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How to Design Solar PV System
What is solar PV system?
Solar photovoltaic system or Solar power system is one of renewable energy system which uses
PV modules to convert sunlight into electricity. The electricity generated can be either stored or used
directly, fed back into grid line or combined with one or more other electricity generators or more
renewable energy source. Solar PV system is very reliable and clean source of electricity that can suit
a wide range of applications such as residence, industry, agriculture, livestock, etc.
Major system components
Solar PV system includes different components that should be selected according to your system type,
site location and applications. The major components for solar PV system are solar charge controller,
inverter, battery bank, auxiliary energy sources and loads (appliances).
• PV module – converts sunlight into DC electricity.
• Solar charge controller – regulates the voltage and current coming from the PV panels going to
battery and prevents battery overcharging and prolongs the battery life.
• Inverter – converts DC output of PV panels or wind turbine into a clean AC current for AC
appliances or fed back into grid line.
• Battery – stores energy for supplying to electrical appliances when there is a demand.
• Load – is electrical appliances that connected to solar PV system such as lights, radio, TV,
computer, refrigerator, etc.
• Auxiliary energy sources - is diesel generator or other renewable energy sources.
Solar PV system sizing
1. Determine power consumption demands
The first step in designing a solar PV system is to find out the total power and energy consumption
of all loads that need to be supplied by the solar PV system as follows:
1.1 Calculate total Watt-hours per day for each appliance used.
Add the Watt-hours needed for all appliances together to get the total Watt-hours per day which
must be delivered to the appliances.
1.2 Calculate total Watt-hours per day needed from the PV modules.
Multiply the total appliances Watt-hours per day times 1.3 (the energy lost in the system) to
get the total Watt-hours per day which must be provided by the panels.
2. Size the PV modules
Different size of PV modules will produce different amount of power. To find out the sizing of PV
module, the total peak watt produced needs. The peak watt (Wp) produced depends on size of the PV
module and climate of site location. We have to consider “panel generation factor” which is different
in each site location. For Thailand, the panel generation factor is 3.43. To determine the sizing of PV
modules, calculate as follows:
2.1 Calculate the total Watt-peak rating needed for PV modules
Divide the total Watt-hours per day needed from the PV modules (from item 1.2) by 3.43 to
get the total Watt-peak rating needed for the PV panels needed to operate the appliances.
2.2
Calculate
the
number
of
PV
panels
for
the
system
Divide the answer obtained in item 2.1 by the rated output Watt-peak of the PV modules
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available to you. Increase any fractional part of result to the next highest full number and that will be
the number of PV modules required.
Result of the calculation is the minimum number of PV panels. If more PV modules are installed, the
system will perform better and battery life will be improved. If fewer PV modules are used, the system
may not work at all during cloudy periods and battery life will be shortened.
3. Inverter sizing
An inverter is used in the system where AC power output is needed. The input rating of the inverter
should never be lower than the total watt of appliances. The inverter must have the same nominal
voltage as your battery.
For stand-alone systems, the inverter must be large enough to handle the total amount of Watts you
will be using at one time. The inverter size should be 25-30% bigger than total Watts of appliances. In
case of appliance type is motor or compressor then inverter size should be minimum 3 times the
capacity of those appliances and must be added to the inverter capacity to handle surge current during
starting.
For grid tie systems or grid connected systems, the input rating of the inverter should be same as PV
array rating to allow for safe and efficient operation.
4. Battery sizing
The battery type recommended for using in solar PV system is deep cycle battery. Deep cycle
battery is specifically designed for to be discharged to low energy level and rapid recharged or cycle
charged and discharged day after day for years. The battery should be large enough to store sufficient
energy to operate the appliances at night and cloudy days. To find out the size of battery, calculate as
follows:
4.1 Calculate total Watt-hours per day used by appliances.
4.2 Divide the total Watt-hours per day used by 0.85 for battery loss.
4.3 Divide the answer obtained in item 4.2 by 0.6 for depth of discharge.
4.4 Divide the answer obtained in item 4.3 by the nominal battery voltage.
4.5 Multiply the answer obtained in item 4.4 with days of autonomy (the number of days that you
need the system to operate when there is no power produced by PV panels) to get the required
Ampere-hour capacity of deep-cycle battery.
