### Watt Per Square Feet Method

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:

Space-by-Space Method (functional area method),
The Whole Building Method.

1- Space-by-Space Method (functional area method)

In the Space-by-Space 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 Space-by-Space Method

The Space-by-Space Method is used only for individual spaces in the building.
The Space-by-Space 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 Space-by-Space 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 high-rise office building with shopping arcade, a 10-story 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.

 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 space-by-space 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 Space-by-Space Method, but the Space-by-Space Method offers greater flexibility.
In addition, the Space-by-Space 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 multi-occupancy 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.

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.