Cable Ampacity Explained: How to Calculate Current Capacity (And Why It Matters)

You're selecting a cable for a 40-amp load. The catalog says a 10 AWG (6mm²) cable is rated for 55 amps. Perfect, right? Wrong. By the time you account for ambient temperature, bundling with other cables, and installation method, that same cable might only handle 28 amps—and now you have a fire hazard.

cable ampacity

Cable ampacity (current-carrying capacity) isn't a single number—it's a calculation that depends on how and where you're using the cable. Get it wrong and you risk equipment damage, fire, or code violations. Get it right and your installation is safe, efficient, and code-compliant.

This guide explains how ampacity actually works, how to calculate it correctly, and which factors matter most in real-world installations.

What is Ampacity and Why It's Not Simple

Ampacity is the maximum current a conductor can carry continuously under specific conditions without exceeding its temperature rating.

The key phrase is 'under specific conditions.' The same cable can have vastly different ampacity depending on:

• Ambient temperature (hotter environment = lower capacity)
• Installation method (in conduit vs. open air)
• Number of cables bundled together (more cables = more heat)
• Insulation temperature rating (higher rating = more current)
• Soil conditions (for buried cables)
• Duty cycle (continuous vs. intermittent)

Critical concept: Ampacity is fundamentally about heat management. Current flowing through a conductor generates heat (I²R losses). If that heat can't dissipate fast enough, the conductor temperature rises until the insulation degrades or fails.

The Basic Ampacity Formula (Simplified)

At its core, ampacity calculation balances heat generation against heat dissipation:

Heat Generated = I² × R × L

Where:

• I = Current (what we're solving for)
• R = Resistance per unit length
• L = Cable length

Heat Dissipated = (T_conductor - T_ambient) / Thermal_Resistance

Where:

• T_conductor = Maximum allowed conductor temperature
• T_ambient = Surrounding temperature
• Thermal_Resistance = Combined resistance of insulation, jacket, and surrounding medium

When heat generated equals heat dissipated, you've found the maximum safe current (ampacity).

In practice: Nobody calculates this from scratch. Instead, we use standardized tables (NEC, IEC, GB standards) that already account for typical conditions, then apply correction factors for your specific installation.

Standard Ampacity Tables: Your Starting Point

Every electrical code provides base ampacity tables. These assume standard reference conditions:

NEC (United States) Reference Conditions:

• 30°C (86°F) ambient temperature
• Not more than 3 current-carrying conductors
• Specific installation method (in conduit, cable tray, etc.)
• Insulation temperature rating (60°C, 75°C, or 90°C)

IEC (International) Reference Conditions:

• 30°C ambient for cables in air
• 20°C ground temperature for buried cables
• Specific installation methods (defined in IEC 60364)
• Insulation temperature ratings

GB (China) Reference Conditions:

• 25°C or 30°C ambient (depending on standard version)
• Specific laying methods
• Insulation types and voltage levels

Example: NEC Ampacity Table (Copper Conductors, 75°C Insulation)

Notice: The same cable carries significantly more current in free air (better cooling) than in conduit (restricted airflow).

This table is just your starting point. Now we apply correction factors.

Correction Factor 1: Ambient Temperature

Higher ambient temperature means less temperature difference between conductor and surroundings, reducing heat dissipation capacity.

Temperature Correction Formula

Corrected Ampacity = Base Ampacity × Temperature Correction Factor

NEC Temperature Correction Factors (for 75°C insulation)

Real-world example:

• Base ampacity: 10 AWG copper = 35A (in conduit, 75°C insulation, 30°C ambient)
• Your installation: 45°C ambient (inside a hot equipment room)
• Corrected ampacity: 35A × 0.71 = 24.85A ≈ 25A

You just lost 29% of your cable capacity due to temperature alone.

Why This Matters

Common high-temperature locations:

• Attics in summer (can reach 50-65°C)
• Near furnaces, boilers, or industrial ovens
• Outdoor installations in hot climates
• Inside electrical enclosures with poor ventilation
• Near heat-generating equipment

Critical mistake: Using standard 30°C table values for cables in 45°C+ environments. This is a major cause of cable overheating in industrial installations.

Correction Factor 2: Number of Conductors (Bundle Derating)

Multiple current-carrying conductors in the same conduit, cable tray, or bundle generate cumulative heat, reducing each cable's ampacity.

