AmpSizerCopper against aluminium
Aluminium conducts about 61% as well as copper, so the same current needs roughly two sizes more of it: 4/0 aluminium and 2/0 copper are both about 180 amps at 75 °C.
It weighs about half as much for the same current and costs substantially less, which is why every utility service drop and most 100 amp sub panel feeders in the US are aluminium.
Where it is genuinely different is at the terminations. Aluminium expands more with heat and keeps deforming under pressure, so a connection that was tight goes loose over years unless it is made with listed connectors, the right compound and the right torque. That is the whole of aluminium’s bad reputation, and it belongs to branch circuits rather than to feeders.
Size for size, and amp for amp
| Size | kcmil | mm² | Cu 60 °C | Cu 75 °C | Cu 90 °C | Al 75 °C | Cu Ω/kft | Al Ω/kft |
|---|---|---|---|---|---|---|---|---|
| 14 AWG | 4.11 | 2.08 | 15 A | 20 A | 25 A | — | 3.14 | — |
| 12 AWG | 6.53 | 3.31 | 20 A | 25 A | 30 A | 20 A | 1.98 | 3.18 |
| 10 AWG | 10.38 | 5.261 | 30 A | 35 A | 40 A | 30 A | 1.24 | 2 |
| 8 AWG | 16.51 | 8.367 | 40 A | 50 A | 55 A | 40 A | 0.778 | 1.26 |
| 6 AWG | 26.24 | 13.3 | 55 A | 65 A | 75 A | 50 A | 0.491 | 0.808 |
| 4 AWG | 41.74 | 21.15 | 70 A | 85 A | 95 A | 65 A | 0.308 | 0.508 |
| 3 AWG | 52.62 | 26.67 | 85 A | 100 A | 115 A | 75 A | 0.245 | 0.403 |
| 2 AWG | 66.36 | 33.62 | 95 A | 115 A | 130 A | 90 A | 0.194 | 0.319 |
| 1 AWG | 83.69 | 42.41 | 110 A | 130 A | 145 A | 100 A | 0.154 | 0.253 |
| 1/0 AWG | 105.6 | 53.49 | 125 A | 150 A | 170 A | 120 A | 0.122 | 0.201 |
| 2/0 AWG | 133.1 | 67.43 | 145 A | 175 A | 195 A | 135 A | 0.0967 | 0.159 |
| 3/0 AWG | 167.8 | 85.01 | 165 A | 200 A | 225 A | 155 A | 0.0766 | 0.126 |
| 4/0 AWG | 211.6 | 107.2 | 195 A | 230 A | 260 A | 180 A | 0.0608 | 0.1 |
| 250 kcmil | 250 | 127 | 215 A | 255 A | 290 A | 205 A | 0.0515 | 0.0847 |
| 300 kcmil | 300 | 152 | 240 A | 285 A | 320 A | 230 A | 0.0429 | 0.0707 |
| 350 kcmil | 350 | 177 | 260 A | 310 A | 350 A | 250 A | 0.0367 | 0.0605 |
| 400 kcmil | 400 | 203 | 280 A | 335 A | 380 A | 270 A | 0.0321 | 0.0529 |
| 500 kcmil | 500 | 253 | 320 A | 380 A | 430 A | 310 A | 0.0258 | 0.0424 |
| 600 kcmil | 600 | 304 | 350 A | 420 A | 475 A | 340 A | 0.0214 | 0.0353 |
| 700 kcmil | 700 | 355 | 385 A | 460 A | 520 A | 375 A | 0.0184 | 0.0303 |
| 750 kcmil | 750 | 380 | 400 A | 475 A | 535 A | 385 A | 0.0171 | 0.0282 |
Read that table twice. Once across a row, to see what the same size gives you in each metal — 4 AWG is 85 A in copper and 65 A in aluminium. Then down the columns, to find the aluminium size that matches a copper one: 85 A in aluminium is 2 AWG, two steps up from 4.
Where each one belongs
| Job | Usual choice | Why |
|---|---|---|
| Service entrance | Aluminium | Large, few terminations, big cost saving |
| Feeder to a sub panel | Aluminium (SER) | Same reasoning; 4-conductor SER is a standard product |
| Long feeder to an outbuilding | Aluminium | The size penalty costs less than the copper would |
| Branch circuits | Copper | Many terminations, small sizes, low cost difference |
| Receptacle and switch legs | Copper | Devices are listed for copper; aluminium needs special terminals |
| Motor and equipment leads | Copper | Vibration and flexing |
| Grounding conductors | Copper | Usually, and required in some locations |
| Underground direct burial | Aluminium URD | Purpose-made and cheap |
The 1970s, briefly
Between about 1965 and 1973 copper prices spiked and US houses were wired with solid aluminium branch-circuit conductors in 12 and 10 AWG. Three things went wrong at once: the alloy of the day crept badly under a screw, the receptacles and switches were listed for copper only, and the connection was made with a plain steel screw that expanded at a different rate from the wire.
The result was loose, hot connections behind faceplates — and enough fires that the CPSC investigated it. The industry response was a better alloy (AA-8000 series), devices listed CO/ALR for the purpose, and a general retreat from aluminium in small sizes.
None of that applies to a 4/0 aluminium service conductor in a listed lug, torqued to specification. It is worth knowing the history because it explains why an inspector will look hard at aluminium in a house — and why they will not blink at it on the outside of the meter.
What to actually do with aluminium
Use listed connectors and read the torque figure. It is printed on the lug or in the instructions, and a torque screwdriver is the cheapest tool in this whole subject.
Use antioxidant compound where it is specified. Not everywhere — some connectors are listed without it — but where the instructions call for it, it is not optional.
Do not mix metals under one screw. AL/CU rated connectors exist for a reason.
Re-torque is not a maintenance schedule. A properly made modern aluminium termination does not need annual attention; a badly made one cannot be saved by it.
Compare the two metals on your own run with the wire size calculator — switch the metal and watch both the size and the voltage drop move.
Frequently asked questions
Is aluminium wire safe?
Modern aluminium alloy conductors in feeder and service sizes are safe, standard and used everywhere. What earned the reputation was the small solid aluminium branch-circuit wiring installed in US houses between roughly 1965 and 1973, which used an older alloy in 12 and 10 AWG at receptacle terminals never designed for it. That specific combination caused fires; 4/0 aluminium in a service is not the same product or the same problem.
How many sizes bigger is aluminium?
Two, as a rule of thumb, and it holds across the range: 100 A is 3 AWG copper or 1 AWG aluminium, 200 A is 3/0 copper or 250 kcmil aluminium. In the small sizes the ratio is a little worse than two sizes, which is one reason aluminium is uncommon below 8 AWG.
Can I connect aluminium to copper?
Only with a connector listed for the combination — marked AL/CU — and with antioxidant compound where the manufacturer requires it. Putting the two metals in direct contact under a screw sets up a galvanic cell that corrodes the joint and heats it. Purpose-made splice connectors for repairing old aluminium branch circuits exist and are the accepted fix.
Why is aluminium used for service entrances if copper is better?
Because at that size the cost difference is large and the disadvantage is manageable. A service or feeder has a handful of terminations, all made once by an electrician with a torque screwdriver, in accessible lugs. A branch circuit has dozens of terminations in receptacles behind furniture — the same metal, a completely different risk profile.
Does aluminium make voltage drop worse?
For the same size, yes, by about 1.6 times. For the same ampacity — that is, two sizes up — it is very close to copper, and sometimes slightly better. So aluminium does not cost you anything on voltage drop once you have sized it correctly for current; it only punishes a size-for-size substitution.
How we calculate this
Sources: