Copper vs Aluminum Wiring: Ampacity, Upsizing, and Safety
Copper and aluminum conductors compared — NEC ampacity at equal size, the upsize rule, resistance, terminations, and when aluminum makes sense.
Updated August 20, 2026
Copper and aluminum move electrons equally well per ampere — the difference is how much metal each needs to do it. Aluminum conducts about 61% as well as copper by volume, so NEC Table 310.16 gives an aluminum conductor of any given AWG size roughly one column less ampacity than the same-size copper, and the practical rule of thumb becomes "go up about two sizes when switching to aluminum."
What keeps both materials in service is that they win on different axes. Aluminum, even after upsizing, weighs about half as much as the equivalent copper and costs dramatically less per ampere-foot — which is why utilities and service-entrance cable went aluminum decades ago and never came back. Copper terminates forgivingly on everything, needs no oxide ritual, and is the only material with a clean history in 15- and 20-amp branch-circuit devices.
The safety story is specific, not general: the aluminum branch-circuit fires of the late 1960s involved small solid AA-1350-series conductors at device terminations, a combination the modern rules no longer permit — today’s NEC requires AA-8000 series alloy, and aluminum lives almost entirely in large stranded feeder and service sizes with listed AL-rated lugs. Every ampacity and resistance figure below is mapped from the same NEC-table constants the ampacity and voltage-drop calculators use.
Side by Side
| Spec | Copper | Aluminum (AA-8000) |
|---|---|---|
| Ampacity at 75 °C — 12 AWG (A) | 25 A | 20 A |
| Ampacity at 75 °C — 6 AWG (A) | 65 A | 50 A |
| Ampacity at 75 °C — 4/0 AWG (A) | 230 A | 180 A |
| Size for a 100 A feeder (75 °C terminations)Smallest NEC Table 310.16 size whose 75 °C ampacity meets the load — the "upsize about two AWG" rule in table form. Computed at 100 A; other ampere targets can shift the gap by a size. | 3 AWG (100 A) | 1 AWG (100 A) |
| AC resistance — 6 AWG (Ω/km at 75 °C)NEC Chapter 9-derived values from the voltage-drop calculator: at equal size, aluminum drops roughly 60% more voltage. | 1.61 | 2.66 |
| Relative conductor weightAluminum is ≈30% of copper’s density at ≈61% of its conductivity, so equal-conductance aluminum still weighs about half. | Baseline — dense metal (≈8.9 g/cm³) | Roughly half the weight of the copper equivalent, even after upsizing |
| Termination requirements | Standard terminals; no special devices or compounds | AA-8000 series alloy required (NEC); lugs listed and marked for aluminum (AL7CU/AL9CU); oxide inhibitor and torque per the listing; CO/ALR devices only for 15/20 A replacements |
| Oxidation & creep behavior | Surface oxide is conductive; negligible cold flow at terminations | Oxide is insulating and re-forms instantly; old AA-1350 solid wire crept under screws — AA-8000 alloys were developed specifically to tame creep |
| Typical applications | Branch circuits, device wiring, equipment grounding, anywhere terminations are numerous | Service-entrance cable, large feeders, utility drops — few terminations, big sizes, long runs |
Which One for the Job
200 A service-entrance feeder
Aluminum (AA-8000)
Two terminations, a large stranded conductor, and meter-to-panel gear whose lugs are all AL-rated: this is exactly the duty aluminum was alloyed for, at a fraction of copper’s cost and half the pulling weight. Virtually every residential service in North America is wired this way.
15 and 20 A branch circuits
Copper
Small-gauge branch wiring means dozens of device terminations, the historical failure point for aluminum — CPSC Publication 516 documents pre-1972 aluminum branch circuits reaching hazardous connection conditions far more often than copper. NM cable in these sizes is effectively copper-only in the market anyway.
Long feeder to a detached garage where voltage drop governs
Aluminum (AA-8000)
When the conductor is being upsized for voltage drop rather than ampacity, both materials end up oversized — and the price gap widens with every size. Aluminum buys the same ohms per foot for far less money on a long run; the only cost is larger conduit and AL-rated terminations at each end.
Panel-swap tails and gutter taps in tight enclosures
Copper
Copper’s smaller size for equal ampacity and its tighter allowed bending make it the easier conductor inside crowded cans, and any terminal accepts it. Where every cubic inch of wiring space counts, the two-size aluminum penalty costs real room.
Frequently Asked Questions
Is the aluminum wiring sold today the same as the 1960s hazard?
No. The fire-prone installations used small solid conductors of AA-1350 utility-grade alloy on devices never designed for aluminum. Modern aluminum building wire must be AA-8000 series alloy under the NEC, is sold almost exclusively in large stranded feeder sizes, and lands on lugs listed and marked for aluminum — a materially different system from pre-1972 branch wiring.
How many sizes up must aluminum go to match a copper conductor?
About two AWG steps at typical feeder sizes — the mapped table row above shows a 100 A feeder needing 3 AWG in copper but 1 AWG in aluminum at 75 °C. It is a rule of thumb, not a law: always size from NEC Table 310.16 for the actual ampere target, since the gap can be one size or three depending on where the columns fall.
What should be done about existing aluminum branch circuits in a house?
CPSC Publication 516 recognizes a full copper repipe, COPALUM crimp splices, or AlumiConn connectors as acceptable repairs, and regards CO/ALR devices as at best a temporary measure. Anything involving those terminations is diagnostic and remedial work for an electrician familiar with the program — not a DIY device swap.
Why do utilities run aluminum if copper conducts better?
Because conductors are bought by the ohm, not by the AWG. Matching copper’s conductance takes a fatter aluminum wire, but that wire still weighs about half as much and costs far less — decisive economics for kilometres of overhead line and for service drops, where the weight also unloads poles and attachment hardware.
Try the Calculators
Sources
- NEC 2023 Table 310.16 and 310.14 — allowable ampacities (via the conductor-ampacity calculator’s verified table) and ampacity rules
- NEC 110.14 and the AA-8000 alloy requirement for aluminum building wire; UL listing markings for AL-rated terminations (AL7CU/AL9CU, CO/ALR)
- CPSC Publication 516 — Repairing Aluminum Wiring (pre-1972 branch-circuit hazard data; COPALUM/AlumiConn repairs; CO/ALR as temporary)
- NEC Chapter 9 Tables 8/9-derived conductor resistances (via the voltage-drop calculator’s verified table); copper ≈61%-conductivity/density relationship per standard CDA and Aluminum Association data