Battery Capacity (Ah) = Total Watt-hours per day used by appliances x Days of autonomy
(0.85 x 0.6 x nominal battery voltage)
5. Solar charge controller sizing
The solar charge controller is typically rated against Amperage and Voltage capacities. Select the
solar charge controller to match the voltage of PV array and batteries and then identify which type of
solar charge controller is right for your application. Make sure that solar charge controller has enough
capacity to handle the current from PV array.
For the series charge controller type, the sizing of controller depends on the total PV input current
which is delivered to the controller and also depends on PV panel configuration (series or parallel
configuration).
According to standard practice, the sizing of solar charge controller is to take the short circuit
current (Isc) of the PV array, and multiply it by 1.3
Solar charge controller rating = Total short circuit current of PV array x 1.3
Remark: For MPPT charge controller sizing will be different. (See Basics of MPPT Charge
Controller)
Example: A house has the following electrical appliance usage:

One 18 Watt fluorescent lamp with electronic ballast used 4 hours per day.
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One 60 Watt fan used for 2 hours per day.
One 75 Watt refrigerator that runs 24 hours per day with compressor run 12 hours and
off 12 hours. The system will be powered by 12 Vdc, 110 Wp PV module.
1. Determine power consumption demands
Total appliance use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 24 x 0.5 hours)
= 1,092 Wh/day
Total PV panels energy needed = 1,092 x 1.3
= 1,419.6 Wh/day.
2. Size the PV panel
2.1 Total Wp of PV panel
capacity needed
= 1,419.6 / 3.4
= 413.9 Wp
2.2 Number of PV panels needed = 413.9 / 110
= 3.76 modules
Actual requirement = 4 modules
So this system should be powered by at least 4 modules of 110 Wp PV module.
3. Inverter sizing
Total Watt of all appliances = 18 + 60 + 75 = 153 W
For safety, the inverter should be considered 25-30% bigger size.
The inverter size should be about 190 W or greater.
4. Battery sizing
Total appliances use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)
Nominal battery voltage = 12 V
Days of autonomy = 3 days
Battery capacity = [(18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)] x 3
(0.85 x 0.6 x 12)
Total Ampere-hours required 535.29 Ah
So the battery should be rated 12 V 600 Ah for 3 day autonomy.
5. Solar charge controller sizing
PV module specification
Pm = 110 Wp
Vm = 16.7 Vdc
Im = 6.6 A
Voc = 20.7 A
Isc = 7.5 A
Solar charge controller rating = (4 strings x 7.5 A) x 1.3 = 39 A
So the solar charge controller should be rated 40 A at 12 V or greater.
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3-Core Cables
These cables are used generally for a perfect balanced 3-phase system. When the currents on the 3-live wires
of a 3-phase system are equal and at an exact 120° phase angle, then the system is said to be balanced. The 3phase loads are identical in all respects with no need of a neutral conductor.
An important example of 3-phase load is electric motor and that is why, they are fed through 3-Core cables in
most cases.
3.5-Core Cables
A 3-phase system may have a neutral wire. This wire allows the 3-phase system to be used at higher voltages
while it will still support lower voltage single phase loads.
It is not likely in such cases that the loads will be identical, so the neutral will carry the out-of-balance current of
the system. The greater the degree of imbalance, the larger the neutral current.
When there is some degree of unbalance and the amount of fault current is very small, then 3.5 core cables are
used. In these types of cables, a neutral of reduced cross section as compared to the 3-main conductors is
used, which is used to carry the small amount of unbalanced currents.
