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I don't know too much about replacing outlets, only what I've read online or through YouTube videos. The house was built in the early 50s. Some outlets have been replaced (brand new kitchen) but some are still the old 2 prong. I bought a receptacle tester and tested the newer style outlets, which all tested as correct wiring. Upon opening and inspecting, none of these new 3 prong outlets had no ground wires. Does this mean the boxes are already somehow grounded? I'm even more confused now that the receptacle tester said that they were correct. Please help!

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    Was there a jumper between neutral and ground on the receptacle? – BMitch Oct 8 '14 at 15:06
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    Are there ground wires in the box (you'll have to remove the cover plate, and possibly the receptacle to see)? Is there a brass colored tab on the yoke where the mounting screw attaches the receptacle to the box (you'll have to remove the cover plate to see)? Maybe turn the power of before taking the cover plate of, definitely turn it off of you're pulling the receptacle out. If the box is metal, and grounded, it will provide enough of a ground through the yoke for an outlet tester to call it grounded. Whether or not the yoke provides an adequate ground, depends completely on the device. – Tester101 Oct 8 '14 at 15:17
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    Post a picture of inside the box and the receptacle. Two thing could be happening here: The 'ground' was bound to the neutral or you have a metal box and it is grounded which would then ground the receptacle. – diceless Oct 8 '14 at 15:27
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    BE CAREFUL. Quite a bit of older AC (metal sheathed) cable will show a ground on a tester but the sheathing IS NOT acceptable for use as a grounding conductor. In order for the sheathing to be used as a ground the cable MUST have the thin aluminum bonding strip run within the sheathing with the power conductors. – Speedy Petey Oct 8 '14 at 22:14
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    By "none of the new outlets had no ground wires" you mean "none of the new outlets had ground wires", yes? And again, please post photos. – Eric Lippert Oct 8 '14 at 23:07
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There are only 2 correct and safe ways to add a 3-prong outlet where formerly there was only a 2-prong one*:

  • EDIT: now allowed in Code since this answer was written: Retrofit just a ground wire, using proper wiring methods. There is no need for the ground wire to follow the same route as the power cable. Grounds can be shared: You don't need to homerun back to the panel if you can reach another grounded circuit with the same size or larger ground wire also coming out of that panel.
  • Safe way #1: run a new cable with an appropriately sized ground all the way back to the breaker panel.
  • Safe way #2: Use a GFCI outlet (with the "test" and "reset" buttons), which will cut off power to the outlet if it senses an imbalance of current. (EDIT: As @EricLippert correctly points out in the comments, you can also replace the circuit breaker with a GFCI breaker instead of using a GFCI outlet.)

There are many wrong ways to add a 3-prong outlet, most of which are dangerous and some of which are hard to spot or test for:

  • Wrong way #1: Using a 2-prong to 3-prong adapter without properly grounding the tab (common but obvious).
  • Wrong way #2: Replace the 2-prong outlet with a 3-prong one, but only hook up the black and white wires (this is hard to see unless you open up the outlet box, but an outlet tester with reveal the problem).
  • Wrong way #3: Replace the 2-prong outlet with a 3-prong one and short together the neutral and ground wires (this is probably the sneakiest because it will test as OK with an outlet tester but is just as unsafe as #1 and #2).
  • Wrong way #4: Replace the outlet with a 3-prong one and connect the ground to a random piece of metal (the outlet box, a pipe, metal cladding on an armored cable). This too may check out as OK with an outlet tester but is not safe.

*There are actually a few other ways to ground an outlet safely, e.g. with armored cable with a bonding strip or a grounded box plus an outlet that allows grounding through the strap. But those are both pretty uncommon.

