How to Choose Cable Size? A Beginner's Guide

I. Introduction

Whether you're wiring a new home, setting up industrial equipment, or installing a renewable energy system, choosing the correct cable size is crucial for safety and efficiency. Undersized cables can overheat, create dangerous voltage drops, and even cause fires, while oversized cables waste money unnecessarily.

The proper cable size ensures that electrical current flows safely through your system while maintaining voltage levels within acceptable parameters. This guide will walk you through the essential factors to consider when selecting electrical cables, helping you make informed decisions for your projects.

II. Key Factors in Choosing Cable Size

1. Current Carrying Capacity (Amperage)

Definition and relevance

Current carrying capacity (ampacity) refers to the maximum current a conductor can safely carry continuously without exceeding its temperature rating. This is the primary factor in determining cable size.

How to estimate or calculate the current load

To determine current load:

• For specific appliances: Check the nameplate for amperage or wattage ratings

• For circuits: Add up the expected loads or use the formula I = P/V (Current = Power/Voltage)

• For motors: Account for starting current, which can be 3-7 times the running current

• Always include a safety margin of 20-25% above your calculated load

2. Voltage Drop

What it is and why it matters

Voltage drop is the reduction in voltage that occurs as electricity travels through a cable. When voltage drops significantly, equipment performance suffers, motors can overheat, and electronic devices may malfunction or fail prematurely.

Acceptable voltage drop limits

• General applications: Maximum 3% voltage drop

• Lighting circuits: Maximum 2% drop recommended

• Motor starting: Up to 5% drop may be acceptable temporarily

• Critical equipment: Consider tighter restrictions (1-2%)

The longer the cable run, the more significant the voltage drop becomes, requiring larger cable sizes to compensate.

3. Cable Length

How cable length affects resistance and voltage drop

Cable resistance increases proportionally with length. Doubling the cable length doubles the resistance and voltage drop. This means that what works for a 10-meter run may be completely inadequate for a 50-meter run, even with the same load.

Special considerations for long-distance runs

• For runs exceeding 30 meters, voltage drop often becomes the determining factor rather than ampacity

• Very long runs may require jumping up multiple cable sizes

• Consider alternative approaches for very long distances:

  • Higher voltage transmission with step-down transformers

  • Multiple feed points

  • DC power for some applications

4. Type of Load

Resistive vs inductive loads

• Resistive loads (heaters, incandescent lights): Draw consistent current based on their power rating

• Inductive loads (motors, compressors, transformers): Draw high inrush current at startup and create power factor considerations

Impact on sizing

• Motors typically require oversized cables to handle starting current

• For circuits with multiple motors, consider diversity factors based on the likelihood of simultaneous operation

• Power factor correction equipment may reduce overall current requirements

5. Ambient Temperature and Installation Conditions

Temperature correction factors

The standard ampacity ratings for cables assume an ambient temperature of about 30°C (86°F). For higher temperatures, cables must be derated:

• At 40°C (104°F): Reduce capacity by approximately 10-15%

• At 50°C (122°F): Reduce capacity by approximately 20-30%

• At 60°C (140°F): Reduce capacity by approximately 40-45%

Installation methods affecting capacity

• Buried cables: Heat dissipation depends on soil type and moisture content

• Conduit use: Restricts heat dissipation, reducing capacity, especially for multiple cables

• Cable grouping: Bundles of cables generate more heat, requiring derating

• Insulation type: Different insulation materials have different temperature ratings

6. Material of the Conductor

Copper vs aluminum comparison

Aluminum is often chosen for large feeders and service entrances where cost and weight are significant factors, while copper is preferred for most branch circuits and critical applications.

III. Common Cable Size Standards

Overview of standards

American Wire Gauge (AWG)

• Used primarily in North America

• Counter-intuitive: Larger numbers indicate smaller wires

• Common sizes: 14 AWG (15A circuits), 12 AWG (20A circuits), 10 AWG (30A circuits)

Metric system (mm²)

• Used internationally and in IEC standards

• Directly represents the cross-sectional area of the conductor

• Common sizes: 1.5 mm², 2.5 mm², 4 mm², 6 mm², 10 mm², etc.

Regulatory standards

• National Electrical Code (NEC): Used in the US

• International Electrotechnical Commission (IEC): International standard

• BS7671: British standards

• Local building codes and regulations may have specific requirements

Example cable sizing table

Note: This table is for general reference only. Actual requirements depend on all factors discussed in this guide and local electrical codes.

