ΩOhmReady

NEC Wire Size Reference: Copper vs Aluminum Ampacity and When to Upsize

wire sizingnec codeampacitycopper vs aluminum

Every circuit you pull starts with the same question: copper or aluminum? The NEC ampacity tables in Table 310.16 list both conductor materials side by side, but choosing between them involves more than comparing numbers in a column. Material cost, termination compatibility, available conduit space, and long-run voltage drop all factor into the decision—and getting it wrong means either a failed inspection or money left on the table.

This NEC wire size chart puts copper vs aluminum ampacity data side by side at all three temperature ratings, then walks through the real-world decision factors that determine which material wins for your specific installation.

NEC Table 310.16: Copper vs Aluminum Ampacity Side by Side

The following table compiles conductor ampacity from NEC (NFPA 70) Table 310.16 for both copper and aluminum conductors across the three standard temperature ratings. All values assume not more than three current-carrying conductors in a raceway, cable, or directly buried, with an ambient temperature of 30°C (86°F).

Wire Size Cu 60°C Cu 75°C Cu 90°C Al 60°C Al 75°C Al 90°C
14 AWG15A20A25A———
12 AWG20A25A30A15A20A25A
10 AWG30A35A40A25A30A35A
8 AWG40A50A55A30A40A45A
6 AWG55A65A75A40A50A60A
4 AWG70A85A95A55A65A75A
3 AWG85A100A115A65A75A85A
2 AWG95A115A130A75A90A100A
1 AWG110A130A145A85A100A115A
1/0 AWG125A150A170A100A120A135A
2/0 AWG145A175A195A115A135A150A
3/0 AWG165A200A225A130A155A175A
4/0 AWG195A230A260A150A180A205A
250 kcmil215A255A290A170A205A230A
350 kcmil260A310A350A210A250A280A
500 kcmil320A380A430A260A310A350A
NEC Reference These values come from NEC Table 310.16 (2023 edition) and Table 310.16 (2020 edition)—the table number and values are unchanged between these editions. The 2026 NEC retains the same ampacity data in its reorganized structure. Always confirm your AHJ’s adopted edition before finalizing conductor selection.

The Real Difference: Ampacity per Dollar

Aluminum conductors carry roughly 78–84% of the ampacity of same-size copper, depending on the gauge. But aluminum costs approximately 50–60% less per foot than copper at comparable sizes. For large feeders and service entrance conductors, this cost advantage overwhelms the ampacity difference.

Cost Comparison: 200A Service Entrance

A 200A residential service requires a minimum of 2/0 AWG copper (175A at 75°C) or 4/0 AWG aluminum (180A at 75°C) per the NEC 310.12 allowance for service conductors. In practice, many contractors run 4/0 copper for headroom.

  • 4/0 copper THWN-2: approximately $3.50–$5.00/ft × 4 conductors (2 hots + neutral + ground)
  • 4/0 aluminum THWN-2: approximately $1.20–$2.00/ft × 4 conductors

On a 50-foot service lateral, that’s a material difference of roughly $460–$600. On a 150-foot run to a detached garage, the savings can exceed $1,400.

This is why most residential service entrance conductors are aluminum—the NEC allows it, the cost savings are substantial, and modern aluminum alloy conductors (AA-8000 series per NEC 310.106) have eliminated most of the historical connection problems associated with older AA-1350 alloy wire.

When Copper Wins: Branch Circuits and Small Feeders

For 15A and 20A branch circuits, copper dominates. NEC 310.106(B) prohibits solid aluminum conductors smaller than 12 AWG, and stranded aluminum in small gauges is impractical for device terminations. Beyond the code restriction, there are practical reasons copper is the standard for branch circuits:

  • Termination compatibility: most residential devices (receptacles, switches, breakers) under 30A are rated for copper only, or require CO/ALR-rated devices if aluminum is used
  • Conductor size in the box: upsizing from 12 AWG copper to 10 AWG aluminum means larger conductors in the same device box, complicating NEC box fill calculations
  • Cost delta is small: at 14 and 12 AWG, the price difference between copper and aluminum is negligible relative to the labor cost of the installation
Common Mistake Using aluminum conductors on terminals rated for copper only (marked “CU” or “75°C CU”). This creates a high-resistance connection that overheats over time. Always check the terminal rating—it’s stamped on the device or listed in the manufacturer’s documentation. For connections to copper bus bars, use approved anti-oxidant compound and proper torque values.

When Aluminum Wins: Feeders, Services, and Long Runs

Aluminum is the economical choice for conductors 4 AWG and larger, particularly for:

  • Service entrance conductors: 100A, 200A, and 400A services are routinely wired with aluminum. NEC 310.12 provides a special residential service conductor sizing allowance (the “83% rule”) that applies to both materials.
  • Feeder runs: a 100-foot feeder to a subpanel in a detached workshop can save hundreds of dollars with aluminum conductors
  • Parallel conductor runs: for 400A+ services requiring parallel sets, aluminum’s weight advantage (aluminum weighs about one-third of copper) makes pulling physically easier

Upsizing Rule of Thumb

When switching from copper to aluminum, the common approximation is to upsize by two AWG numbers (e.g., 6 AWG copper → 4 AWG aluminum for a 65A circuit). This is a rough guideline—always verify against Table 310.16 for your specific temperature rating and load.

Key Relationship $$\text{Ampacity ratio} = \frac{\text{Al ampacity at size } n}{\text{Cu ampacity at size } n} \approx 0.78 \text{ to } 0.84$$

This means aluminum consistently carries about 78–84% of the current that same-size copper carries, regardless of gauge. The ratio holds across all three temperature columns in Table 310.16.

