This guide is intended for street lighting that is grid powered.

Why street lighting.

• For safety or security reasons.
• Clear view of objects for comfortable movement.

It should be noted that the above are the main objectives and thus the lighting scheme should not be expected to give a day light lighting level.

Design considerations:

• Spacing of the street lights.
• Wattage of the street lights.
• Light levels required.
• Type of lighting colour: Warm or Daylight?
• Warm light (yellow) penetrates darkness more than day light and hence it is a common choice when it comes solar street lighting.

Lamps used in street lighting:

• High pressure sodium lamp.
• Metal Halide Lamps.
• Low pressure sodium lamps.
• Incandescent Lamp (not recommended).
• LED.
• CFL.

What you will find in this guide:

• Street illumination level calculations.
• Spacing between two light poles calculations.
• Street Light Luminaire calculations.
• Cable Sizing
• How to estimate the required power for the street light area:
• Energy Cost Calculations
• Pole Arrangement Schemes in Street (Road) Lighting Design
• Street Lighting Bills of Quantities

 NOTE: This guide has been made as simple and practical as possible. It mostly applies to street lighting though some of its principles also apply to road lighting.  Some concepts are mentioned in passing and the reader can research and read more on the same. There are details that have been deliberately omitted such as “Light intensity distribution of the lighting luminaire”, “Spread and Throw Angles of the luminaire” etc. In the near future, I may write a more detailed guide and also another one for “Market Lighting” and share the respective BOQs as well.

 A.Street illumination level in Lux:

Where;

• AI=Average lumens.
• E = The illumination in Lux
• w = Width of the roadway
• d = Distance between luminaries
• UF = Utilization. Which is dependent on the type of fixture, mounting height, width of roadway and the length of mast arm of outreach.
• MF=Maintenance factor (normally 0.7 to 0.9).

The typical value of Al is

• 20500 lumens for 400 watts
• 11500 lumens for 250 watts
• 5400 lumens for 125 watts
• The value of Al varies depending upon the type of lamp specified.

B.Spacing between two light poles

Suppose you have two poles of street light having lamps rated at 300W, what is the ideal space between them if;

• Lamp output of the Lamp (LL) is= 33200 lumens.
• Required Lux Level (E) is =5 lux.
• Width of the road (W) = (11.48feet) 3.5 M
• Height of the pole (H) = (26.24feet) 8 M
• Utilization factor=0.19
• Lamp Lumen Depreciation Factor (LLD) = 0.8
• Luminaries dirt Depreciation Factor (LDD) = 0.9

The ideal spacing would be 24M. Note that the formula is based on “feet” as the unit of length.

C.Street Light Luminaire.

Suppose you have all other details required and you wanted to figure out the proper wattage of your lamps. For example, what would be the street light wattage of each luminaire of street light pole having the following considerations?

• Road Details: The width of road is 7 Meter. Distance between each Pole (D) is 50 Meter.
• Required Illumination Level for Street Light (E) is 6.46 Lux per Square Meter.
• Luminous efficacy is 28 Lumen/Watt.
• Maintenance Factor (mf) 0.29, Utilization Factor (U.F) is 0.9.

Average Lumens (AI),

D.Cable Sizing:

It is important to size your cables properly so as to ensure that voltage drops do not exceed the recommended values.

To properly size your cables, you must consider these factors: the total load, load current, the cable length and the respect constant for the given load current.

The voltage drop on load conditions at a final sub-circuit;

Where:

(i)  The current carrying capacity of the cable, = I_R     Amps

 (ii)  The load current;   =I_L   Amps

 (iii)  The rated Voltage drop per Ampere per meter = Vd/A/m Volts

 (iv)  The length of run of the cable =L meters.

According to the IEE wiring regulation (BS 7671: 2008), 525-01-02 (17th Edition);

The maximum permissible voltage drop to a final sub-circuit= 4% of the nominal supply voltage.

For single phase (240V) the maximum permissible voltage drop; V_d =4%X240 =       9.6Volts

For Three phase (415V) the maximum permissible voltage drop; V_d =4% X415 =16.6Volts

For example, suppose that you have 6, 150W rated lamps. 

Say, the total cable length that will be used is 100m (that’s, the last lamp will be located 100m from the transformer.

Total load= 150WX6=900W

According to BS7671-2008, load current of 3.75A can be well accommodated by a 1.5mm² copper cable. We are using BS7671-2008, Table 4E4A for cable sizing, and Table 4E4B for voltage drop constants, pages 348 and 349 respectively. Note that for the voltage drop (per ampere per metre) constants are in mV/A/m, thus you have to divide the constant by 1000. Check the table references at the end of the post.

