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


You'll want 14/2 NM...

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.

...and a torque screwdriver

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.)

  • C'mon upvoters, this is a good answer! – Harper - Reinstate Monica Jun 6 '20 at 6:11
  • Just as an FYI, they do have a non-torque method too (quarter turn past resistance, if memory serves), but the torque is preferred. I bought a torque screwdriver to do mine (I still work with them from time to time in my house) and it was prohibitively expensive for a one-off project. – Machavity Jun 7 '20 at 14:57

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.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.