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There's a 20A circuit that passes right by a spot I need an additional outlet for a lamp/general use near laundry. Currently it serves about 8 devices, either single bulb light fixtures or 15A 3-prong receptacles. Because the receptacle I'd add on would be near laundry, it needs to be GFCI. It would be the only GFCI on the circuit from what I can tell.

Since the circuit is rated for 20A, and the wiring is 12/2, can I put a 20A GFCI in? On the other hand, since most of the receptacles the circuit serves are 15A, for all I know some past amatuers have 14/2 in some segments of the circuit, would I be better off installing a 15A GFCI if I don't intend to use it for anything intensive?

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    Nonononono! "15A" GFCIs are not fuses and are not circuit breakers. They will cheerfully flow 30A. Detecting overcurrent is Not Their Job. Their job is detecting Ground Faults. If you have the slightest suspicion of #14 wire on that branch, downbreaker to 15A until that is disproven. – Harper - Reinstate Monica Aug 27 '18 at 22:34
  • @Harper - You can still put a 20A receptacle on a 15A circuit, right? Even if it's a true 20A with the combo sideways plug? – Mazura Aug 27 '18 at 23:45
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    @Mazura absolutely not. 15A circuits can only accommodate 15A receptacles. You may be confused by a nearby rule: 20A circuits can have 15A or 20A receptacles. – Harper - Reinstate Monica Aug 27 '18 at 23:49
  • Right. I'll confirm all segments on the circuit are 12/2 before doing anything else. Much of it is the old NM 12/2 so it's a little misleading at first, and other wiring in this house has had problems w/ unsafe wire gage combos on other circuits (which have since been fixed). Haven't had any work done on this circuit yet. – cr0 Aug 28 '18 at 0:12
  • Still, once I confirm the circuit is wired for 20A, mu question stands about which GFCI I'd install. – cr0 Aug 28 '18 at 0:13
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There's two problems I see here

  1. There's probably no need for a 20A plug. Let's be honest, how often have you looked at your outlets and gone "Man, I really wish I had a 20A plug right here!" in your laundry room? Washers are 15A, gas dryers are 15A and electric dryers need a dedicated 30A two phase plug. What are you going to use that's 20A? Don't add 20A just for the sake of having 20A. For a shop or garage, maybe, but inside your home you'll rarely need anything more than 15A.
  2. I would just GFCI the outlet you need, and 15A at that (remember, a 20A GFCI will be more expensive). Too many people think that they must use the LOAD side of their GFCI because it's serial wiring. Or we're wasting the plug somehow if it's only protecting that one outlet. NOT SO! Wire in series only those outlets that need a GFCI. If they don't need GFCI protection then just pigtail the connections and leave the GFCI out of the series.
  • Is GFCI like basic receptacles in that a 15A one can safely be installed on a 20A circuit? Though as @Harper noted the backstabs on a 15A GFCI may not fit the 12/2 wiring of the circuit. – cr0 Aug 28 '18 at 3:37
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    15A outlets can be installed on a 20A circuit provided there's more than one outlet. Look at your existing 20A circuits. Your house should be most, if not all, 15A – Machavity Aug 28 '18 at 3:46
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    Also, it's important to note that when Harper says backstabs, he typically means the cheaper 15A outlets. The 15A Leviton GFCIs I've installed recently don't have 14 gauge backstab holes, they have a side plate that works kinda like a backstab (you screw a plate over the wire and it locks it in place) but easily accommodates 12 gauge wire – Machavity Aug 28 '18 at 3:48
  • Makes sense, thanks. There's a surprising amount of 20A circuits and most if not all at those have proper wiring, but they mostly serve 15A outlets. Certainly better than other way around. And good point, the 'good backstabs' of GFCIs with the side plate would work fine for either wire gauge. – cr0 Aug 28 '18 at 4:12

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