4-Core Cables
When there is severe out-of-balance conditions, the amount of fault current will raise to a very high level.
Generally in the case of linear loads, the neutral only carries the current due to imbalance between the phases.
The non-linear loads such as switch-mode power supplies, computers, office equipment, lamp ballasts and
transformers on low loads produce third order harmonic currents which are in the phase of all the supply phases.
These currents do not cancel at the star point of a three-phase system as do normal frequency currents, but add
up, so that the neutral carries very heavy third harmonic currents
THE 3.5 CORE CABLE IS POWER CABLE (NORMALLY HIGH LOAD).
-3 LEADS WILL BE OF SAME SQ.MM
-1 LEADS WILL BE,HALF OF THE ABOVE 3 LEADS
THE 4 CORE CABLE CAN BE USED FOR BOTH POWER AND CONTROL CABLE(NORMALLY LESS LOAD)
-4 CORE WILL BE IN SAME SQ.MM
Neutral conductor must be at least 1/2 the size of other conductor.
It is theoraticaly explained , that algebric sum of all the phases when in balanced
condition
then neutral current will be zero.
Now suppose one phase fail i.e. total unbalanced condition its neutral conductor will
provide
a return path and its current will be half the load on other phases so by this
result on neutral conductor for every unbalanced condition its current will not exceed
the rated value (same as in phase conductors).
The selection of neutral conductor depends on the nature of loads, such as the load harmonics
(more than 20% loads are electronic) or the load balancing (most of the loads are 3 phase motor) etc, etc.
So, there are few general cases to consider here:
1. If the loads are almost balanced and there is no significant electronic loads, you can select 3.5 (3 + 0.5)
2. If the loads are not balanced and there is no significant electronic loads, you can select 4 (3 + 1)
3. If the loads are balanced and there is significant electronic loads, you can also select 4 (3 + 1)
4. If the loads are not balanced and there is significant electronic loads, you need to select even higher size for
neutral (3 + 1.5)
With modern equipment it is getting to be a major problem especially with IT gear.
Some cable manufacturers can now supply cable with a neutral 1.5 X phase conductor.
S.K.B.P. EE
Structured Design Process //
To achieve the best overall outcome in a lighting installation, it is important to avoid the tendency of
rushing straight into luminaire selection before determining more broadly what is required from the
system. The use of a structured design process helps to avoid this.
The key steps in the design process are:
1.
2.
3.
4.
5.
6.
7.
Identify the requirements
Determine the method of lighting
Select the lighting equipment
Calculate the lighting parameters and adjust the design as required
Determine the control system
Choice of luminaire
Inspect the installation upon completion
(and if possible, a few months after occupation, to determine what worked and what didn’t. This is the
only way to build up experience to apply to future designs)
The five initial stages are considered in more detail in the following lines.
1. Identifying the requirements
This involves gaining a full understanding of what the lighting installation is intended to achieve. This
includes the following:
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Task Requirements ?
 Illuminance
 Glare
Mood of the space
Relation to shape of space
Things to be emphasised
Things to hide
Direction of light
Interaction of daylight
2. Determine the method of lighting
At this stage, consideration is given to how the light is to be delivered, e.g. will it be recessed, surface
mounted, direct or indirect, or will up-lighting be used, and its primary characteristics, e.g. will it be
prismatic, low brightness or mellow light.
Consideration should be given at this stage to the use of daylight to minimise the need for artificial light
3. Select the lighting equipment
Once the method of lighting has been selected, the most appropriate light source can then be chosen
followed by the luminaire.
