Copper vs Aluminum Conductors: When to Use Each
The Core Difference: Conductivity
Copper conducts electricity approximately 61% better than aluminum. This means aluminum conductors need to be larger (higher gauge) to carry the same current as copper. A 4 AWG copper conductor has roughly the same ampacity as a 2 AWG aluminum conductor.
This difference flows through every other consideration: cost, weight, termination requirements, and installation difficulty.
Where Aluminum Makes Sense
Aluminum dominates in three applications.
Service entrance cables. The conductors between the utility meter and the main panel are typically aluminum. The runs are short (reducing voltage drop concerns), the wire sizes are large (where aluminum’s cost advantage is greatest), and the connections are made once by a qualified electrician.
Large feeder runs. Subpanel feeds, building feeds, and other large-ampacity runs benefit from aluminum’s lower cost per amp-foot. When you’re pulling 200A feeders across 150 feet, the material cost difference between copper and aluminum is significant.
Utility distribution. Overhead power lines are almost exclusively aluminum (often aluminum-clad steel for strength). The weight savings at scale are enormous — copper overhead lines would require substantially heavier support structures.
Where Copper Dominates
Branch circuits (30A and under). Residential and light commercial branch circuits are copper. The wire sizes are small enough that aluminum’s cost advantage is minimal, and the termination concerns with aluminum (discussed below) make it impractical for the volume of connections in branch circuit work.
Environments with corrosion. Aluminum oxidizes faster than copper and the oxide layer is resistive, which creates heat at connections. In humid, corrosive, or outdoor environments, copper’s superior corrosion resistance reduces maintenance.
Tight spaces. Because aluminum conductors are physically larger at the same ampacity, they require larger conduit, larger boxes, and larger terminals. In retrofit work where space is constrained, copper’s smaller footprint matters.
The Connection Problem
Aluminum’s reputation problem traces to the 1960s-70s, when aluminum branch circuit wiring was installed in residential construction using connections and devices designed for copper. The result was widespread connection failures — aluminum expands and contracts more than copper with temperature cycling, loosening connections over time. Loose connections create resistance, resistance creates heat, and heat creates fire risk.
Modern aluminum installations avoid this by using AL-CU rated devices, anti-oxidant compound on all connections, and proper torque specifications. For large feeders and services (where aluminum is standard), these precautions are routine and well-understood.
For branch circuits, the risk-reward calculation doesn’t favor aluminum. The cost savings are small, and the connection requirements add labor that erases most of the material savings.
Voltage Drop Comparison
Aluminum’s higher resistance means greater voltage drop at the same gauge. The K value for copper is 12.9; for aluminum it’s 21.2 at 75°C. On a long run, this difference can push you to a larger aluminum conductor than expected.
A 100-foot run at 20A: copper 10 AWG gives 4.14% voltage drop at 120V. The same run with aluminum 10 AWG gives 6.8% — well above the 3% recommendation. You’d need 8 AWG aluminum to get comparable performance.
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Cost Comparison
Aluminum costs roughly 30-50% less than copper per pound, but the price advantage per ampere-foot is smaller because you need a larger aluminum conductor. For feeders above 100A, aluminum typically saves 25-40% on conductor cost. For branch circuits under 30A, the savings are 10-15% — not enough to justify the additional connection requirements.
Material prices fluctuate with commodity markets. Copper prices are more volatile than aluminum, so the relative advantage shifts over time.
The Decision Framework
Use copper for branch circuits, short runs, retrofit work, and any application where terminations will be made by multiple electricians over the life of the building. Use aluminum for service entrance cables, large feeders, and long runs where the material cost savings are meaningful and connections are made by qualified electricians using proper techniques.
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