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I've purchased a house with an outside outlet about 100 feet from the house. It is a GFCI, wired with 10 AWG outside rated wire (I assume because of the distance), run through conduit underground to the basement, where it ends at a 20 amp breaker. Inside the basement, there is a 12 AWG circuit that serves a nearby outside outlet on the deck, tapped into the 10 AWG wire inside a box. It all looks good, but question 1 is: Is it legit to tap into the 10 AWG wire with 12 AWG to serve nearby outside outlet? If so, can I tap another 12 AWG wire into the box to add another nearby outside outlet? (I would probably use WAGO flat connectors rather than try to twist two 10 AWG and two 12 AWG wires together.). Thanks in advance!

  • This is a good case for either a GFCI breaker, or for one GFCI as the first device (inside the house) and the other devices fed from its LOAD terminals. All three of the outlets need GFCI protection, and GFCIs (even "weatherproof" ones) tend to fail when outside, even with proper "in use" covers on the outlets. Far better to keep that inside (and have just one, or a breaker) rather than needing 3 (and your description implies that the existing nearby outlet is NOT GFCI which is a no-no for an outdoor outlet.) – Ecnerwal May 21 at 4:04
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You can combine whatever wires you want provided they are all large enough - 12 or larger for a standard 20A circuit, so 10 is fine.

You need to check each specific type to make sure, but there are definitely Ideal and other brands of wire nuts that can handle ranges of wires up to 4 #10.

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Mixing wire sizes is OK as long as the overcurrent protection is sized to the minimum sized wire.

None of the outlets you described seem to be circuits that require dedicated circuits so adding a receptacle tap should not be a problem. You may have box fill issues, each #10 requires 2.50 cu in, and each #12 requires 2.25 cu in, all grounds count as one of the largest size, and if there is a device mounted in the box it counts as two wires.

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