# How to Calculate Spacing Between Lighting Units in Highway || Formula & Mathematical Example

## How to Calculate Spacing Between Highway Lighting Units

In this article, we will discuss the formula and solve the mathematical problem about the spacing between two poles of street light units:

### Formula

The following relationship or formula is used to determine the spacing between the lighting units

Spacing Between Lighting Units = ( Lamp lumen ✖ Coefficient of utilization ✖ Maintenance factors) / ( Average lux ✖ Width of road)

### Mathematical Example

A street lighting system is to be designed for the following conditions:

• Street width – 15 m
• Mounting height – 7.5 m
• Lamp size – 6000 lumen
• Luminaire type – II

Calculate the spacing between lighting units to produce an average lighting intensity of 6.0 lux, using the given coefficient of utilization chart(see. fig. below).

Solution:

The ratio of road width and mounting height is = (Street width/ Mounting height) = (15/7.5) = 2

From the above chart, the coefficient of utilization for the ratio of road width to height = 3.6

Assume a maintenance factor = 0.8 [ if this value is not given 80 % may be assumed].

Now,

Spacing = ( Lamp lumen ✖ Coefficient of utilization ✖ Maintenance factors) / ( Average lux ✖ Width of road)

Spacing = (6000 ✖ 0.36 ✖ 0.8) / ( 6 ✖ 15) = 19.2 m (Answer.)