The following attributes should be studied when choosing the light source:
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Light output (lumens)
Total input wattage
Efficacy (lumens per Watt)
Lifetime
Physical size
Surface brightness / glare
Colour characteristics
Electrical characteristics
Requirement for control gear
Compatibility with existing electrical system
Suitability for the operating environment
A number of factors also affect luminaire choice:
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Characteristics of the light source and control gear
Luminaire efficiency (% lamp light output transmitted out of the fixture)
Light distribution
Glare control
Finish and appearance
Size
Accessibility of components for maintenance
Ability to handle adverse operating conditions
Aesthetics
Thermal management
4. Calculate the lighting parameters
Lighting calculation methods fall into three broad categories:
1. Manual calculation methods
2. Three dimensional modelling
3. Visualisation
Photometric data for light sources and luminaires is commercially available to contribute to these
calculations
4.1 Manual calculation methods
There are a wide range of manual computation methods for the calculation of different lighting aspects.
These include complex methods for calculating the illuminance from a wide variety of shapes of luminous
objects. The majority of these have now been superseded by computer programs (check our free software).
The Lumen Method was the mainstay for interior lighting and has remained in use as a quick and
relatively accurate method of calculating interior illuminance.
The Lumen Method calculates the average illuminance at a specific level in the space, including an
allowance for the light reflected from the interior surfaces of the room. The calculation method has a set
of assumptions that, if followed, gives a reasonable visual environment.
Inadequate attention to the assumptions will produce poor results.
The basic assumptions are:
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All the luminaires in the room are the same and have the same orientation
The luminaires do not have a directional distribution and are aimed directly to the floor
The luminaires are arranged in a uniform array on the ceiling and have the same mounting height
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
The luminaires are spaced less than the maximum spacing to mounting height ratio nominated in
the coefficient of utilisation tables
The average illuminance produced by a lighting installation, or the number of luminaires required to
achieve a specific average illuminance, can be calculated by means of utilization factors (UF), a UF
being the ratio of the total flux received by a particular surface to the total lamp flux of the installation.
Lumen method formula
The average illuminance E(h) over a reference surface s can be calculated from the “lumen method”
formula.
where:
F – the initial bare lamp flux (lumens)
n – the number of lamps per luminaire
N – the number of luminaires
LLF – the total light loss factor
UF(s) – the utilization factor for the reference surface s of the chosen luminaire
Utilization factors can be determined for any surface or layout of luminaires. The “UF” symbol is
normally shown followed by an extra letter in brackets, to denote the surface, for example, UF(F) is the
utilisation factor for the floor cavity and UF(W) is the utilisation factor for the walls.
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Utilization factors are, in practice, only calculated for general lighting systems with regular arrays of
luminaires and for three main room surfaces. The highest of these surfaces, the C surface (for ceiling
cavity), is an imaginary horizontal plane at the level of the luminaires having a reflectance equal to that of
the ceiling cavity.
The lowest surface, the F surface (for floor Cavity), is a horizontal plane at normal working height (i.e.
table height), which is often assumed to be 0.85 m above the floor.
The middle surface, the W surface (for walls), consists of all the walls between the C and F planes.
Although the lighting designer can calculate utilization
factors, lighting companies publish utilization factors for
standard conditions for their luminaires. The standard
method of presentation is shown below. To use this table, it
is only necessary to know the Room Index and the effective
reflectance of the three standard surfaces (floor cavity, walls
and ceiling cavity).
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Room Index
The Room Index is a measure of the angular size of the room, and is the ratio of the sum of the plan
areas of the F and C surfaces to the area of the W surface. For rectangular rooms the room index is given
by:
Where:
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L – the length of the room
W – the width of the room
Hm – the height of the luminaire plane above the horizontal reference plane
 If the room is re-entrant in shape, for example L shaped, then it must be divided into two or more
non-re- entrant sections, which can be treated separately.

Spacing to Mounting Height Ratio (SHR)

The Spacing to Mounting Height Ratio (SHR) is the spacing between luminaires divided by
their height above the horizontal reference plane.
It affects the uniformity of illuminance on that plane. When the UF tables are determined, for a nominal
spacing to height ratio SHR NOM, the maximum spacing to height ratio SHR MAX of the luminaire is
also calculated, and is a value that should not be exceeded if the uniformity is to be acceptable.
