Copper vs. Aluminum Wire: Resistance, Ampacity, Cost, and Where Each Is Used
Most residential wiring in North America uses copper. Most large commercial and utility feeders use aluminum. This isn't an accident, each material has real advantages at different scales, and understanding the tradeoffs lets you make sensible choices rather than defaulting to one without thinking about it.
The resistance difference
Copper conducts electricity better than aluminum. The resistivity of annealed copper is about 1.72 microohm-centimeters at 20°C. Aluminum is roughly 2.65 microohm-centimeters, about 60% more resistive than copper for the same cross-sectional area.
In practical terms: an aluminum conductor needs to be about two AWG sizes larger than a copper conductor to carry the same current with the same resistance. 2 AWG aluminum is roughly equivalent to 4 AWG copper in ampacity. This is the starting point for all aluminum conductor sizing.
Ampacity comparison
The NEC tables reflect this directly. For conductors at 75°C in conduit:
| Application | Copper | Aluminum equivalent |
|---|---|---|
| 15 A | 14 AWG | 12 AWG |
| 20 A | 12 AWG | 10 AWG |
| 30 A | 10 AWG | 8 AWG |
| 50 A | 8 AWG | 6 AWG |
| 100 A | 3 AWG | 1 AWG |
| 200 A | 3/0 AWG | 4/0 AWG |
These are estimates based on NEC Table 310.12. Actual sizing depends on installation conditions, temperature rating, and local code.
Cost
Aluminum wire costs less per pound than copper, and because aluminum has about one-third the density of copper, a given length of aluminum conductor weighs significantly less than the equivalent copper conductor. The cost savings on large feeders can be substantial, on a 200 A service entrance run of 100 feet, aluminum may cost 40–50% less than copper.
This is why utilities and commercial electrical contractors routinely use aluminum for feeders, service entrances, and distribution cables. The economics make sense at that scale.
At smaller sizes (12 AWG and below), the savings shrink and the handling differences matter more, which is why small-gauge residential wiring is almost always copper.
Where aluminum works well
Aluminum conductors are the standard choice for:
- Service entrance conductors from the utility to the meter and main panel
- Feeder conductors from a main panel to subpanels
- Large branch circuits in commercial and industrial settings (100 A and above)
- Overhead utility spans where the low weight matters for sag calculations
For these applications, aluminum Type XHHW-2 or USE-2 conductors in conduit are reliable and code-compliant when installed correctly.
Where copper is the better choice
Small gauge wiring, the 14 and 12 AWG circuits that make up most of a home's branch circuits, is almost always copper. Several reasons:
First, the cost difference at small gauges doesn't justify the hassle. Second, small aluminum conductors are more difficult to terminate. The wire is softer and more prone to nicking during stripping, and nicks create local hot spots. Third, most devices (outlets, switches, light fixtures) are listed for copper or copper-clad aluminum only, using aluminum in a device listed for copper is a code violation.
If you're using aluminum on a branch circuit, the device must be marked CO/ALR (copper/aluminum rated). These aren't always easy to find and cost more than standard devices.
The termination problem and antioxidant compound
Aluminum oxidizes on contact with air, forming aluminum oxide, which is a poor conductor. At connections, this oxide layer increases resistance, which generates heat, which can cause the aluminum to creep and loosen the connection, the cycle that caused problems in aluminum branch circuit wiring of the 1960s and 1970s.
The fix for terminations is antioxidant compound (NoAlox is a common brand). Apply it to the stripped conductor before making the connection. This isn't optional for aluminum; it's the reason aluminum terminations in panels hold up well when done correctly. Most panel manufacturers specify it.
On large conductors (1 AWG and above), aluminum terminations are generally reliable when properly torqued and treated with antioxidant. The 1960s–1970s problems were primarily with small-gauge wiring terminated in devices not designed for aluminum.
Derating aluminum
The two-sizes-up rule is the quick guide, but the actual NEC tables give precise ampacity values. When you derate for conduit fill, ambient temperature, or bundling, the process is the same for aluminum as for copper, multiply the base ampacity by the appropriate correction factor.
A cable size calculator that handles both copper and aluminum will apply these factors based on the installation parameters you enter, which is faster than working through the NEC tables manually.
The short version
For branch circuit wiring inside a home, use copper. For service entrances and large feeders, aluminum is the standard and works well. The two-sizes-up rule gives you a starting point for equivalent ampacity. Always use antioxidant compound at aluminum terminations and verify that any devices are rated for aluminum.
Conductor sizing and material selection should be verified against the current NEC and local adopted code. Consult a licensed electrician before making permanent installations.