August 29, 2014 3:45 PM   Subscribe

I need to replace a bunch of two-prong outlets with three-prong outlets and I have a few questions.

The old owners of our new apartment replaced some, but not all, of the two-prong outlets with three-prong. According to the engineer who did the inspection, all the three-prong outlets are properly grounded and the apartment is wired with BX cable and has a metal box, so switching out the remaining outlets should be simple. Should be. What I need a hand with is:

-The outlets are painted over with at least a few layers of paint, I'm not concerned with the paint job, so what's the best way to free the faceplate?

-Can someone link to a visual guide on what to expect and how to correctly hook up the outlets? I am not good at all comfortable with written instructions for stuff like this.

-One of the outlets has a very large faceplate that has two knobs on it, maybe half an inch long and roughly the width of a Sharpie. I've completely failed at Googling what that is, or how to replace it. Any ideas?
posted by griphus to Home & Garden (20 answers total) 3 users marked this as a favorite
Here's exactly the email the engineer sent me in case I misphrased something:
Because NYC used BX cable and metal receptacle box, the armor of the bx wiring and the metal boxes act as the ground all the way to the panel so there is no need for a separate ground wire. If it were romex and plastic boxes like in Long Island, then you would need a separate ground. The three prong outlets that I did test during the inspection were fine.
posted by griphus at 3:47 PM on August 29, 2014

You shouldn't try to do this yourself. Hire an electrician.
posted by Chocolate Pickle at 3:52 PM on August 29, 2014 [7 favorites]

I'm not an electrician, and I nth the advice to hire one. But I will say that when I've replaced sockets, I've run a short piece of extra wire from the grounding lug on the receptacle to a screw on the metal box. I guess it's supposed to be grounded by the screws that hold it on place, but in an old house where many things don't fit well, the screws are not always reassuringly tight.
posted by SemiSalt at 4:01 PM on August 29, 2014 [1 favorite]

Yes hire someone.
posted by trip and a half at 4:06 PM on August 29, 2014

I don't have a good visual guide, but it really isn't that hard. I can recommend Rex Cauldwell's excellent "Wiring A House" book.

The box itself should have a threaded hole for a #10 (I think) ground "screw" (it's a bolt), you can buy green colored ground screws, exactly the right thing for the job, in any hardware store. You run a piece of bare or green insulation THHN wire, #14 or #12 (for 20A circuits, although in my house I use #12 for everything), form a loop on each end, and screw one end into the box, the other on to the appropriate bolt on the socket.

The third picture in the "Test for a ground wire" section under the "Step-by-step" tab of this page shows it.
posted by straw at 4:07 PM on August 29, 2014 [1 favorite]

I concur with straw. Done carefully (throw the breakers), this isn't a big deal.

The two pole plate is probably a TV antenna connection. Do you have a photo? If there's no 110 outlet, you can just leave it or you can find a cover plate with no holes.

You can pick up one of these testers. That will let you know you've turned off the correct breaker when you start and test your work when you're done.
posted by humboldt32 at 4:17 PM on August 29, 2014 [1 favorite]

You can do this task. Make sure that the power to the plug is off. Get a circuit tester, which looks like a plug and tells you if the plug is wired right. The face plates can be removed from the wall by cutting around the edge with a utility knife, to break the paint seal. The screws may need to be chipped free. You may want to replace the plates if they are ugly.

Look at one of the current 3-prong outlets as your guide to how to wire it. Could the plate with the knobs be an old-style phone plate? After you have replaced the plug and plate, turn on the power and test with your circuit tester.
posted by Midnight Skulker at 4:17 PM on August 29, 2014

If you open up the grounded outlets ("3-prong") you are going to see that the ground prong is wired to the metal box that holds the outlet in the wall, no surprise there. What you may or may not be able to see, but is absolutely there, is some sort of wire connecting all those grounds to a true ground, apparently back at your circuit breaker panel.

I would certainly not assume from this, nor from your electrician's email (at least not the part you've quoted), that the enclosures for the ungrounded outlets ("2-prong") are likewise wired back to true ground, and that is the real issue for whether this is all a good idea for you to approach yourself. To me the email only seems to say that he checked the 3-prong, apparently grounded, outlets and determined that they are in fact grounded. Without more context, it does not appear to say that you'll be able to properly ground any 3-prong outlets you install yourself.
posted by Joey Buttafoucault at 4:31 PM on August 29, 2014 [1 favorite]

Are your two buttons a switch?
posted by advicepig at 4:52 PM on August 29, 2014

I would free face plates by running a utility knife around the edge before trying to pry it off.
posted by advicepig at 4:54 PM on August 29, 2014 [1 favorite]

BX or AC cable looks like this (but in your case there will only be two wires inside). The metallic sheath is connected to ground; it serves as the grounding conductor. At each electrical box (like this one), the sheath is (hopefully) tied to the box using some sort of connector/fitting (see this video for how they fit together). So your boxes are bonded to ground through the cable's sheath.