NEC Adjustment Factors for Multiple Conductors

Important notes:

• Only current-carrying conductors count (neutral doesn't count if balanced, ground never counts)
• In a standard 3-phase circuit: 3 phases = 3 conductors (neutral doesn't count if balanced)
• In a circuit with significant neutral current: neutral counts as a conductor

Real-world example:

• Base ampacity: 10 AWG = 35A
• Your installation: 12 power cables in same conduit
• Adjustment: 35A × 0.50 = 17.5A

You just lost 50% capacity due to bundling.

Common Bundling Scenarios

Scenario 1: Multiple circuits in one conduit

• 4 three-phase circuits = 12 current-carrying conductors
• Adjustment factor: 0.50
• Each cable loses half its capacity

Scenario 2: Cable tray with many cables

• 30 power cables in tray
• Adjustment factor: 0.45
• Severe derating required

Scenario 3: Single circuit, properly spaced

• 3 conductors (or 4 if neutral carries current)
• Adjustment factor: 1.00
• No derating needed

Pro tip: Sometimes it's cheaper to install multiple smaller conduits than to upsize cables for one large conduit. Calculate both options.

Correction Factor 3: Installation Method

How the cable is installed dramatically affects cooling and therefore ampacity.

Installation Methods Ranked by Cooling Efficiency

Best (Highest Ampacity):

1. Free air, spaced from surfaces - Maximum airflow, best cooling
2. Single cable on surface - Good airflow on three sides
3. Cable tray with spacing - Decent airflow between cables

Moderate: 4. Cable tray, touching - Cables touch, reducing cooling 5. Conduit, above ground - Limited airflow but some convection 6. Buried in duct bank - Soil provides cooling but limited

Worst (Lowest Ampacity): 7. Direct burial, multiple cables - Poor heat dissipation, cumulative heating 8. Concrete-encased conduit - Very poor heat dissipation

Example Comparison: 4/0 AWG Copper (107mm²)

Key insight: The same conductor can have 95% more capacity (350A vs 180A) depending solely on installation method.

Correction Factor 4: Soil Conditions (For Buried Cables)

Buried cables depend on soil thermal conductivity for cooling. Soil type, moisture content, and depth all matter.

Soil Thermal Resistivity

Good Heat Dissipation (Low Thermal Resistivity):

• Wet clay: 0.5-0.7°C·m/W
• Moist sand: 0.7-1.0°C·m/W
• Normal soil with moisture: 1.0-1.5°C·m/W

Poor Heat Dissipation (High Thermal Resistivity):

• Dry sand: 2.0-3.0°C·m/W
• Dry clay: 2.5-4.0°C·m/W
• Very dry soil: 4.0-6.0°C·m/W

Burial Depth Effect

Typical correction factors by depth:

• 0.5m depth: 1.00 (reference)
• 0.8m depth: 0.94
• 1.0m depth: 0.89
• 1.5m depth: 0.82
• 2.0m depth: 0.77

Why depth matters: Deeper burial means heat must travel farther through soil to dissipate. More thermal resistance = less ampacity.

Practical consideration: In dry climates, consider installing buried cables in thermally-enhanced backfill (special sand mixtures) or using larger cables to compensate.

Putting It All Together: Complete Calculation Example

Let's work through a real-world scenario:

Requirements:

• Load: 80 amps continuous
• Location: Industrial facility in Arizona
• Installation: 6 three-phase circuits (18 current-carrying conductors) in one conduit
• Environment: 40°C inside building near machinery
• Insulation: 75°C THHN

Step 1: Estimate required base ampacity

We need to work backwards from our required 80A.