If we used the 1.5mm² copper cable, the voltage drop would be:

Voltage drop=0.031×3.75×100=11.625V

11.625V exceeds the maximum voltage drop of 9.6V

We move to the next cable size, i.e, 2.5mm^2,

If we used the 2.5mm² copper cable, the voltage drop would be:

Voltage drop=0.019×3.75×100=7.125V

7.125V is very close to the limit of 9.6V, and since we must account for losses by the lamps themselves, temperature related constants, a 4mm² cable would be appropriate as it would take care of such.

Thus, for this example, we will use a 4mm² copper cable whose current rating is 49A for a single phase ac load. Based on our hypothetical load current of 3.75A.

Voltage drop=0.012×3.75×100=4.5V

Sure, the 4.5V drop is well within the limit of 9.6V. Thus, the 4.5mm²   is more suitable for our example.

Next is the MCCB sizing.

According to IEE regulations:

I_l ≤ I_b ≤ I_c

Where:

• I_l = Design current of the circuit.
• I_b = Nominal current or current setting of the protective device.
• I_c = Current carrying capacity of the conductor in the particular installation conditions.

In this case:

I_l = 3.75A

I_c = 49A

Thus, you can choose any MCCB between 5A-30A. So we can choose 30A.Note that the choice of the MCCB is also dependent on the short circuit currents calculations. So, we can have two MCCBs rated 30A but with different short circuit current breaking capacity, say 16kA and 32kA. These calculations are beyond the scope of this guide though. But you can search online how to do that, or contact me.

E.How to estimate the required power for the street light area:

Suppose you have the following data, what will be the street light watt?

• Required Illumination Level for Street Light (L) is 6 Lux per Square Meter.
• Luminous efficacy (En) is 20 Lumen per Watt.
• Required Street Light Area to be illuminated (A) is 1 Square Meter.

F.Energy Cost Calculations:

Simply, the cost of running a light is directly related to the wattage of the lamp plus any associated fitting such as the ballast, transformer etc. The higher the wattage, the higher the running cost.

The running cost of lamp over its lifetime:

G.Overall Average Lux Level for Street Lighting:

The Average Lux Level of Street Light is measured by 9 point method.

Steps.

Make two equal quadrants between two street light poles on the lane of light poles (one side pole to road).

We have 3 points P1, P2 and P3 under the light pole then P4 & P7 are points opposite pole 1 or Point P3 same is applicable for P6 and P9 for Pole 2.

Note that there is another more accurate method, 12 point as the 9 point assumes linear distribution of lux which is not the case in reality. Check another example at the end of the post.

H.Pole Arrangement Schemes in Street (Road) Lighting Design:

Single Sided

When the width (W) of the street (road) is nearly equal to the pole height (H), i.e. W = H then the poles are arranged in one side only. Generally pole height is available of 8.5-10 meter.

The span between two poles is equal to the road width.

Double Sided

When the width (W) of the street (road) is nearly double the pole height (H), i.e. W = 2H then the poles are arranged along both sides in opposite to each other manner.

The span between two poles may not be equal to the road width.

Staggered Sided or Zigzag Pattern

When the width (W) of the street (road) is nearly 1.5 times of the pole height (H), i.e. W = 1.5 H then the poles are arranged in both sides in zigzag manner.

The span between two poles may not be equal to the road width.

I.Calculate Uniformity Ratio:

Once luminaries spacing has been decided, it is necessary to check the uniformity of light distribution and compare this value to the selected lighting

Uniformity Ratio ( UR) = E_av /E_min

• E_av= average maintained horizontal luminance.
• E_min = maintained horizontal luminance at the point of minimum illumination on the pavement.

J.Street Lighting Bills of Quantities

• The bills of quantities for street lighting normally capture the following:
• Lighting points.
• The lighting fittings.
• Poles used and their type.
• Consumer units that may be used.
• Earthing facilities.
• Automatic Photocell cells units to automatically switch the lights on and off.
• Type and size of the holes for the poles.

The items mentioned above must be well described and their quantities well capture.

As a guide, you can download these two blank street lighting bills of quantities below in EXCEL and PDF formats .

Excel Format:

Street Lighting BoQ-Blank BoQ 1

Street Lighting- BoQs -Blank BoQ 2

PDF formats:

Street Lighting- BoQs -Blank BoQ 2

Street Lighting BoQ-Blank BoQ 1