Can aluminum and copper go under the same wire nut?

Only if the connector is listed for intermixed aluminum and copper, such as a purple AL/CU rated twist connector. Standard wire nuts are not listed for that use, per NEC 110.14.

Why does aluminum to copper contact cause problems?

The two metals expand at different rates and set up galvanic corrosion where they touch. The joint loosens, oxidizes, heats up under load, and can eventually arc.

Can You Connect Aluminum and Copper Wires Together?

Yes, but only with a connector listed for the job. NEC 110.14 requires terminals and splicing devices to be identified for the conductor material, and it prohibits intermixing aluminum and copper in the same terminal unless the device is identified for that exact condition. Direct twist them together under a standard wire nut and you have built a slow failure into the wall.

Why the metals fight each other

Aluminum expands and contracts more than copper as loads cycle, so a mixed joint works itself loose over time. Where the dissimilar metals touch, galvanic corrosion forms a high resistance oxide layer. Loose plus resistive equals heat, and heat in a buried junction box is how aluminum branch wiring earned its reputation in the 1970s.

Connectors that are listed for AL to CU

Several products carry the right listing. Insulated multiport connectors rated AL9CU, commonly called Polaris style, are the cleanest choice for larger conductors and feeder taps. For branch circuit sizes, purple twist connectors listed for aluminum to copper combinations exist, though many electricians prefer the crimp style COPALUM repair, which is a listed permanent splice installed with a dedicated tool, or simply a small listed multiport. Split bolts marked AL9CU also qualify for the splice itself, wrapped and insulated properly. Whatever you use, the AL and CU rating must be printed on the connector or its listing sheet.

Good field practice

Strip aluminum carefully so you do not nick the conductor, because nicks concentrate stress and aluminum breaks where copper would survive. Brush the strands and apply an oxide inhibitor where the connector manufacturer calls for it. Torque lugs to spec per 110.14(D) rather than by feel. And where an aluminum feeder lands on equipment, check that the lug says AL9CU or CU7AL; nearly all modern panel and breaker lugs do, but device screws on receptacles and switches generally do not, which is why CO/ALR devices exist for aluminum branch circuits per 406.3.

If you are extending old aluminum branch wiring to add a receptacle, the copper pigtail with a listed connector is the standard approach, and some jurisdictions want the COPALUM or AlumiConn method specifically, so ask your inspector before you close up.

Questions like this one come straight from electricians using Ask BONBON in the field. Ask yours and get a cited NEC answer before the inspector shows up.