I see some discussion about aluminum wiring. I have a bit of experience here. In car audio installations, it's common to use 4AWG and even 1AWG in higher power systems. When the price of copper went up to $10K a metric ton, several companies started selling CCA or Copper Clad Aluminum wire. CCA wire is aluminum with a thin layer of copper on it. It looks like copper wire but it's much lighter and if you look at the cut cross section you see mostly silver. I was involved with evaluating and testing these wires and we found that there was a 30% difference in resistance and current handling. This means you need to upsize your wire by
over 30% for the same performance as OFC copper. Of course virtually all of the companies selling CCA wire were selling them in the same gauge as the pure OFC copper gauges and they weren't mentioning the performance drop.
Anyway, the research I did was for DC voltage and those wires had an amount of copper in them. I've seen numbers for AC wiring being closer to a 40% drop in current capacity and a need for around 50% more cross sectional area. If I was adding a 240v receptacle, I would definitely use copper, regardless of code and supply wire material.
This has some interesting info:
Aluminum vs. Copper Conductors | TES Engineering
"
For uses below 200 amps –
Copper is the better choice because there is minor cost saving (if any) by using the alternative: aluminum conductors. Additionally, breakers, fuse blocks, and receptacles are listed to only terminate with copper and will not accept aluminum connections.
Between 200 and 800 amps –
Complete case-by-case study and evaluate the building characteristics such as voltage, distance, load variations and type of load. Within installations of this size, aluminum can often be used from the utility transformer to the building distribution, then from the
building distribution to the house distribution panel. Significant savings can be achieved based on the distances involved.
Aluminum conductors often offer larger cost savings above 800 amps. For example, in one large multi-building lifestyle center project, the estimated savings to use aluminum instead of copper building service conductor was $100,000."