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Elliott Electric Supply

Calculations: Floodlighting & Mounting Height


Use the following formula to calculate floodlighting and mounting height depending on lamp type and wattage.

Mounting Height Guide
Watts H.P. Sodium Metal Halide Mercury Vapor Tungsten Halogen
70 10-15'      
100 15-20'      
150 20-25'      
175     18-23'  
200 20-25'      
250 22-27' 20-25' 20-25'  
400 30-35' 25-30' 20-25'  
500       15-20'
1000 40-50' 40-45' 35-40' 20-25'
1500       25-30'

Light Depreciation Chart (Cleaning every 3 years) Environment
Clean Medium Dirty
.96 1.01 1.06

Step 1: Determine foot-candle level from outdoor candle table below.

Step 2: Determine square footage of area to be lighted (length x width).

Step 3: Determine type of fixture to be used and proper mounting height.

Step 4: Identify initial lumens of fixture type (sese product description)

Step 5: Determine dirt depreciation factor of fixture from Light Depreciation Chart below.

Step 6: Use this data in the formula below to calculate the number of fixtures.

Light Fixtures Formula:

Number of fixtures = 3 x the number of footcandles needed x the square footage of the area divided by the initial lumens of the fixture (lamp) type x dirt depreciation.

Example:

Using 400 watt hps on 30' pole. 3 x 2 x 40,000 divided by 50,000 x 1.01 = 4.85. (Final fraction should be rounded up.) Required number of fixtures = 5.

Note: This guide is meant to help in making lighting estimations, not precise calculations.