Aluminum carries about 61% more resistance per size than copper — here is what that means for sizing, terminations and cost.
| Size | Cu Ω/kft | Al Ω/kft | Al/Cu ratio | Al size to match Cu |
|---|---|---|---|---|
| 12 AWG | 1.93 | 3.18 | 1.65× | 8 AWG |
| 10 AWG | 1.21 | 2 | 1.65× | 6 AWG |
| 8 AWG | 0.764 | 1.26 | 1.65× | 4 AWG |
| 6 AWG | 0.491 | 0.808 | 1.65× | 3 AWG |
| 4 AWG | 0.308 | 0.508 | 1.65× | 1 AWG |
| 3 AWG | 0.245 | 0.403 | 1.64× | 1/0 AWG |
| 2 AWG | 0.194 | 0.319 | 1.64× | 2/0 AWG |
| 1 AWG | 0.154 | 0.253 | 1.64× | 3/0 AWG |
| 1/0 AWG | 0.122 | 0.201 | 1.65× | 4/0 AWG |
| 2/0 AWG | 0.0967 | 0.159 | 1.64× | 250 kcmil |
| 3/0 AWG | 0.0766 | 0.126 | 1.64× | 300 kcmil |
| 4/0 AWG | 0.0608 | 0.1 | 1.64× | 350 kcmil |
| 250 kcmil | 0.0515 | 0.0847 | 1.64× | 500 kcmil |
| 300 kcmil | 0.0429 | 0.0707 | 1.65× | 500 kcmil |
| 350 kcmil | 0.0367 | 0.0605 | 1.65× | — |
| 400 kcmil | 0.0321 | 0.0529 | 1.65× | — |
| 500 kcmil | 0.0258 | 0.0424 | 1.64× | — |
Aluminum costs a fraction of copper per pound and weighs 30% of copper per unit volume, so for large feeders — services, subpanel feeds, anything 1/0 and up — aluminum two sizes larger is usually cheaper, lighter and entirely code-compliant. Modern AA-8000 series alloy with anti-oxidant compound and properly torqued AL-rated lugs has none of the reputation problems of 1960s branch-circuit aluminum.
Branch circuits (15–30 A) are effectively copper territory — aluminum smaller than 12 AWG is not listed, device terminations are CU-only, and the size penalty erases the savings. Copper also tolerates more termination abuse and fits smaller conduit for the same ampacity.
Run both materials through the wire size solver and the energy loss calculator to compare lifetime cost, not just purchase price.