IV. Cable Sizing Calculation Methods

1. Manual Calculation

Basic voltage drop formula:

VD = (2 × L × I × R) / 1000

Where:

• VD = Voltage Drop (V)

• L = Cable length (one-way, in meters)

• I = Current (A)

• R = Cable resistance (Ω/km)

Simplified cable size selection:

1. Determine the maximum current (including safety margin)

2. Select cable that exceeds this current rating

3. Calculate voltage drop based on length

4. If voltage drop exceeds limits, increase cable size

2. Using Online Calculators or Software

Many free and paid tools are available to simplify cable sizing:

• Manufacturer-provided calculators (Southwire, General Cable)

• Professional electrical design software (e.g., ETAP, EasyPower)

• Mobile apps for quick field calculations

These tools typically account for factors like:

• Cable type and material

• Installation method

• Ambient temperature

• Grouping factors

• Voltage drop requirements

3. When to Consult an Electrician or Engineer

Consider professional help when:

• Working with three-phase power systems

• Designing critical infrastructure

• Handling large loads (>30A)

• Working in hazardous environments

• Dealing with unusual installation conditions

• When local codes require professional certification

V. Real-World Examples

Example 1: Choosing cable for a 10A light circuit at home

Scenario:

• Circuit: 120V lighting circuit drawing 10A

• Distance: 15 meters from panel to furthest fixture

• Installation: In-wall, normal residential conditions

Solution:

1. Current requirement: 10A × 1.25 (safety margin) = 12.5A

2. Initial cable size based on ampacity: 14 AWG (15A rating)

3. Voltage drop check:

   • 14 AWG copper has resistance of approximately 8.3 Ω/km

   • VD = (2 × 15 × 10 × 8.3) / 1000 = 2.49V

   • As percentage: 2.49V / 120V × 100% = 2.08%

4. Decision: 14 AWG is suitable as voltage drop is under 3%

Example 2: Selecting cable for a 5kW motor 50 meters away

Scenario:

• 5kW three-phase motor, 400V, power factor 0.85

• Distance: 50 meters

• Installation: In conduit, ambient temperature 35°C

Solution:

1. Current calculation:

   • I = 5000 / (√3 × 400 × 0.85) = 8.5A

   • Starting current: 8.5A × 6 = 51A (momentary)

   • With 25% margin: 8.5A × 1.25 = 10.6A continuous

2. Initial cable size based on ampacity: 2.5 mm² (carrying capacity ~20A)

3. Temperature derating: At 35°C, derate by ~5% → 19A (still sufficient)

4. Voltage drop check:

   • 2.5 mm² has resistance approximately 7.41 Ω/km

   • VD = (√3 × 50 × 10.6 × 7.41) / 1000 = 6.8V

   • As percentage: 6.8V / 400V × 100% = 1.7%

5. Starting voltage drop check:

   • VD during start = (√3 × 50 × 51 × 7.41) / 1000 = 32.7V

   • As percentage: 32.7V / 400V × 100% = 8.2%

6. Decision: Consider increasing to 4 mm² to reduce voltage drop during starting, especially if motor starts frequently or under load

VI. Safety and Regulations

Importance of adhering to electrical codes

Electrical codes exist to protect life and property. They represent the minimum safety requirements based on extensive research and experience. Common codes include:

• NEC (National Electrical Code): Updated every three years in the US

• CEC (Canadian Electrical Code): Canadian equivalent of NEC

• IEC standards: International standards adopted by many countries

• Local building codes: May have additional requirements specific to your location

Certification and labeling of cables

Look for these markings to ensure cables meet safety standards:

• UL (Underwriters Laboratories) or ETL certification

• CE marking in Europe

• Type designations (e.g., THHN, XHHW, NM-B)

• Temperature rating (e.g., 60°C, 75°C, 90°C)

• Voltage rating (e.g., 300V, 600V)

• Size markings (AWG or mm²)

Fire safety and insulation types

Different insulation types offer varying levels of protection:

• PVC (Thermoplastic): Common, economical, but produces toxic smoke when burned

• XLPE (Cross-linked polyethylene): Better heat resistance and durability

• Low-smoke zero-halogen (LSZH): Safer in fires, required in many public buildings

• Mineral insulated (MI): Highest fire rating, used in critical circuits

For critical applications or public buildings, always consider fire performance alongside electrical requirements.

VII. Conclusion

Key takeaways

• Safety first: When in doubt, choose the larger cable size

• Consider all factors: Current, length, voltage drop, environment, and load type all matter

• Follow codes: Local electrical codes set minimum requirements

• Think long-term: Future expansion may justify larger cables now

• Consult professionals: For critical or complex installations

Final tips

• Always overestimate rather than underestimate your needs

• For DIY projects, have your work inspected by a professional

• Document your installations with diagrams and calculations

• Consider future needs when sizing cables for new construction

• Remember that the cost difference between cable sizes is often minor compared to the cost of reinstallation

VIII. Resources and Tools

Useful calculators and references

• Southwire Voltage Drop Calculator

• Cerrowire Electrical Calculator

• BlueSeaSystem's Circuit Wizard

Standards and guidance documents

• NFPA 70 (National Electrical Code) - Available at NFPA.org

• IEEE 141 (Red Book) - Power distribution for industrial plants

• IEEE 242 (Buff Book) - Protection and coordination of industrial power systems

Books for further reading

• "Ugly's Electrical References" - A compact reference guide

• "Practical Electrical Wiring" by Herbert P. Richter and W. Creighton Schwan

• "National Electrical Code Handbook" published by NFPA

Remember that while this guide gives you the fundamentals of cable sizing, electrical work can be dangerous and often requires professional licensing. Always consult with qualified electricians or engineers for critical installations and follow all local codes and regulations.