Three dimensional modelling
Although it was possible to calculate the luminance of
all the surfaces in a room, the calculations were
extremely laborious and could only be justified in the
most special cases. However, the advent of computer
modelling enabled a more flexible approach to lighting
design and significantly increased the information
available to the designer.
In contrast to the Lumen Method, lighting programs enable the lighting designer to broaden the
assumptions:
 A mixture of luminaires can be used
 The luminaires no longer have to be arranged in a regular array
 Directional luminaires can be modelled
 A large number of calculation points can be considered to give a meaningful uniformity calculation
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The illuminance and luminance of all surfaces can be calculate
This gives the lighting designer a much greater understanding of what is happening in the room
However there has been considerable research, experience and documentation over the past 80 years that
has developed the current thinking in the adequacy of various illuminance levels for various tasks and
functions.
Although there is some general understanding of the need for appropriate luminance distribution in
the vertical plane, there is little information, experience or understanding for many designers to
determine:
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What the luminance of surfaces should be in varying situations
What is an acceptable luminance uniformity
Whether there should there be a maximum luminance uniformity
What is the desired graduation in luminance
At what point is the luminance distribution of the wall unacceptable
It is important in using a lighting calculation program that the output records the type of luminaire used,
the location of the luminaires, the assumed lumen output of the lamp, the light loss factor and the aiming
points. If this is not recorded you have a pretty picture of the installation and no way of making it a
reality.
4.3 Visualisation
These are programs that create a perspective rendering of the space in levels of detail that vary from a
block representation of the space, to photographic quality renderings, depending on the sophistication of
the program and the level of detail of the interior to be entered.
The programs fall into two basic types:
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Flux transfer or radiosity calculations
Ray tracing calculations
The major difference being in how they interpret light from reflective surfaces.
A Lambertian surface is a perfect diffuser, where light is reflected in all directions, irrespective of the
angle of incidence of the light such that irrespective of the viewing angle the surface has the same
luminance. A specular surface is a mirror like surface, where the angle of reflection of the light is the
same as the angle of incidence.
Left: Lambertian
surface; Middle: Specular surface; Right: Semi-specular surface
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A real life surface is a combination of both surfaces (semi-specular) and has both specular and diffuse
characteristics. Some materials are more specular while others are more diffuse.
A flux transfer or radiosity program treats all surfaces as diffuse or Lambertian surfaces, as a result
their rendering tends to appear flat with soft shadow details. It will tend to overestimate the
uniformity. Ray tracing traces the individual rays of light from the source to the eye as it reflects from
surface to surface around the room. As a result ray tracing can allow for the specular component of the
surfaces.
Some programs calculate the entire lighting by ray tracing while others calculate the space on a flux
transfer basis and have an overlay of ray tracing of specific areas to improve the quality of the rendering.
When ray tracing is added, reflections are added in polished surfaces and shadows become sharper.
Visualisation programs are a useful tool in the presentation of a design, as a tool for the designer to
check that the design is consistent with his own visualisation of the space, and to model specific lighting
solutions. The programs are still calculation tools and not design programs.
The programs can show the designer how a specific design will perform but that they cannot
reliably be used to assess the acceptability of a design.
Irrespective of the form of the visualisation output, it is important that the program provides adequate
information to enable the construction and verification of the lighting design.
The output should include:
Installation information – the type and location of all luminaires and the aiming information. The
lamp details should be included as well as the specific catalogue number of photometric file that has
been used.
Light technical parameters – the illuminance, uniformity and other parameters that have been
calculated to achieve the design.
Verification information – adequate details to enable the lighting calculation to be verified. This
should include the luminaire type, the photometric file, surface reflectances that were assumed, light
loss factors, lumen output of lamps and mounting and aiming locations.
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Determine the control system
The effectiveness and efficiency of any lighting installation is affected as much by the control system as
by the light sources and fixtures chosen.