Voltage Drop: The Hidden Upsizing Factor

Aluminum has higher resistivity than copper (K = 21.2 for aluminum vs. K = 12.9 for copper in the standard voltage drop formula). That 64% higher resistivity means aluminum conductors produce significantly more voltage drop on long runs.

$$V_D = \frac{2 \times K \times I \times L}{CM}$$

where K is the resistivity constant (12.9 for copper, 21.2 for aluminum), I is the load current in amps, L is the one-way distance in feet, and CM is the circular mil area of the conductor.

Worked Example: Voltage Drop on a 150-ft Feeder

60A load, 240V single-phase, 150-foot run to a workshop subpanel. NEC recommendation: 3% maximum voltage drop on the feeder.

3% of 240V = 7.2V maximum allowable drop.

Option A: 6 AWG copper (26,240 CM)

$$V_D = \frac{2 \times 12.9 \times 60 \times 150}{26{,}240} = \frac{232{,}200}{26{,}240} = 8.85\text{V} = 3.69\%$$

Exceeds 3%—need to upsize to 4 AWG copper (41,740 CM):

$$V_D = \frac{2 \times 12.9 \times 60 \times 150}{41{,}740} = \frac{232{,}200}{41{,}740} = 5.56\text{V} = 2.32\%$$

Option B: 4 AWG aluminum (41,740 CM)

$$V_D = \frac{2 \times 21.2 \times 60 \times 150}{41{,}740} = \frac{381{,}600}{41{,}740} = 9.14\text{V} = 3.81\%$$

Exceeds 3%—need to upsize to 2 AWG aluminum (66,360 CM):

$$V_D = \frac{2 \times 21.2 \times 60 \times 150}{66{,}360} = \frac{381{,}600}{66{,}360} = 5.75\text{V} = 2.40\%$$

Result: copper requires 4 AWG; aluminum requires 2 AWG. Even with the upsizing, 2 AWG aluminum typically costs less than 4 AWG copper per foot.

Temperature Correction and Derating: Material Matters

When conductors run through high-temperature environments (attics above 40°C, rooftops, engine rooms) or are bundled with many other conductors in a conduit, the ampacity from Table 310.16 must be derated. The correction factors from NEC Table 310.15(B)(1) apply equally to copper and aluminum—but because aluminum starts with lower ampacity, the derated value drops to a smaller absolute number.

Derating Example: Attic Run at 50°C

Per NEC Table 310.15(B)(1), the correction factor for 90°C-rated conductors at 46–50°C ambient is 0.82.

  • 6 AWG copper THWN-2 (90°C column): 75A × 0.82 = 61.5A derated
  • 6 AWG aluminum THWN-2 (90°C column): 60A × 0.82 = 49.2A derated

For a 50A circuit in a hot attic, copper still works at 6 AWG (61.5A > 50A), but aluminum needs to be upsized to 4 AWG (75A × 0.82 = 61.5A derated).

Copper vs Aluminum Decision Framework

Factor Copper Advantage Aluminum Advantage
Ampacity per size16–22% higher ampacity—
Material cost—50–60% lower per foot
Voltage drop64% lower resistivity (K=12.9 vs 21.2)—
Weight—~1/3 the weight of copper
Termination easeUniversal device compatibilityRequires AL/CU-rated equipment
Conductor size in racewaySmaller diameter per ampere—
Small branch circuits (<30A)Only practical optionNEC restricts solid Al < 12 AWG
Service entrance (≥100A)—Industry standard, major cost savings
Long feeder runsLess upsizing needed for VDStill cheaper even after upsizing

Modern Aluminum: AA-8000 Series Alloys

The aluminum conductor problems of the 1960s and 1970s (overheated connections, house fires) were caused by AA-1350 alloy and incompatible terminations. NEC 310.106(B) now requires that aluminum building wire use AA-8000 series alloys (per ASTM B800), which have superior creep resistance and connection reliability. Combined with properly rated AL/CU connectors and anti-oxidant compound, modern aluminum conductors are a reliable and code-compliant choice for feeders and services.

Tip When using aluminum conductors, always apply anti-oxidant compound (also called “no-ox” or joint compound) to the stripped conductor before termination. Most AL/CU-rated breakers and lugs come with anti-oxidant pre-applied or included. Torque all connections to manufacturer specifications—a calibrated torque wrench is not optional for aluminum terminations.

Quick Reference: Common Circuit Sizes

Circuit Copper Size Aluminum Size Notes
20A branch12 AWGNot practicalCopper standard for branch circuits
30A dryer/AC10 AWG8 AWGCheck terminal rating
50A range6 AWG4 AWGAL/CU rated terminals required
60A subpanel6 AWG4 AWGCheck voltage drop on long runs
100A subpanel3 AWG1 AWGVerify VD if >75 ft
200A service2/0 AWG*4/0 AWG**NEC 310.12 residential allowance

Use a wire size calculator that accounts for ampacity, voltage drop, temperature correction, and conduit fill simultaneously. Checking only one factor and missing another is the most common wire sizing error.

Related tools

Voltage Drop Calculator

Calculate voltage drop for single-phase and three-phase circuits per NEC 2026 standards.

Electrical Load Calculator

Calculate residential service size per NEC 2026 optional method with demand factor breakdown.

Wire Size Calculator

Calculate wire size per NEC Table 310.16 with temperature and conduit fill derating.

← More electrical calculation tools guides