Copper vs Aluminum Wire: Voltage Drop, Cost & When to Use Each

Copper vs Aluminum Conductivity

The electrical properties of copper and aluminum make each material suitable for different applications. Understanding these fundamental differences helps explain why copper is preferred for some installations while aluminum dominates others.

Electrical Conductivity

Copper is one of the best electrical conductors among common metals, second only to silver. At 20°C, copper has a conductivity of approximately 59.6 MS/m (megasiemens per meter). Aluminum's conductivity is about 37.7 MS/m—roughly 63% that of copper. This means aluminum requires a larger cross-sectional area to carry the same current with equivalent resistance.

The conductivity difference translates directly to voltage drop. For the same wire size (AWG), length, and current, aluminum will have approximately 1.6 times the voltage drop of copper. This is why aluminum conductors must be larger than copper to achieve comparable performance.

Weight Comparison

While copper conducts better, aluminum is significantly lighter. Copper weighs about 8.96 g/cm³, while aluminum weighs only 2.70 g/cm³—about 30% of copper's weight. Even when using larger aluminum conductors to match copper's ampacity, the aluminum installation is typically lighter.

For example, 4/0 AWG copper weighs approximately 640 pounds per 1000 feet, while 250 kcmil aluminum (roughly equivalent in ampacity) weighs about 230 pounds per 1000 feet. This weight difference matters for overhead lines, long conduit runs, and applications where structural support is a concern.

Thermal Properties

Both materials expand when heated, but at different rates. Aluminum has a higher coefficient of thermal expansion than copper, meaning it expands and contracts more with temperature changes. This characteristic requires special consideration at terminations and connections to prevent loosening over time.

Aluminum also has different thermal conductivity than copper, affecting how quickly heat dissipates from the conductor. These thermal properties influence how each material performs under varying load conditions and ambient temperatures.

Resistance Comparison Table

The following table compares resistance values for copper and aluminum conductors of the same AWG size. Values are from NEC Chapter 9, Table 8 at 75°C.

As the table shows, aluminum consistently has about 1.64-1.65 times the resistance of copper at every wire size. This ratio remains constant across the AWG range, making it straightforward to compare voltage drop between the two materials for any application.

When to Use Copper

Copper is the preferred choice in several key situations where its superior conductivity and connection reliability outweigh its higher cost.

Branch Circuits

For typical residential and commercial branch circuits (15A and 20A), copper is the standard choice. The wire sizes involved (14 AWG and 12 AWG) are small enough that cost difference is minimal, and copper's easier termination makes it more practical for the many connections in branch circuit wiring. Most device terminals in receptacles and switches are designed for copper.

Space-Constrained Installations

When conduit space is limited, copper's smaller size for equivalent ampacity becomes valuable. Using copper may mean the difference between fitting conductors in existing conduit versus replacing with larger conduit—a significant cost and labor consideration in retrofit work.

Wet or Corrosive Environments

Copper resists corrosion better than aluminum in most environments. In wet locations, coastal areas with salt exposure, or industrial settings with corrosive atmospheres, copper's durability makes it the safer choice. While aluminum can be used with proper terminations and anti-oxidant compound, copper requires less maintenance attention in challenging environments.

Frequent Termination Changes

In applications where connections are frequently made and remade—such as temporary installations, equipment that's regularly moved, or training facilities—copper's easier termination and greater tolerance for handling makes it preferable. Aluminum connections require more care and proper techniques that may not always be followed during quick changes.

Motor Leads and Flexible Connections

For flexible cord, motor leads, and applications requiring frequent flexing, copper is essentially mandatory. Aluminum wire is more prone to fatigue failure when flexed repeatedly. All portable cord and flexible cable standards specify copper conductors.

When to Use Aluminum

Aluminum is the economical choice for many larger installations where its cost advantages outweigh the need for larger conductors and careful termination practices.

Service Entrance Conductors

The conductors between the utility meter and main panel carry the entire building load and are often the largest conductors in an installation. For 100A, 200A, and larger services, aluminum provides substantial cost savings. Most utility meters and main breaker panels have terminals rated for aluminum, making this application straightforward.

Feeders to Subpanels

Feeders to detached buildings, subpanels, and other major distribution points carry significant current over distances where conductor cost becomes meaningful. A 100-foot run of 2/0 AWG aluminum for a garage subpanel costs significantly less than the equivalent 1/0 AWG copper, with minimal difference in voltage drop when properly sized.

