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When they built my house, than ran 10/2 wire from the 20 amp breakers. I’d like to pigtail into one of those and make a new outlet. But I’d prefer to use 12/2 as it’s easier to work with and it’s easier to find outlets that accept 12 gauge wire.

Can I safely pigtail (under a wire nut) #10 wire and #12 wire on a 20 amp circuit?

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    Hmmm. Is this wire you have in place Aluminum or Copper? Makes a big difference, and Aluminum wire is by far the Most Common Reason you'd see 10Ga on 20A. – Ecnerwal May 9 at 14:12
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    Can you post photos of the inside of the box you're working in please? – ThreePhaseEel May 9 at 19:07
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If copper, no problem, easy, yes you can use 12Ga copper to add to it, it was just overbuilt. Using larger wire than required is fine, code only specifies the minimum size required.

Do make sure you use wire nuts (or other wiring connectors) rated for the combination and number of wires you use. There is normally a little table on the container listing acceptable combinations of wires for this particular size connector.

If Aluminum, it's complicated and not burning your house down is a main consideration.

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The #2 reason for a wire size bump is a long-distance run (>70 feet @ 120V; > 150 feet at 240V).

The #1 reason for #10 on 20A circuits is aluminum wire.

Note that copper-clad aluminum is a thing - that looks copper from the sides, but looking at the end of the wire you will see the gray of aluminum. The metallurgy will also be stated on the cable sheath if that's readable.

If you have aluminum:

  • First, seriously consider an AFCI breaker for that circuit. That will catch most of the problems people worry about with aluminum wire.
  • The wires must land on aluminum-rated receptacles and switches, meaning modern CO-ALR rated devices with notably gray terminals. Any Al-Cu receps and switches must be junked. (or used for copper applications).
  • Breakers should be reasonably modern so you benefit from the latest terminal designs. Aluminum was a "bit of a learning curve" when it comes to terminations, which is why they had to revise the CO-ALRevised rating.
    • If it's an obsolete panel type such as Pushmatic or the Crouse-Hinds double-stuffs, stay with the breakers you have.
    • If it's a dangerous type like Zinsco or FPE, plan a panel replacement ASAP, or at least thru-lug to a modern panel and move circuits over to it as able.
  • Splices must be done with competent aluminum lug-style connectors, such as Alumiconn or MAC Block. There are several wire-nut connectors that are UL-listed, but burn up a lot anyway.
  • All screws should be torqued properly with a torque screwdriver.
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