Step 2: Apply temperature correction

• 40°C ambient
• Temperature correction factor: 0.82

Step 3: Apply bundling adjustment

• 18 conductors
• Bundling adjustment: 0.50

Step 4: Calculate required base ampacity

Required Base = Actual Need ÷ (Temp Factor × Bundle Factor) Required Base = 80A ÷ (0.82 × 0.50) Required Base = 80A ÷ 0.41 Required Base = 195A

Step 5: Select conductor from table

From NEC Table 310.16 (75°C copper in conduit):

• 2/0 AWG (67mm²) = 175A base (too small)
• 3/0 AWG (85mm²) = 200A base ✓ (adequate)

Final selection: 3/0 AWG copper conductors

Verification:

• Base ampacity: 200A
• After temperature correction: 200 × 0.82 = 164A
• After bundling adjustment: 164 × 0.50 = 82A
• Final capacity: 82A ✓ (meets 80A requirement)

Cost reality check:

• If you ignored corrections and used the '55A cable' for '80A load': FIRE HAZARD
• If you used 2/0 AWG (175A base): Still inadequate after corrections
• Proper selection (3/0 AWG): Safe, code-compliant installation

The 80% Rule: Why You Need Margin

Most electrical codes require conductors be sized so the load doesn't exceed 80% of the conductor's ampacity for continuous loads (3+ hours).

Why 80%, not 100%?

1. Safety margin for calculation uncertainties
2. Voltage drop considerations (covered separately)
3. Future load growth
4. Real-world variations in manufacturing, temperature, etc.

Application:

• If your load is 40A continuous, you need cable rated for 40A ÷ 0.80 = 50A minimum
• This 80% rule applies AFTER all correction factors

Example:

• Load: 40A continuous
• Required corrected ampacity: 40A ÷ 0.80 = 50A
• Temperature factor: 0.82
• Bundling factor: 0.80
• Required base ampacity: 50A ÷ (0.82 × 0.80) = 76A minimum

Special Considerations

High-Frequency Loads (VFDs, Harmonics)

Variable frequency drives (VFDs) and non-linear loads generate harmonics that increase effective heating:

• Harmonics increase RMS current above fundamental
• Skin effect concentrates current on conductor surface
• Additional heating not captured in standard tables

Rule of thumb: Derate cables by additional 10-20% for VFD applications, or consult manufacturer.

Intermittent vs Continuous Loads

Standard ampacity tables assume continuous loading (100% duty cycle).

For intermittent duty:

• Duty cycle < 50%: May use higher ampacity
• Must calculate thermal time constants
• Typically requires engineering analysis

Common mistake: Assuming 'it only runs sometimes' means you can use smaller cable. Unless properly calculated, always use continuous rating.

Voltage Drop vs Ampacity

Two separate but related calculations:

Ampacity: Limits based on temperature/heat Voltage drop: Limits based on acceptable voltage loss

You must satisfy BOTH requirements. Often:

• Short runs: Ampacity determines size
• Long runs: Voltage drop determines size (often requiring larger cable than ampacity alone)

Example:

• 50A load, 150 feet (45m) from source
• Ampacity calculation: 6 AWG adequate
• Voltage drop calculation: 2 AWG required
• Final selection: 2 AWG (larger of the two)

Parallel Conductors

For very high currents, multiple conductors per phase in parallel:

Requirements:

• All conductors same length
• Same material and size
• Same insulation type
• Properly balanced current sharing

Ampacity: Each conductor carries its pro-rata share of total current.

Example:

• Need 600A capacity
• Two 4/0 AWG in parallel per phase
• Each cable: 230A base × 2 = 460A theoretical
• Apply correction factors to total
• Usually only practical for large industrial installations

Quick Reference: Ampacity Decision Checklist

Before selecting cable size, answer these questions:

1. What is the actual load current?

• Nameplate rating
• Measured current if available
• Include all loads on circuit

2. Is the load continuous (3+ hours)?

• Yes → Apply 80% rule (multiply load by 1.25)
• No → Use actual load (if truly intermittent)

3. What is the ambient temperature?

• Measure at cable location
• Consider worst-case (summer peak)
• Inside enclosures: can be 10-20°C higher than room temp

4. How many conductors in conduit/bundle?

• Count only current-carrying conductors
• Include neutral if it carries significant current
• Don't count ground

5. What is the installation method?

• In conduit
• Cable tray
• Free air
• Buried
• Choose appropriate ampacity table

6. What insulation temperature rating?

• 60°C, 75°C, or 90°C
• Must match termination temperature rating
• Higher rating = higher ampacity

7. Any special conditions?

• High altitude (>2000m: additional derating)
• Soil conditions (for buried)
• VFD loads (additional derating)
• High ambient temperature near equipment

Common Ampacity Mistakes and How to Avoid Them

Mistake 1: Using Base Table Values Directly

Problem: Ignoring correction factors Result: Undersized cable, overheating, potential fire Solution: Always apply temperature and bundling corrections

