Copper vs Aluminum Wire: Sizing Differences Explained

Why the same gauge doesn't carry the same current

Aluminum has roughly 1.6 times the electrical resistance of copper for the same cross-sectional area. That's why every ampacity table — including the NEC wire size chart — lists a lower current rating for aluminum than copper at the same AWG size, and it's also why an aluminum conductor needs to step up roughly one gauge size larger than copper to carry the same load at a comparable voltage drop. This site's calculators apply that difference automatically via the material toggle — the underlying reason is the resistivity constant used in the voltage-drop formula (12.9 for copper, 21.2 for aluminum in the simplified calculation most published calculators use).

Where each is actually used

Copper dominates residential branch-circuit wiring — it's more forgiving to work with, has a longer track record in smaller gauges, and the ampacity/cost trade-off usually favors it at typical branch- circuit currents. Aluminum shows up most often in larger conductors: service entrance cables, feeders to subpanels, and utility-side wiring, where its significant weight and cost advantage at large sizes outweighs the need for a slightly bigger gauge. It's rare (and, practically, discouraged) to see aluminum used for small branch-circuit gauges like 12 or 14 AWG.

The safety history worth knowing about

Aluminum branch-circuit wiring installed in the 1960s-70s (a period when copper prices spiked) developed a real reputation for connection failures — the metal expands and contracts differently than copper under heating/cooling cycles, which loosened connections at outlets and switches over time and became a documented fire-risk factor. Modern aluminum conductors (AA-8000 series alloy) and proper antioxidant-compound, aluminum-rated connectors address the underlying problem, and aluminum wiring installed correctly today is considered safe. That history is exactly why aluminum branch-circuit connections specifically (not aluminum feeders/service conductors, which have a much better track record) are one of the clearer cases where hiring a licensed electrician familiar with proper aluminum-compatible hardware is worth it over a DIY approach — see when to DIY vs call a licensed electrician.