Guide · NEC 2023 Table 310.16 · 75°C terminals
Copper vs aluminum — what size Al replaces your Cu
Aluminum needs one size up to ~50A and two sizes up past 100A for equal ampacity — and the same step equalizes voltage drop. Base conditions below (≤3 conductors, 30°C); derating and long runs can push both metals up.
| Load | Copper min | Aluminum min | Note |
|---|---|---|---|
| 15A lighting | #14 Cu (20A) | #12 Al (20A) | Al needs the same 15A cap — no savings at this size |
| 20A receptacle | #12 Cu (25A) | #10 Al (30A) | One size up; Al #12 is capped at 15A and cannot serve 20A |
| 30A dryer | #10 Cu (35A) | #8 Al (40A) | One size up; Al #10 is capped at 25A |
| 40A range | #8 Cu (50A) | #8 Al (40A) | Same size works — Al lands exactly on 40A |
| 50A EV / RV | #6 Cu (65A) | #6 Al (50A) | Same size works — then check drop for the run length |
| 60A subpanel | #6 Cu (65A) | #4 Al (65A) | One size up past 50A — the pattern for feeders |
| 100A feeder | #3 Cu (100A) | #1 Al (100A) | Two sizes up; installers often pull #1/0 Al |
| 150A feeder | #1/0 Cu (150A) | #3/0 Al (155A) | Two sizes up past 100A |
| 200A service | #3/0 Cu (200A) | #250 Al (205A) | Two sizes up; compare conduit + lug cost, not just wire |
Why one size up covers voltage drop too
#10 Al ≈ #12 Cu per foot
Al K 21.2 ÷ 10380 CM = 0.00204 V/A·ft vs Cu 12.9 ÷ 6530 = 0.00198 — within 3%. The 64% higher resistivity is cancelled by ~60% more circular mils.
#8 Al ≈ #10 Cu per foot
21.2 ÷ 16510 = 0.00128 vs 12.9 ÷ 10380 = 0.00124 — again ~3%. The pattern holds up the sizes: #6 Al ≈ #8 Cu, #4 Al ≈ #6 Cu.
So: size for ampacity, verify drop
Pick aluminum from the table above, then run the exact distance through the voltage drop calculator. Long 12V/120V runs may still force both metals up a size.
Price the whole pull, not the wire
Aluminum wire at 100A+ is a third to half the copper price — but add one conduit trade size and AL-rated lugs. Copper wins under ~50A where material savings vanish into fittings.
Rules for safe aluminum installs
AA-8000 alloy + AL-rated terminations
Modern AA-8000 Al on CU-AL listed lugs, torqued to spec, with antioxidant. Never land aluminum on copper-only devices — that was the 1970s failure.
No aluminum for 15–20A branches
#10 Al can serve 20A on paper, but device compatibility, box fill, and near-zero savings make #12 Cu the sane default. Start Al at 4 AWG feeders.
Upsize the conduit too
Equal-ampacity Al is physically fatter (Table 5). Re-run conduit fill — 40% limit on the larger bundle usually costs one trade size.
Torque and recheck
Aluminum creeps under pressure. Use a torque screwdriver to the lug spec and re-torque per manufacturer guidance after the first thermal cycles.
Copper vs aluminum questions, answered
What Al size replaces my Cu?
One size up to ~50A, two sizes up past 100A. #12→#10, #10→#8, #6→#4/#6, #3→#1, #3/0→250 kcmil.
Aluminum on 20A circuits?
Legal (#10 Al) but pointless — cost, boxes, and device ratings favor #12 Cu. Aluminum starts paying at feeders.
Is old aluminum wiring dangerous?
AA-1350 on copper-only devices, yes. Modern AA-8000 on AL-rated lugs, torqued, is a different system entirely.
Bigger conduit for Al?
Usually one trade size — fatter wire at equal ampacity plus 40% fill math. Quote conduit + lugs with the wire.
When does Al win?
100A+ feeders and services, longer runs. Below ~50A copper wins on total installed cost and simplicity.
Does drop change the conversion?
No — one size up equalizes drop within ~3% (#8 Al ≈ #10 Cu per foot). Verify long runs in the calculator anyway.
Size it in aluminum →Wire size chart →Ampacity vs drop →
Related: Voltage drop distances ·Conduit fill chart ·All electrical calculators