Mistake 2: Not Accounting for Ambient Temperature

Problem: Using 30°C table for 45°C+ environment Result: Cable operating beyond temperature rating Solution: Measure actual ambient temperature, apply correction

Mistake 3: Miscounting Current-Carrying Conductors

Problem: Not counting neutral in circuits with harmonics Result: Undersized cable for actual heating Solution: Count neutral if it carries significant current (>50% of phase)

Mistake 4: Confusing Ampacity with Breaker Size

Problem: '20A breaker so I need 20A cable' Result: Potentially undersized after corrections Solution: Calculate required ampacity, then select breaker

Mistake 5: Ignoring Installation Method

Problem: Using 'free air' ampacity for conduit installation Result: Cable rated 30-40% higher than actual capacity Solution: Use correct table for actual installation method

Mistake 6: No Safety Margin

Problem: Selecting cable at exactly calculated ampacity Result: No room for load growth, calculation uncertainties Solution: Round up to next standard size, apply 80% rule

Mistake 7: Not Considering Voltage Drop

Problem: Focusing only on ampacity Result: Cable thermally adequate but excessive voltage drop Solution: Calculate both ampacity AND voltage drop, use larger size

Software and Online Calculators

For complex installations, consider using ampacity calculation software:

Free Options:

• NEC Ampacity Calculator (various online tools)
• IEC 60364 calculators
• Manufacturer-specific tools (Southwire, etc.)

Professional Software:

• ETAP (comprehensive electrical analysis)
• SKM PowerTools
• EDSA (full power system design)

When to use software:

• Complex installations with multiple factors
• Large industrial projects
• When code compliance documentation needed
• Unusual conditions (multiple correction factors)

When manual calculation is OK:

• Simple installations
• Standard conditions
• Using conservative assumptions (oversizing)

Practical Tips for Real-World Applications

Tip 1: Round Up, Don't Round Down

When in doubt, go to the next larger cable size. The cost difference is usually minimal compared to reinstallation costs if undersized.

Tip 2: Document Your Assumptions

Write down:

• Ambient temperature used
• Number of conductors counted
• Correction factors applied
• Load calculations

This documentation helps future troubleshooting and code inspections.

Tip 3: Measure Actual Temperatures

Don't guess ambient temperature. Use a thermometer in the actual location:

• Inside enclosures
• In attics/ceilings
• Near heat sources
• During peak temperature times

Tip 4: Plan for the Future

Consider:

• Potential load growth (next 5-10 years)
• Additional circuits in same conduit
• Changing ambient conditions (new equipment nearby)

Oversizing cable slightly now is cheaper than replacing it later.

Tip 5: Verify with Thermal Imaging

After installation, use thermal imaging to verify cables aren't running hot:

• Cables should be close to ambient temperature under load
• Hot spots indicate problems (poor connections, undersized, etc.)
• Routine thermal surveys catch problems early

Tip 6: Consider Installation Ease

Sometimes a larger cable with adequate ampacity is harder to install than two smaller cables:

• Bend radius requirements
• Conduit fill limitations
• Termination space

Balance ampacity requirements with practical installation considerations.

The Bottom Line: Ampacity Is About Safety

Cable ampacity isn't just about meeting code—it's about preventing fires, equipment damage, and safety hazards.

Key principles to remember:

1. Base ampacity is just a starting point - Always apply correction factors for your specific installation
2. Heat management is everything - Current capacity is fundamentally limited by how fast heat can dissipate
3. Multiple factors compound - Temperature correction AND bundling adjustment can reduce capacity by 60-70%
4. Measure, don't guess - Actual ambient temperature, actual number of conductors, actual installation method
5. When in doubt, size up - The cost difference between cable sizes is small compared to failure costs
6. Document everything - Your future self (or inspector) will thank you

Before you buy cable:

• Calculate actual required ampacity with ALL correction factors
• Select from appropriate table for your installation method
• Verify termination temperature ratings match
• Check voltage drop separately
• Round up to next standard size
• Document your calculations

Done correctly, your cable will safely carry its load for decades. Done incorrectly, you're installing a fire hazard.

Now you know how to calculate ampacity properly—use this knowledge to design safe, compliant electrical installations.