Copper and aluminum are the only two conductors you will realistically meet in building wiring, and the choice between them has swung back and forth over the last century — driven by copper prices, wartime shortages and some hard lessons about how aluminum behaves in small connections. Here is a practical comparison: what actually differs, what the risks are, and where each metal earns its place.
Conductivity, size and weight
Copper is the better conductor per unit of cross-section: aluminum carries roughly 60% of the current copper does for the same size. In practice that means an aluminum conductor must be about one to two sizes larger than the copper conductor it replaces.
Aluminum wins decisively on weight. Even upsized to match copper's ampacity, an aluminum conductor weighs roughly half as much — which is why the electricity grid loves it. Nearly all overhead transmission and distribution lines are aluminum (often steel-reinforced), because towers and poles carry the weight.
If you're sizing conductors for a project, our guide on how to calculate wire cross-section covers the fundamentals; just remember the aluminum upsizing rule when comparing tables.
Cost
Aluminum is consistently cheaper — often dramatically so when copper prices spike. For large cross-sections (service cables, feeders, industrial runs) the savings are substantial, which is why aluminum dominates there. For small branch circuits the material saving is minor, and copper's handling advantages usually win.
Mechanical behaviour — where the trouble lives
The real difference between the metals isn't conductivity; it's what happens at connection points over years:
- Oxide layer. Aluminum instantly forms a hard, poorly-conducting oxide skin. Copper oxide, by contrast, still conducts reasonably. Poorly made aluminum joints therefore develop resistance — and resistance means heat.
- Creep (cold flow). Under the pressure of a terminal screw, aluminum slowly deforms and relaxes. A joint that was tight in 1975 can be loose today. Loose joints heat, oxidise further, and can ultimately arc.
- Thermal expansion. Aluminum expands and contracts more than copper with each heating cycle, working connections loose over thousands of cycles.
- Galvanic corrosion. Where aluminum touches copper in the presence of moisture, the aluminum corrodes preferentially.
None of these is a problem when connections are designed for aluminum — large lugs, anti-oxidant compound, correct torque, connectors rated for the combination. All of them are problems when aluminum lands in devices designed for copper.
The legacy problem: small-gauge aluminum branch wiring
In the 1960s and early 1970s, high copper prices pushed builders in several countries to run small aluminum conductors to outlets and switches. Combined with devices designed for copper, the failure modes above produced a well-documented record of overheated terminations. That era is why "aluminum wiring" alarms home inspectors and insurers to this day.
If your home dates from that period and hasn't been inspected, put it on your list — warning signs like warm outlet plates, flickering and sparking sockets deserve immediate professional attention. Approved remediation methods exist that don't require rewiring the whole house; a licensed electrician can advise what your local rules accept.
Where each conductor is the right choice today
| Application | Typical choice | Why |
|---|---|---|
| Overhead power lines | Aluminum (steel-reinforced) | Weight and cost dominate at grid scale |
| Service entrances & feeders | Often aluminum | Large sizes where savings are big; lugs designed for Al |
| Branch circuits (outlets, lights) | Copper | Small joints, many terminations, standard devices |
| Flexible cords & appliance leads | Copper | Flexibility and fatigue resistance |
| Industrial busbars | Either | Engineered case by case |
The bottom line
Neither metal is "bad". Aluminum built the modern grid and remains the rational choice for big conductors; copper remains the default for the small, termination-dense wiring inside buildings. The danger zone is history's leftovers — small aluminum conductors in copper-era devices — and mixed-metal joints made without the right connectors. Both are exactly the kind of thing a periodic professional inspection catches, and both are covered in more depth by our guide to electrical cable classification.
Safety note: assessing and remediating aluminum wiring involves opening terminations — licensed-electrician territory in every jurisdiction. Standards for connectors and remediation methods vary by country; never rely on a general article for a specific installation decision.
