I
indicted before in our course, that we can design interior lighting by using
any method from the following three ones:
The
Zonal Cavity (Lumen) method
Point
by point method
Watt
per square feet method
I explained the first and second
method in the following articles:
You can review the following
previous articles for more information and good following:
Today
I will explain the third method for lighting design which is “Watt per square
feet method” as follows.
Third: Watt per square feet method (as per ASHRAE/ANSI/IESNA 90.1)
We have
two cases for lighting design by using Watt per square feet method which are:
SpacebySpace
Method (functional area method),
The Whole
Building Method.
1 SpacebySpace Method
(functional area method)
In the SpacebySpace Method, the building will be divided
into different space based on its function like offices, conference halls,
corridors and lobbies, shops, parking areas, workshops and etc (see fig.1). The lighting power density (LPD) in (W/ft2)
or/and (VA/ft2) is prescribed for these different spaces, these lighting power
densities (LPDs) in addition to spaces area will be used to estimate the
preliminary lighting load of this building as described in below.
Usage conditions of SpacebySpace
Method
The SpacebySpace Method is used only for individual spaces
in the building.
The SpacebySpace Method may be used for any building or
portion of a building.
Area Measurement in space by
space method
The square footage is measured from the outside surface of
exterior walls to the centerline of walls between interior partitions of the
spaces. The sum of the Gross Interior Area equals the total Gross Area of the
building.
Method of estimation by using SpacebySpace
Method
 Divide the building into different space based on its
function (for
example, office, storage, mechanical, and corridor).
Calculate the gross interior area of each space.
Determine the Lighting power density in (W/ft2) or/and (VA/ft2) for
this space from the following table(as
per ASHRAE/ANSI/IESNA 90.1):
Consider requirements for supplementary lighting (for
example, floodlighting, security lighting, and special task lighting).
Multiply the gross interior area of the space by the
assigned Lighting load density to get the estimated lighting electrical
load for this space.
Example#1:
For a highrise office building with shopping arcade, a 10story
building (12 floors) with a floor area of approx. 25 m x 60 m.
There is a car park for customers in front of the building, the
access way to the parking garage (levels 1 to 3 for employees) and a fountain
(80 m x 20 m).
Real floor area approx. 1350 m2 (14 levels + technical equipment
on roof level).
Floor heights of levels 00 to +10: 4 m, levels 1 to 3: 3 m
Calculate Preliminary lighting power for this building.
Answer:
Space

Area (sqft)

Load Density
Watt/sqft

Estimated lighting Load
For Space


Parking Garage /
Utilities Areas (Incl. Roof Area)

46,391

0.2

9,278

watt


Shopping Center / Bank

14,701

1.55

22,787

watt


Offices

14,701

1.13

16,612

watt


Total Estimated Lighting
load

48,677

watt


Application Of Cos Phi =
0.85

57,267

watt


Application Of Load Level
= 70%

81,810

watt


Preliminary lighting load

81.81

KW

2 The Whole Building Method
The Whole Building Method is
used for buildings, to some extent, bigger than that can be handled by the Space
by space method, although it is easier than the Space by space method as you
estimate the electrical load based on the activity of the whole building unlike
the Space by space method which need dividing the building into multi areas
with different activities.
Comparison between
spacebyspace and building type methods
Lighting Power densities (LPDs) under the two methods are
similar for buildings with typical space distributions, but may vary depending
on the space distributions in particular buildings.
In general The Whole Building Method is easier to use
than the SpacebySpace Method, but the SpacebySpace Method offers greater
flexibility.
In addition, the
SpacebySpace Method allows the use of the Additional Interior Lighting Power
Allowance for certain specific uses.
Usage conditions of The Whole Building
Method
The Whole Building Method may be used only for:
An entire building (using the primary occupancy), or;
Single, independent, and separate occupancies in a
multioccupancy building.
Area Measurement in The Whole Building
Method
Total Gross Area will be used in The Whole Building
Method and the square footage is measured from the outside surface of
exterior walls, or from the outside surface of exterior walls to the centerline
of walls between building types.
Method of estimation by using The
Whole Building Method
Calculate the total Gross Area for the building;
Determine the building type/usage (use a reasonable
equivalent if the specific building type is not listed);
Determine the load density in (W/ft2) or/and (VA/ft2) for
that building from the following Table:
Multiply the total gross area of the building by
the assigned Lighting power density to get the estimated lighting power
loads for this building.
Take into your consideration the following points:
Consider requirements
for supplementary lighting (for example, floodlighting, security lighting, and
special task lighting).
Example#2:
(4) floors office building with a floor area
2000 SF, calculate the total preliminary electrical load for the transformer(s)
feeding this building By using The following tables from ASHRAE and IEEE.
Answer:
The total gross area of the building = 2000 x 4 = 8000 SF^{}
The building type/usage is Office
From above ASHRAE and IEEE tables, the load densities will be as
follows:
Lighting = 1 W/SF
The
subtotals estimated electrical loads will be as follows:
Lighting power
load = 1 W/SF x 8000 = 8000 Watt
Conversions between
different units of lighting quantities:
You can use the following table to convert
between different units of lighting quantities as follows:
In the next article, I will explain the
quick estimate charts and complete lighting design examples. Please, keep
following.
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