*400) = 57.8 A As
thumb rule, we can consider 2A/sq mm to arrive the cross section of the
required cables. Cross
Section of Cables in sq mm = Rated Current in A(I)/2 = 57.8/2 = 28.9 A
3 Battery
cable Selection: Battery Current = UPS Power Rating / UPS DC Voltage rating Then,
use Uninyvin cables tables5
to get the proper size as per applied conditions.
For 3 bunched cables, the minimum cable size that can be
used for this battery current should be 25mm2.

Sizing a generator set for UPS
system 
An uninterruptible power supply (UPS) is used in data centers to prevent loss of service, data
loss and damage to sensitive IT and communications equipment. This is done
during shortterm power disturbances and blackouts, but is not designed
to deliver backup power indefinitely, so for extended blackout, a standby generator will
be needed as a backup source for the UPS. The two systems (standby diesel generator and UPS) are connected
by an automatic mains failure (AMF) panel (see
Fig1). This is installed upstream of the UPS system.  When the AMF panel
senses a mains power failure: The UPS system feed the critical load from
its battery, A signal contact initiates the generator to start after an extended mains failure time; delays of 2
s to 10 s are typical Once the generator is up to full cycle speed and capable of
supplying power, the UPS system
input synchronizes to the output supply to the generator. When this is achieved the UPS system draws on the supply from the
generator and its batteries go into recharge.  When the mains power supply
returns: The generator will continue to operate for several cycles (may
be 2 minutes) to ensure the mains power supply is stable. The UPS system
then synchronizes back to the mains power supply and The generator is issued a signal contact from the AMF panel to power off. The UPS uses the mains power
to recharge its batteries again. Fig.1: Typical protected power installation with Mains Failure
Detection and Change Over A
typical continuous power sequence for a power outage and UPS ridethrough to
generator set power is shown above for a UPS is shown in Fig2. Fig2: A
typical continuous power sequence 
When sizing UPS systems
and generators it is important to classify the loads to be protected into
critical, essential and nonessential loads. So,
When sizing the generator system the load normally consists of the: Critical loads + Essential Loads The critical load is generally supported by the UPS system and so the sizing becomes: UPS system
+ Essential Loads 
1 Ups
system

2 Essential Loads

The
sizing of a diesel generator to feed an uninterruptible power supply maybe
calculated from one of the following methods:

First: the sizing formula KVA
gen = (KVA UPS + KVA battery charge)/ (0.8 x ∩) + KVA Essential loads Where:
∩
is the efficiency of the UPS 0.8
is the deration factor KVA
battery charge generally ranges from 0 to 25 percent of KVA UPS (15 percent
is typical). If it’s unknown, use 25 percent of KVA UPS for an approximation If
UPS efficiency is unknown, we recommend the following guidelines:

Second:
the Rule of thumb Combining a UPS system with a gen
set presents some special considerations to ensure compatibility. We use the
following fourstep procedure to size Cat gen sets that have static
UPS systems as part of all of their load: Step#1: Find the UPS input kW Step#2: Set minimumsize gen set In
this step, establish the gen set size needed to contain waveform distortion as follows:
Step#3: Consider other loads Be
sure to size the gen set to accommodate other loads in the application.
Establish the kW of the other loads, then add it to the UPS input kW x 1.15
for battery charge. Step#4: Gen set sizing Round
up to the nearest larger size standby gen set. Example#4: The UPS system is rated 200 kVA/180
kW sixpulse rectifier; other loads connected to the gen set total 100 kW.
Size the proper generator to feed these loads? Solution: Step#1:
From
supplier data, the UPS input is 255 kW, including battery recharge. Step#2: 255
kW x 1.6 (sixpulse rectifier) = 408 kW minimum standby rated gen set. Step#3: (255
kW x 1.15) + 100 kW = 393 kW minimum standby rated gen set with other loads. Step#4: 408
kW standard is larger than 393 kW, therefore a standby gen set of at least
408 kW is recommended. 
Important Notes

In the next Article, we will
explain the following:
 Battery room ventilation calculation,
 Installation and testing of UPS.
So, please keep following.
Subject
Of Pervious Article 
Article 
Applicable Standards for UPS Systems
Classification of UPS: 1Voltage range, 2No. of phases, 3 Mobility, 4 Technological design, 

5 Physical Size/capacity, 6 Form factor/ configurations: 6.1 “N” System Configuration 

6.2 “N+1” System Configuration, which
includes:
6.3 Parallel Redundant with Dual Bus Configuration
(N+1 or 1+1) 
Classification and Types of
UPS – Part Three 
6.4 Parallel Redundant with STS Configuration
6.5 System plus System 2(N+1), 2N+2, [(N+1) + (N+1)],
and 2N 
Classification and Types of
UPS – Part Four 
7 According to UPS Topology 7.1
Offline or Standby UPS, 7.2 Line
Interactive UPS, 7.3
StandbyFerro UPS, 7.4 Online
Double Conversion UPS, 7.5 The
Delta Conversion OnLine UPS. 
Classification and Types of
UPS – Part Five 
8 According to UPS Distribution Architecture 8.1 Centralized UPS Configuration, 8.2 Distributed (Decentralized) UPS
Configuration, 8.2.1 Distributed UPSZonewise
Configuration 8.3 Hybrid UPS Configuration. Conventional (Monolithic) Vs Modular UPS System:

Classification and Types of
UPS – Part Six 
Three Basic Configurations Of Mains
And Bypass For A UPS System:
9According to Use of transformers
with the UPS:

Classification and Types of
UPS – Part Seven 
Transformer Arrangements in Practical UPS Systems: 1Transformer options for the “single mains”
configuration 2Transformer Options for the “Dual Mains”
Configuration 
Classification and Types of
UPS – Part Eight 
3 Transformer options for “single mains without
bypass” 

Components of Online Double Conversion UPS: 1 Rectifier, 2 Inverter, 3 Energy Storage system: 3.1 Battery 
Components of Online Double
Conversion UPS– Part One

3.1.1 Battery Configurations
3.1.2 Battery Size and Location 3.1.3 Battery Transition Boxes 3.1.4 Battery Monitoring 3.2 Energy Storage System – Flywheel 3.3 Energy Storage system – Super Capacitors 3.4 Hydrogen Fuel Cells 4 Static switch Earthing Principles of UPS Systems 
Components of Online Double
Conversion UPS – Part Two 
Evaluation Criteria for Selecting an UPS 
Evaluation Criteria for
Selecting an UPSPart One

Example: Selecting an Uninterruptible Power Supply
(UPS) UPS System Ratings and Service Conditions First: from IEC 601464 Second: according to American
standards 
Evaluation Criteria for
Selecting an UPSPart Two 
The UPS sizing calculations steps 
Stationary UPS Sizing
Calculations – Part One

2 Rectifier/Charger Sizing Calculations 3 Inverter sizing calculations & Static Switch
Sizing 4 The Battery sizing calculations First: The Manufacturers’ methods, which include:

Stationary UPS Sizing
Calculations – Part Two 
Second: The IEEE methods of Battery Sizing Calculations
which includes:

Stationary UPS Sizing
Calculations Part Three 
 UPS Backup time calculation  Selection and sizing of UPS protective devices (CBs
or Fuses) 
Stationary UPS Sizing Calculations – Part Four 
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