Give consideration to:

Providing multiple switches to control the number of lights that come on at any one time. Using
one switch to turn on all the lights in a large room is very inefficient.

Placing switches at the exits from rooms and using two-way switching to encourage lights to be
turned off when leaving the room.
Using ‘smart’ light switches and fittings which use movement sensors to turn lights on and off
automatically. These are useful in rooms used infrequently where lights may be left on by mistake,
or for the elderly and disabled.
Make sure they have a built-in daylight sensor so that the light doesn’t turn on unnecessarily. Models
which must be turned on manually and turn off automatically, but with a manual over-ride, are preferable
in most situations. Be aware that the sensors use some power continuously, up to 5W or even 10W in
some cases.
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Using timers, daylight controls and motion sensors to switch outdoor security lights on and off
automatically. controls are particularly useful for common areas, such as hallways, corridors and
stairwells, in multi-unit housing.
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Using solar powered lighting for garden and security lights.

Using dimmer controls for incandescent lights (including halogens). This can save energy and
also increase bulb life. Most standard fluorescent lamps cannot be dimmed, but special dimmers and
lamps are available. If lamps are to be dimmed it is important to ensure that the correct equipment is
used, especially when retrofitting more energy efficient lamps.
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Choice of Luminaire
The performance of a luminaire should be considered just as carefully as its cost. In the long term a well
designed, well constructed luminaire will be cheaper than a poor quality unit; and the salient features of a
good quality luminaire are:
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Sound mechanical and electrical construction and a durable finish
Adequate screening of high luminance lamps to minimise discomfort and glare
Adequate heat dissipation to prevent over-heating of the lamp, wiring and ancillary equipment
High light output ratio with the appropriate light distribution
Ease of installation, cleaning and maintenance
input information
These are the input data for the following calculation:
1. An office area has length: 20 meter; width: 10 meter; height: 3 meter.
2. The ceiling to desk height is 2 meters.
3. The area is to be illuminated to a general level of 250 lux using twin lamp 32 watt CFL luminaires with
a SHR of 1.25.
4. Each lamp has an initial output (Efficiency) of 85 lumen per watt.
5. The lamps Maintenance factor (MF) is 0.63 ,Utilization Factor is 0.69 and space height ratio (SHR)
is 1.25.
Calculation
1. Total wattage of fixtures:
Total wattage of fixtures = Number of lamps x each lamp’s watt.
Total wattage of fixtures = 2 × 32 = 64 Watt.
2. Lumen per fixtures
Lumen per fixtures = Lumen efficiency (Lumen per Watt) x each fixture’s watt
Lumen per fixtures = 85 x 64 = 5440 Lumen
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3. Number of fixtures
Required number of fixtures = Required Lux x Room area / MF x UF x Lumen per fixture
Required number of fixtures = (250 x 20 x 10) / (0.63 × 0.69 × 5440)
We will need 21 fixtures
4. Minimum spacing between each fixture
The ceiling to desk height is 2 meters and space height ratio is 1.25, so:
Maximum spacing between fixtures = 2 × 1.25 = 2.25 meter.
5. Number of required rows of fixtures along with width of
room
Number of rows required = Width of room / Max. spacing = 10 / 2.25
Number of rows required is therefore = 4.
6. Number of fixtures required in each row
Number of fixtures required in each row = Total Fixtures / Number of rows = 21 / 4
Therefore, we have 5 fixtures in each row.
7. Axial spacing between each fixture:
Axial spacing between fixtures = Length of room / Number of fixtures in each row
… and that would be: 20 / 5 = 4 Meter
8. Transverse spacing between each fixture:
Transverse spacing between fixtures = Width of room / Number of fixtures in row
… and that would be: 10 / 4 = 2.5 Meter.
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Number of rows with lighting fixtures = 4
Number of lighting fixtures in each row = 5
Axial spacing between fixtures = 4.0 meter
Transverse spacing between fixtures = 2.5 meter
Required total number of fixtures = 21
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