Electrical Properties Compared
Copper conductivity: 100% IACS (International Annealed Copper Standard). Aluminum conductivity: 61% IACS. This means aluminum requires approximately 1.6× the cross-sectional area to carry the same current as copper.
In practical NEC terms: 4 AWG copper (85A at 75°C) performs comparably to 2 AWG aluminum (90A at 75°C). The aluminum conductor is physically larger but weighs less — aluminum is only 30% the weight of copper per unit volume.
Resistance comparison at 75°C: 10 AWG copper = 1.29 Ω/1000 ft; 10 AWG aluminum = 2.13 Ω/1000 ft. This ~65% higher resistance directly impacts voltage drop calculations.
Cost Analysis
Copper pricing has risen dramatically — as of 2024-2025, copper feeders cost 2-3× more than equivalent aluminum feeders per ampere of capacity. For a 200A residential service entrance, the difference can be $500-$1500 in conductor cost alone.
Where aluminum saves the most: large feeders (100A+), service entrance conductors, and parallel runs. A 400A service using 500 kcmil copper costs roughly $8-12 per foot per phase, while the equivalent aluminum (750 kcmil) costs $3-5 per foot per phase.
Where copper is cost-competitive: small branch circuits (14-10 AWG) where the absolute price difference is small and aluminum is not permitted by many AHJs for these sizes. NEC 240.4(D) limits small conductors (14, 12, 10 AWG) which are predominantly copper.
NEC Code Requirements
NEC Table 310.16 lists separate ampacity columns for copper and aluminum. You must use the correct column — mixing them up is a common error that produces undersized conductors.
Terminal Temperature: Most residential equipment (panels, breakers, receptacles) is rated 75°C. Both copper and aluminum conductors must be sized using the 75°C column regardless of insulation rating.
Connection Requirements (NEC 110.14): Aluminum conductors require terminals or connectors rated for aluminum (marked AL or AL-CU). Using copper-only terminals with aluminum wire causes galvanic corrosion, increased contact resistance, and eventual fire.
Anti-Oxidant Compound: While not strictly required by NEC, aluminum connections should use anti-oxidant compound (e.g., Noalox, Penetrox) to prevent oxide buildup. Most equipment manufacturers require it in their installation instructions, making it effectively mandatory per NEC 110.3(B).
When to Use Aluminum
Service Entrance Conductors: Aluminum is the de facto standard for residential and commercial service entrances. 200A service: 4/0 AWG aluminum (the industry standard choice).
Large Feeders (100A+): Feeders to subpanels, HVAC equipment, and motor loads. The cost savings at larger sizes are substantial.
Parallel Runs: Aluminum is excellent for parallel conductor installations (1/0 AWG minimum per NEC 310.10(G)) where the savings multiply.
Utility Connections: Utilities exclusively use aluminum for overhead and underground service drops. The lighter weight reduces pole loading and installation costs.
When to Use Copper
Branch Circuits (14-10 AWG): Copper is the standard for 15A, 20A, and 30A branch circuits. Most receptacles and switches are not rated for aluminum at these sizes.
Motor Circuits: Motor terminals are typically copper-only. Using aluminum requires additional transition splices, adding cost and potential failure points.
Wet or Corrosive Environments: Copper resists corrosion better than aluminum in damp environments, saltwater proximity, and chemical exposure.
Space-Constrained Installations: Copper's smaller size for equivalent ampacity matters in tight junction boxes, panel boards, and conduit runs where every 1/4 inch counts.
Fire Alarm and Low-Voltage Systems: Copper is universally required for fire alarm circuits (NFPA 72) and virtually all low-voltage systems.
Common Mistakes to Avoid
Using copper lugs on aluminum wire — This causes galvanic corrosion and is the #1 cause of aluminum wiring fires. Always use AL-CU or AL-rated connectors.
Incorrect torque — Aluminum expands more than copper when heated. Under-torqued connections loosen over thermal cycles. Always use a calibrated torque wrench to the manufacturer's specified value.
Applying copper ampacity tables to aluminum — Reading the wrong column in NEC Table 310.16 can result in undersizing by 2+ AWG sizes.
Not applying anti-oxidant paste — Aluminum oxide (which forms instantly on exposed aluminum) is an insulator. Without anti-oxidant compound, connection resistance increases over time, causing overheating.