Cable Size Calculator

What is Cable Sizing?

Cable sizing involves selecting the appropriate conductor size and type for electrical installations based on current load, voltage drop, installation method, and safety requirements. Proper cable sizing ensures safe, efficient, and code-compliant electrical systems.

Why Proper Cable Sizing Matters

• Safety: Prevents overheating, fire hazards, and equipment damage
• Efficiency: Minimizes power losses and voltage drop
• Compliance: Meets NEC and local electrical code requirements
• Cost-effectiveness: Balances upfront costs with long-term performance
• Reliability: Ensures stable power delivery to loads

Key Factors in Cable Sizing

1. Current Load

Calculate the total current the cable must carry:

• Sum of all connected loads
• Include diversity factors for multiple loads
• Account for future expansion (typically 25% spare capacity)
• Apply continuous load factor (125% for loads running 3+ hours)

2. Voltage Drop Limitations

NEC recommends maximum voltage drop:

• Branch circuits: 3% maximum
• Feeders: 2% maximum
• Combined: 5% total maximum

3. Installation Method

Different installation methods affect cable ampacity:

• Direct burial: Underground installation
• Conduit: Enclosed in raceway
• Cable tray: Open tray installation
• Free air: Open installation (highest capacity)

4. Environmental Conditions

• Ambient temperature (affects ampacity derating)
• Number of conductors in conduit/tray
• Soil thermal resistivity (for buried cables)
• Exposure to moisture, chemicals, or sunlight

Cable Sizing Process

Step 1: Determine Load Current

Step 2: Apply Safety Factors

• Multiply by 1.25 for continuous loads
• Add margin for future expansion (typically 25%)
• Consider simultaneous operation of multiple loads

Step 3: Select Cable from Ampacity Tables

Choose cable size with ampacity ≥ calculated current, considering:

• Conductor material (copper or aluminum)
• Insulation temperature rating (60°C, 75°C, 90°C)
• Installation method

Step 4: Verify Voltage Drop

Calculate voltage drop for selected cable size and verify it's within limits.

Step 5: Check Short Circuit Rating

Ensure cable can withstand available fault current for the clearing time of protective devices.

Common Cable Applications

Residential Applications

Commercial/Industrial Applications

• Motor feeders: Size based on 125% of motor FLA
• HVAC equipment: Consider inrush current and cycling
• Data centers: Low voltage drop critical for sensitive equipment
• Manufacturing: Heavy loads require large conductors and proper derating

Cable Types and Applications

NM Cable (Romex)

Non-metallic sheathed cable for dry, residential locations.

• Common sizes: 14/2, 12/2, 10/2, 8/3, 6/3
• Not suitable for damp locations or outdoor use
• Most economical for residential wiring

UF Cable

Underground feeder cable for direct burial and wet locations.

• Solid or stranded conductors
• Sunlight and moisture resistant
• Ideal for outdoor circuits and landscape lighting

THHN/THWN

Individual conductors for conduit installations.

• Available in wide range of sizes (14 AWG to 1000 kcmil)
• 90°C temperature rating
• Suitable for commercial and industrial use

Armored Cable (AC/MC)

Metal-clad cable for commercial applications.

• Provides mechanical protection
• Can be used in exposed locations
• Common in commercial buildings

Important Considerations

Related Tools

For comprehensive electrical design, use our other calculators:

• Wire Gauge Calculator - Calculate individual wire size
• Voltage Drop Calculator - Verify voltage drop limits
• Ampacity Calculator - Check current carrying capacity