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  • Good answer. I have added some additional explanation in my answer. Also, there is a third way to deal with the situation, which is to put a GFCI breaker on the circuit, instead of a GFCI outlet. – Eric Lippert Oct 9 '14 at 16:37
  • "*There are actually a few other ways to ground an outlet safely, e.g. with armored cable with a bonding strip or a grounded box plus an outlet that allows grounding through the strap. But those are both pretty uncommon."......On the contrary, properly grounded AC cable is very common. Self-grounding devices are also very common, although few people I know use them for this purpose. – Speedy Petey Oct 9 '14 at 19:40
  • @SpeedyPetey: Perhaps the use of grounded armored cable is regional; I've never seen it. But mostly I would be surprised if the wiring was already properly grounded all the way to the outlet but then had a 2-prong receptacle installed. – Hank Oct 9 '14 at 21:29
  • Well, it's not that uncommon to see proper AC cable with 2-prongers since in the days it was originally used 3-prong receptacles were not common. – Speedy Petey Oct 10 '14 at 0:24
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    @glenviewjeff: A continuity tester can be helpful when trying to figure out what's going on but it doesn't necessarily mean the connections are suitable or safe. Some receptacles (you need to check) are called "self-grounding, which means that if the box is metal and properly grounded, that the outlet itself becomes properly grounded just by being screwed to the box... i.e. the metal strap on the outlet can make the ground connection. On the other hand, some receptacles are not considered self-grounding even though the metal-to-metal connections may light up a continuity tester. – Hank Sep 7 '15 at 15:52
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I don't know too much about replacing outlets, only what I've read online or through YouTube videos.

That's a good start but I would strongly recommend that you learn more before you make potentially safety-impacting changes in your electrical wiring. In particular if you cannot immediately state the relationships between amps, volts and watts, if they are in any way unclear to you, don't work on power systems until you've internalized them. You wouldn't want someone who didn't understand the difference between weight and height to work on your house, so don't let someone who doesn't understand the difference between current and potential to work on your wiring.

Henry Jackson's answer describing the right and wrong ways to deal with this situation is good but I would add a few additional details to clarify why various options are unsafe.

First of all, let's talk about the purposes of the three wires. The hot (black) wire provides a voltage and therefore delivers current to the appliance plugged into the plug. You know that the total current through a circuit is constant, so that current has to go somewhere; it goes from the appliance to the neutral (white) wire, which is connected to ground. The neutral has current, but supplies no voltage. By design the safety ground never carries current; if it is carrying current then the wiring is by definition faulty, and you have a ground fault. The safety ground, as its name implies, is there for human life safety; in the event of a fault there is a clear path to ground for the current, rather than it trying to go to ground through the human holding the faulty toaster.

Let's make sure that's clear: the fundamental purpose of the ground wire is to ensure that even in the unlikely event of a fault, the current passes to ground through the wire, not through a human. A system which lacks this property is unsafe in the event of a fault.

So, why is Henry's safe way one safe? Because there is a dedicated copper wire just for the safety ground that goes back to the electrical box, which is typically connected to a copper rod driven into the ground.

Why is Henry's safe way two safe? Here we have no safety ground at all. Rather, the GFCI contains a detector which measures the difference between the hot and neutral currents. Again, current is constant throughout a circuit, so if there is a difference between the current in two halves of a circuit, some of the current must be going to ground not via the neutral, and it might be going through a person. The circuitry cuts the power going to the appliance in this case.

Now, immediately we note that the GFCI outlet does not cut power at the breaker. (You can get GFCI breakers, but let's suppose you do not have one.) It cuts power to the appliance. It therefore does not provide fault protection in the case where the hot wire has electrified the ungrounded box that the GFCI is in, because the box is not the appliance. A human could touch that box and be shocked and the GFCI would provide no protection. You can use GCFIs to increase the safety of a circuit that has no safety ground, but they are not equivalent protections. A GFCI provides less protection than a safety ground because it does not ground an electrified box, and it provides more protection in that there could be a small leak of current to the safety ground but you'd never know if it was not enough to trip the breaker.

In environments like kitchens and bathrooms where there is likely to be a combination of water and electricity, the best practice is to have both a safety ground and a GFCI-protected circuit. Remember that a correctly-wired GFCI provides ground fault protection to every outlet "downstream" of it, that is, every outlet hooked up via the LOAD terminals.

The discussion above shows that there is a third safe way to deal with an ungrounded circuit, and that is to add a GFCI at the breaker box.

Now let's consider the wrong ways.

The first and second wrong ways are the same: the appliance is not grounded via the safety ground at all. In the event of a fault that electrifies the appliance, a human could be the best path to ground.

The third wrong way -- connect the safety ground to the neutral -- is wrong but why is a bit subtle. The neutral is connected to ground, so the appliance is in fact grounded in the event of a fault, right? Well, let's think about that. The assumption of a well-designed safety system should be everything but the safety system has failed, and this is your last chance to protect the human. What if the wiring is faulty such that the hot has electrified the box and the neutral wire has broken? Now the safety ground is not grounded to anything because the neutral wire is broken, and the box is electrified.

