Unit v

BUILDING SERVICES - II:
ELECTRICAL SERVICES:
Unit – V:
Lighting Calculation: NBC standards, nominal
illumination levels in building interiors, lux, lumen,
intensity, lighting schemes.

ILLUMINATION AND LUX:
What is the method for calculating illumination ?
The problem of the Architect / Lighting Designer at the
functional level is to determine how many lights and
where to place them to get the correct level of illumination
for a given activity.
What do we understand by LUX ?
The lux (symbol: lx) is the SI unit of illuminance and
luminous emittance, measuring luminous flux per unit
area. It is used in photometry as a measure of the intensity,
as perceived by the human eye, of light that hits or passes
through a surface. It is analogous to the radiometric unit
watts per square metre, but with the power at each
wavelength weighted according to the luminosity function,
a standardized model of human visual brightness
perception. In English, "lux" is used in both singular and
plural.

One lux is equal to one lumen per square metre:

One lux is equal to one lumen per square metre:
In other words .. LUX = Level of Illumination.
OR
LUX (lx) = Lumens / Sq.M.
Level of Illumination ?
LUMEN METHOD:
This method works perfectly in most cases where the
height of the ceiling is within 3000 mm from floor level,
the Working Plane is between 750 to 1000 mm and

How To Calculate Lux ?
How do I calculate lux - the formula?
Lux level = total light output (lumens) / area (square
meters)
Total Light Output = lumens per fixture * number of
fixtures
Therefore
Number of Fixtures = (Lux level * area) / lumens per

Every Lamp has certain Lumens.
You need to look up the manufacturer data for your light
fitting to get the lumens per fixture.
S.No. Type of Lamp. Lumens Output.
1. 36 W Tube Light. 2400 Lumens.
2. 60 W GLS Lamp. 600 Lumens.
3. 10 W CFL. 600 Lumens.

What we did above is still not sufficient as there are certain
Correction Factors involved. There are basically two
Correction Factors i.e., Utilization Factor = UF and
Maintenance Factor = MF.
UTILIZATION FACTOR: (UF)
reasons viz., RoomThis may be due different
Reflectance's, Dimensions etc.,
The utilization factor (UF) is a Direct ratio. This is the
proportion of the downward luminous flux that reaches
the work area directly without reflection. It depends on
the shape of the room. The Direct ratio has a low value
with a tall narrow room (small RI) and a high value for a
wide room (large RI).

The light distribution from the luminaries. Luminaries
that emit light through a larger solid angle will generally
have a smaller direct ratio that luminaries that emit light in
a narrower beam.
There exist data sheets of UF for rooms of different shapes
and luminaries of a common type.
Catalogues of luminaries often provide data sheets of UF
for rooms.
Normally we take 0.8 as the Utilization Factor.

This utilization factor is somewhat more complicated to
determine than the maintenance factor.
The utilization factor will depend on the following:
The luminarie properties. One needs the light output ratio
(LOR). How much of the light emitted by the lamps
actually leaves the luminarie? An enclosed lamp in a
luminarie with low reflectivity will have a smaller LOR
than a naked lamp.
The light emitted downwards will probably reach the
work area without reflections. However light emitted
upwards can only reach the work area after reflection(s)

The utilization factor will depend on the following:
The reflectance of the room surfaces. Are the surfaces
(walls, ceiling) light or dark in color?
(A color with strong chroma does not reflect other colors).
Reflectance's are available from manufacturers of paints
and furniture finishing's.
It is usual to make the reflectance of the ceiling highest,
walls slightly lower and the floor darker.
Do not chooseReflectance's of desks should be 20-40%.
very dark wood, or bright surfaces.
The geometric proportion of the room. The geometric
shape of the room will affect the UF. A factor called the
room index (RI) is defined from the horizontal vertical
areas of the room.

MAINTENANCE FACTOR: (MF)
It is also known as LLF OR Light Loss Factor. This may be
due different reasons viz., Time, Temperature, Cleanliness
etc.,
The maintenance factor gives an estimate of how lighting
conditions will deteriorate through use.
Some factors are:
• Dust and dirt inside luminaries surfaces.
• Aging of light bulbs emitting less light.
• Cleaning of room surfaces, e.g. ceiling.
Normally we take 0.75 to 0.8 as the Maintenance Factor.

Coming back to our earlier Example where we said that
we need 10 Nos. of 36W Tube Light Fittings.
Now with application of UF and MF it will be as follows:
No. of Fixtures = 10 / 0.8 x 0.8 = 15.625 i.e., 16 Nos.
But practically many times to maintain the balance we
may use upto 12 Nos. of fixtures also.
Formula
for
Number of
Fixtures.
Lux x Area / 300 x 80 /
15.625Lumen
Output x MF
x UF
2400 x 0.8 x 0.8

LIMITATION OF LUMEN METHOD:
There are certain limitations to this method, but all said
and done this works perfectly for 90% of the installations
where the parameters are within the limits.
Height is not considered.
Fixtures or luminaries changes are not considered. (Polar
curve etc.,)
Room Reflectance's are not considered.
It does not distinguish between different fixture behaviors
i.e., point source or linear source etc.,
For this there is one more complicated method known as
POIINT BY POINT METHOD. This is more specific for
experts in Lighting Design and detailing.

LIGHTING STANDARDS:
What are the legal requirements for office lighting? Is
there a legal requirement for daylight?
The only legal requirements are 'sufficient and suitable'
and similar wording in health and safety legislation. There
is no absolute legal requirement for daylight, but it is a
requirement that it be provided if practicable. Various
guidance documents suggest good practice, but are not
themselves mandatory.
What is the recommended lighting level for offices?
The Code for Lighting recommends a maintained
illuminance of 500 lux for general offices (e.g. writing,
typing, reading, data processing, etc.) and for CAD work
stations and conference/meetings rooms. Where the main
task is less demanding, e.g. filing, a lower level of 300 lux
is recommended.

ELECTRICAL FORMULAS:
Demand for Power (kW) = System Input Wattage (W) ÷
1,000
Energy Consumption (kWh) = System Input Wattage (kW)
x Hours of Operation/Year
Hours of Operation / Year = Operating Hours / Day x
Operating Days / Week x Operating Weeks / Year
Lighting System Efficacy (Lumens per Watt or LPW) =
System Lumen Output ÷ Input Wattage
Unit Power Density (W/ Sq.M.) = Total System Input

SUNLIGHT:
400 lux: Sunrise or sunset on a clear day.
1,000 lux: Overcastday.
10,000–25,000 lux: Full daylight (not direct sun)
32,000–130,000 lux: Direct sunlight.

Unit v

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