Industrial and Commercial Feeders

Large commercial and industrial buildings routinely use aluminum for major feeders. The cost savings on 500 kcmil aluminum versus 350 kcmil copper (roughly equivalent ampacity) multiplied across thousands of feet of feeder runs represents substantial budget savings. Professional electricians in these settings are trained in proper aluminum termination techniques.

Overhead Service Drops

Utility service drops from the pole to the building almost universally use aluminum because of its lighter weight. The reduced weight means longer spans between supports and less strain on attachment points. This application is so dominated by aluminum that copper service drops are rare.

Large Motor Circuits

For larger motors (typically 50 HP and above) with long runs, aluminum feeders provide good value. The larger conduit required for aluminum may be offset by conductor cost savings, especially when the conduit size increases only marginally. Many industrial motor control centers have terminals designed for aluminum conductors.

Cost Comparison

Understanding the true cost comparison between copper and aluminum requires considering not just wire price, but also the larger conduit sometimes needed for aluminum and installation labor differences.

Raw Conductor Cost

The following table shows approximate relative costs for copper and aluminum conductors of equivalent ampacity (prices vary significantly over time based on commodity markets):

Note: These are representative ratios. Actual prices fluctuate with copper and aluminum commodity markets. The savings with aluminum generally increase with larger wire sizes.

Total Installed Cost

When comparing total installed cost, consider:

• Conduit cost: Aluminum may require larger conduit due to bigger wire size
• Labor time: Aluminum terminations take more care and time
• Anti-oxidant compound: Required for aluminum connections
• Terminal compatibility: Some equipment requires copper or CU/AL rated terminals

For short runs with many terminations, copper may actually be less expensive overall. For long runs with few terminations, aluminum typically provides significant savings even accounting for larger conduit.

Installation Considerations

Proper installation techniques differ between copper and aluminum, with aluminum requiring more attention to detail at connections.

Aluminum Connection Requirements

Aluminum connections require specific practices to ensure long-term reliability:

• Use only AL or CU/AL rated terminals: Terminals not rated for aluminum will fail over time
• Apply anti-oxidant compound: Also called oxide inhibitor or joint compound, this prevents the oxide layer that forms on aluminum from increasing connection resistance
• Follow torque specifications: Use a calibrated torque wrench to achieve proper connection pressure
• Consider compression fittings: For splices and terminations, compression connectors rated for aluminum provide reliable connections

Why Aluminum Connections Can Fail

Aluminum presents several challenges at terminations. First, aluminum oxidizes quickly when exposed to air, forming a thin aluminum oxide layer that has high resistance. Second, aluminum expands and contracts more than copper with temperature changes, which can loosen connections over time. Third, aluminum is softer than copper and can creep (slowly deform) under sustained pressure, also loosening connections.

These issues led to problems with aluminum wiring in homes during the 1960s and 1970s, when improper terminations caused connection failures and fires. Modern aluminum conductors use different alloys (AA-8000 series) with better properties, and proper termination practices make aluminum safe and reliable when installed correctly.

Copper Connection Advantages

Copper connections are more forgiving of less-than-perfect installation practices. Copper forms a thin oxide layer that's still conductive, unlike aluminum oxide. Copper has lower thermal expansion and doesn't creep, so connections remain tight over time. Standard terminals without special ratings work fine with copper. While proper torque is still important, copper connections have more margin for error.

Mixing Copper and Aluminum

When copper and aluminum conductors must be connected together, special precautions are needed. Direct copper-to-aluminum connections can cause galvanic corrosion, especially in the presence of moisture. Use only connectors specifically rated for copper-to-aluminum connections, or use a transition splice that keeps the two metals from direct contact. Never splice copper and aluminum using standard wire nuts or twist-on connectors not rated for this purpose.

Equivalent Wire Sizes

The following table shows copper and aluminum wire sizes that provide approximately equivalent ampacity and performance. Use these equivalents when substituting one material for the other.

The two-size difference (e.g., 6 AWG copper ≈ 4 AWG aluminum) is a general rule that applies across most of the AWG range. For voltage drop calculations, remember that the equivalent aluminum will have slightly different resistance, so always use the actual resistance values from NEC Table 8 rather than assuming equivalence. Use our voltage drop calculator to compare specific scenarios with either material.

Frequently Asked Questions

Sources

• NFPA 70: National Electrical Code - Chapter 9, Table 8
• Copper Development Association - Electrical Wire and Cable
• The Aluminum Association - Aluminum Building Wire

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