How to Size Cable for Street Lighting Circuits

Choosing the correct cable size for a street lighting circuit is one of the most important steps in electrical design. A wrong cable size can cause voltage drop, overheating, poor lamp performance, and unnecessary power losses. That is why engineers and electricians always need to size cable for street lighting circuits carefully.

Street lighting systems often run over long distances. This makes cable sizing more critical compared to indoor wiring. Whether you are installing LED street lights, solar-powered poles, or conventional lighting systems, understanding how to size cable for street lighting circuits ensures safety, efficiency, and compliance with electrical standards.

In this guide, you will learn the complete cable sizing process with practical examples, tables, and expert tips.

Why Proper Cable Sizing Matters in Street Lighting

Street light circuits are usually distributed across roads, highways, parking lots, and industrial areas. Since the load is spread over long cable lengths, voltage drop becomes a major concern.

Proper cable sizing helps in:

• Reducing energy losses
• Maintaining voltage at each luminaire
• Preventing cable overheating
• Improving system lifespan
• Meeting NEC and IEC standards
• Lowering maintenance costs

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If you fail to size cable for street lighting circuits correctly, the last street light may receive low voltage, causing dim output or early failure.

Key Factors to Size Cable for Street Lighting Circuits

Before selecting the cable, consider these important factors:

1. Total Connected Load

Calculate the total wattage of all street lights.

Formula:

Total Load (W) = Number of Lights × Wattage per Light

Example:

20 lights × 120W = 2400W

Uncover insights on iec standard for lighting lux level, download a calculator in excel to calculate the required lux level according to IEC standards.

2. System Voltage

Common street lighting voltages include:

Higher voltage systems reduce current and allow smaller cable sizes.

3. Cable Length

Longer cable length increases resistance and voltage drop.

Example:

A 300-meter cable run needs larger cable than a 50-meter run for the same load.

4. Allowable Voltage Drop

IEC recommends:

• 3% for lighting circuits
• Maximum 5% in some cases

For street lights, lower voltage drop gives better light consistency.

Use our online tool Lighting Load Calculation per Square Foot: Download Excel Sheet

5. Installation Method

Cable ampacity changes depending on installation:

• Underground buried cable
• Cable tray
• PVC conduit
• Direct laying

Buried cables may require derating.

6. Ambient Temperature

High temperatures reduce cable current carrying capacity.

This is very important in hot climates.

Basic Formula to Calculate Cable Size

To size cable for street lighting circuits, use current and voltage drop formulas.

Current Formula for Single Phase

I = P ÷ (V × PF)

Where:

• I = Current (A)
• P = Power (W)
• V = Voltage
• PF = Power Factor

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Example:

Load = 2400W
Voltage = 230V
PF = 0.95

Current:

I = 2400 ÷ (230 × 0.95)

I = 10.98A

Required current is 11A.

Voltage Drop Formula

For single phase:

VD = (2 × L × I × mV/A/m) ÷ 1000

Where:

• VD = Voltage Drop
• L = Length
• I = Current

This formula helps verify if the cable size is acceptable.

Use our free street light cable size calculator

For easier calculations, you can use our street light cable size calculation tool.

Standard Street Light Cable Size Chart

Below is a general cable size reference:

This table is for copper cables under normal conditions.

Step-by-Step Example to Size Cable for Street Lighting Circuits

Let’s calculate a real-world example.

Given Data:

• Number of poles = 15
• Light per pole = 150W LED
• Voltage = 230V
• Cable length = 180m
• Power factor = 0.95
• Allowable voltage drop = 3%

Download Free Street Lighting Voltage Drop Calculator in Excel

Step 1: Total Load

Total Load:

15 × 150 = 2250W

Step 2: Current Calculation

I = 2250 ÷ (230 × 0.95)

I = 10.29A

Step 3: Select Initial Cable

From ampacity, 2.5 mm² looks suitable.

Step 4: Check Voltage Drop

For 2.5 mm² copper:

mV/A/m = 18

VD = (2 × 180 × 10.29 × 18) ÷ 1000

VD = 66.67V

Voltage drop percentage:

(66.67 ÷ 230) × 100 = 28.9%

This is too high.

Step 5: Increase Cable Size

Try 10 mm²:

mV/A/m = 4.4

VD = (2 × 180 × 10.29 × 4.4) ÷ 1000

VD = 16.3V

Percentage:

(16.3 ÷ 230) × 100 = 7.08%

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Still high.

Try 16 mm²:

mV/A/m = 2.8

VD = 10.37V

Percentage:

4.5%

Better and acceptable in many applications.

This shows why voltage drop matters more than current in long street light runs.

Key Takeaways

• Always calculate total load first
• Voltage drop is the main factor in long street lighting circuits
• Never select cable only based on current
• Copper cables offer lower resistance than aluminum
• Follow IEC or NEC regulations for compliance
• Future load expansion should be considered
• Use calculation tools for quick verification

To compare conductor ratings and dimensions, check our guide on street light wire size.

Find the exact street lights cable size with Street Light Cable Size Chart

Copper vs Aluminum Cable for Street Lighting

Copper is preferred for reliability, but aluminum can reduce project cost.

Common Mistakes in Street Light Cable Sizing

Avoid these mistakes:

• Ignoring voltage drop
• Using undersized neutral cable
• Not applying temperature correction factor
• Skipping diversity factor
• Not considering future extension
• Wrong power factor assumptions

These mistakes can lead to system failure.

Find more Electrical Engineering Tools for easy electrical calculations and estimations.

Best Practices for Safe Installation

To properly size cable for street lighting circuits, follow these best practices:

• Use armored cables for underground installation
• Maintain proper earthing
• Install weatherproof junction boxes
• Use voltage drop software for large projects
• Keep spare capacity for expansion
• Follow local electrical codes

A well-designed cable system improves safety and reduces long-term expenses.

Final Thoughts

When planning outdoor lighting systems, it is essential to size cable for street lighting circuits using both ampacity and voltage drop calculations. Many installers focus only on current, but long-distance street light circuits require deeper analysis.

By calculating load, checking voltage drop, and selecting the right conductor size, you can build a safe and efficient lighting system. Whether it is a small residential road or a large municipal lighting project, proper cable sizing ensures stable performance and lower maintenance.

Using professional tools and standard cable charts makes the process faster and more accurate.

Use this tool if you are trying to calculate cable size for underground cables. Try here Underground Cable Size Calculator – Find Correct Wire Size for Long Distance Runs

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