Now what you need to do is to bond the grounding terminal of your outlet to the box. You would do that by running a wire from the box's grounding screw to the grounding terminal (usually a green screw) of the outlet. See the video on this page.
posted by Monday, stony Monday at 6:51 PM on August 29, 2014

If the box is properly grounded, there are self grounding outlets that will connect ground to the box without a wire from the green ground terminal to the box. If you get these, spend a few bucks on an outlet tester.
posted by advicepig at 7:28 PM on August 29, 2014

Here's a video that pretty much sums it up. Your wiring sounds like the first example in the video.
(Monday- your video link is ... odd...)
posted by TDIpod at 7:35 PM on August 29, 2014 [1 favorite]

screws on new receptacle:
gold-hot-black wire
silver-neutral-white wire
green-ground-green or bare wire. Make sure bare wire doesn't touch gold or silver terminals.
Check with outlet tester. This will tell you if the wires are on the proper terminals and if the ground is connected.
posted by H21 at 11:47 AM on August 30, 2014


I got an outlet tester, which confirmed what the engineer said about the 3-prong outlets being properly grounded. I cracked open one of the 3-prong ones, and there's nothing connected to the green ground screw; just the black/white wires on either side. I replaced one of the other 2-prong ones with a 3-prong outlet, tested it, and it looks like it's properly grounded.

Should I trust the tester?
posted by griphus at 12:29 PM on September 6, 2014

Maybe. The good scenario is that the box is fully grounded, and by screwing the socket to the box you've connected the grounded bits of the socket screw tabs to the box. This is not code, and not optimal, but it's better than nothing.

The bad scenario is that the the box and whatever it's attached to absorbs enough electrons to make the ground tester think it's got ground. A long enough piece of wire will sometimes be sufficient to do this (in my rental years, I often did some heinous things to give myself a somewhat grounded socket).

In either case, most modern devices are 2 prong or (higher capacity laptop power supplies) don't really use that ground line for anything important, so it's not actively dangerous, but it'd be nice to have the peace of mind. At the very least, I'd run an explicit ground wire to the box for any sockets you open up.
posted by straw at 11:14 AM on September 8, 2014

Should I trust the tester?

If there is no ground wire, the ground connection is being made through the BX conduit connections to the box and through the screws to the receptacle. The conduit screws could be loose or the connections corroded. The tester could be reading a very flaky connection as good.
Everything will function correctly without the ground. The purpose of the ground is for safety. If there is no ground and the hot wire somehow contacted a metal case, you could be zapped. With a ground connected, it would blow a fuse or breaker.
I had a light switch with a metal cover that I was getting a shock on when I touched it. I looked inside and found that the hot wire was pinched between the box and the cover. There was a ground wire but it was not hooked up.
So, the right way to do it is to run a ground wire to the fuse box, but that's probably very difficult. I guess one way to look at it is: You're safer than with 2 prong receptacles because you MIGHT/PROBABLY have a ground.
posted by H21 at 12:22 PM on September 8, 2014

If there is no ground wire, the ground connection is being made through the BX conduit connections to the box and through the screws to the receptacle

I am reasonably sure this is what is happening. When I (very, very carefully) tested the outlet without screwing it in, it showed an open ground. I replaced a number of them, all of which show proper ground with the tester and the screws are new and the connection to the box is as clean and solid as it's going to get without a major rewiring.

I'm going to check the wiring of the 3-prong ones that were put in by the previous owners to make sure everything is solid there too.

A knowledgeable friend of mine suggested figuring out which outlet is the closest to each individual breaker line and installing a GFCI outlet on there. If anyone has some advice on that I'm all ears.
posted by griphus at 3:48 PM on September 8, 2014

Also I wrapped the inner part of the outlet with electrical tape because the already-installed ones were wrapped and I assume that's to keep any connection from being made between the hot wire and the box.
posted by griphus at 3:52 PM on September 8, 2014

A knowledgeable friend of mine suggested figuring out which outlet is the closest to each individual breaker line and installing a GFCI outlet on there.

This is a pretty good Idea. A GFCI works by measuring the difference between the current in the hot leg to the neutral leg. Any current going from hot to ground will trip the GFCI. It does need to be on the first outlet from the fuse or breaker box. link link
posted by H21 at 2:10 PM on September 11, 2014

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