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Can AWG 6 be used in the middle of a 20a AWG 12 circuit? Would this affect the performance of a 20 Amp Single-Pole CAFCI Circuit Breaker?

I have an existing, never used, buried, 6-3 Romex NM-B W/G wire leading from (but not connected to) a Square D 100 amp load center approx. 100 ft. to an outside structure with an AWG 12 curcuit used for lighting and one GFCI outlet. All of the outside fixtures are water proof.

I would join the AWG 12 on each end to the AWG 12, in a junction box using split bolts?

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There's no problem connecting a bigger conductor, as long as the circuit breaker protects the smallest conductor in the circuit. In fact, that big wire will reduce voltage drop over that 100 foot run. Actually, it's 100 feet, plus the length of whatever wiring you run to the outlet and lights, plus the length of whatever extension cord(s) you plug into the outlet.

If you were to use a 12 AWG wire for a 100 foot 120V single phase run, you'd experience almost 8% voltage drop if you were actually pushing 20 amperes. Assuming you had 120 volts at the source, you'd have about 112 at the other end, which isn't a problem for a lot of loads (aside from using a little more electricity), but for some types of equipment I've seen it cause some weirdness. A 10 AWG wire would experience 5% voltage drop over the same distance, and 8 AWG wire would give you 3% voltage drop. If you ran a 240V split phase circuit down that 6 AWG wire, you'd experience about 5% voltage drop if you were pushing 40 amperes of current.

It seems that this existing wire was probably originally meant to warm up a subpanel in the other structure? Why not just install a small subpanel with a 40A two pole main breaker, then run a couple of 15A or 20A circuits out of the subpanel with a little bit of room to grow? The panel itself would only cost about $15. The breakers would cost a little more if you're required to install GFCI and/or AFCI breakers.

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  • Thanks Craig for your detailed response. It confirmed that what I'm doing is safe. The 'structure' is outside (no roof) and the lights we installed and occasional fan we plug in are all that's in the plans for now. We have temporarily used an extension cord to light it up. That cable was surplus and buried two years ago 'just-in-case'. I will leave things where it's clear what I did and it's possible grow it safely.
    – George Sheffield
    Commented Aug 25, 2015 at 17:02
  • You're welcome. Yes, safe. I just elaborated a tiny bit on what @Tester101 said, and Tester101 definitely has a point about potentially confusing the next person who comes along. #6 is pretty big wire for a 20A circuit. ;-)
    – Craig Tullis
    Commented Aug 25, 2015 at 19:53
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There's no problem using larger conductors in a circuit, other than it might be confusing to anybody that comes along later. You'd also be wasting money, if you're buying the larger cable.

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  • Questioner states the wire is already in place.
    – Ecnerwal
    Commented Aug 23, 2015 at 13:49
  • @Ecnerwal Just adding a bit more info, for anybody else that stumbles upon this thread.
    – Tester101
    Commented Aug 23, 2015 at 17:07
  • Thanks guys. That cable was surplus and buried two years ago 'just-in-case'. I will leave things where it's clear what I did and it's possible grow it safely.
    – George Sheffield
    Commented Aug 25, 2015 at 17:03

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