Adding 1 extra light total of 2 on circuit. Need to know what gauge of copper wire to add to aluminum 12-2 gauge 3’ span.
I will use AlumiConn connectors
Home Improvement Stack Exchange is a question and answer site for contractors and serious DIYers. It only takes a minute to sign up.Sign up to join this community
The rule for converting between aluminum and copper wire gauges is that in order to carry a given amount of current, you need an aluminum wire two sizes larger than the copper wire you would have used. So, a 12AWG aluminum wire is rated for the same 15A as a 14AWG copper wire is, and NM is apropos since jurisdictions that don't permit NM also generally don't permit the use of aluminum, especially at branch-circuit gauges.
While your choice of AlumiConns for this job is a good one, as they are the most reliable way of making an aluminum-to-copper branch circuit wiring connection available to the rest of us, they, like all mechanical setscrew splice/tap connectors, are sensitive to tightening torque. As a result, a using an inch-pound torque screwdriver to tighten the connector screws to the manufacturer's specified torque value is highly recommended for long-term reliable results.
(Sidenote: if you're having trouble with the relatively chunky Alumiconns taking up too much room in the box, get a Wiremold ceiling starter box and use it as an extension ring to add extra volume.)
The rules for the minimum size wire allowed are determined by the size of the overcurrent protection (i.e. the breaker or fuse). The maximum overcurrent protection allowed for #12 aluminum is 15A, the minimum size copper for 15A is #14. You can use larger than #14 if convenient, but must use the size of overcurrent protection that matches lowest rated wire in the circuit.