The fourth wrong way -- connect the ground to some metal object -- is again subtle in its wrongness.

First of all, the metal object might not actually be grounded. Metal-clad cable, for example, could have been fed itself from an ungrounded box. That pipe could be part of an ancient plumbing system that is disused and has been hacksawed away from the real plumbing (which should be grounded). Or, it might be grounded now, but not grounded in the future. People do not think "I might need to rewire the bedroom because I upgraded the plumbing in the bathroom". Don't go making a hazard for people in the future.

Second, the metal object might be inadequately grounded. The conductors -- which were by assumption not designed to be a part of a safety system -- might be inadequate to take the current imposed by a ground fault, and they could melt, thereby ungrounding the system, or start a fire. Now you're not only being electrocuted, your house is on fire too.

Third, the metal object might be adequately grounded, but not bonded to the ground for the electrical box. Suppose you have two distinct grounds -- the electrical system ground, and a bit of plumbing -- and they actually physically go to ground at opposite ends of the building, and they are grounded in dry, sandy soil. In this scenario it is possible that the two grounds have different potentials, and now a human can be touching two grounded things and still get shocked. All the grounding systems in a structure have to be bonded to each other. (I know this from personal experience; my house had two distinct grounding systems in exactly this manner when I moved in. Fortunately it was never a problem, and it was easily fixed by running a short piece of heavy-duty wire between the plumbing and the electrical box.)

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The only time metal sheathed cable can be used as a grounding conductor is if it has a thin aluminum bonding strip. You MUST assure this is your kind of cable before re-installing grounding type receptacles.

Image ©Mike Holt: enter image description here

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  • Armored cable is not code in Chicago. Is conduit safe for grounding to the box? – glenviewjeff Sep 6 '15 at 14:35
  • It is, but you should use bonding bushings at both ends, bond panel side conduit to ground bus bar in a sub panel or grounded neutral bus bar at main service disconnect. Bond conduit to metal box at other end. The code says you only have to use bonding bushings if the knock outs are the multiple size type. I like the bonding bushings, sure little more $ & time...but I sleep easier knowing my work is as safe as can be. I personally don't like to use the conduit as equipment ground...what if a connector comes loose?=No Safety/Equipment Ground! My 2cents. – user43694 Sep 19 '15 at 3:51
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Late answer, but hopefully helpful to someone. Like the OP I was confused because, although my 60's house has mostly 2 prong there are a couple 3 prong and they test "correct". Figured out just recently that it was because my house was wired with armored wiring to metal boxes. Decided to DIY the 2 prongs to 3 prongs despite not knowing much at first, but did the research and troubleshot a good amount to finally finish today.

I went the GFCI protected receptacles route and I just completed replacing all the 2 prong outlets in my home with GFCI protected 3 prong outlets so I thought I would share some of what I learned in the process. The Sparky Youtube channel, this forum, and some thoughtful troubleshooting helped me get it done. Total cost was less than $300, including tools and parts for about 30 outlets. Disclaimer: I am not an electrician. This is just what worked for me.

Tools: Screwdrivers / Drill with driver bits, Non Contact Tester, Receptacle Tester, Wago Lever Nuts (3 conductor, used for pigtailing), 12/2 wire (to make pigtails), Wire cutter/stripper, Diagonal Cutter, Needle Nose Pliers (to help remove painted over receptacle covers), Tamper Resistant GFCI (Slim) and TR 3 prong receptacles, new receptacle covers, electrical tape.

Tips:

  • I contacted Leviton to get a bunch of extra stickers since there were not near enough "No Equipment Ground" stickers. They mailed them out to me no charge.

  • Make sure to get slim GFCI receptacles. They can be a real pain to get into the boxes

  • This Sparky Channel video explains the process that I based my efforts on https://www.youtube.com/watch?v=16KObgI44UE

Safety first. I checked and rechecked to make sure I was not dealing with any hot wires and never touched a hot wire (used a non contact tester). Test all the wires EVERYTIME before you work on them. Even the white wires can be hot sometimes!

Steps I Took. I don't have any fancy equipment so these are the steps I took to find the start of each branch (where the GFCI goes). This is to replace old 2 prong outlets with no ground wiring present (only black and white). Be sure to know exactly what outlets are on each switch in the electrical panel before starting. My electrical panel labels were way off.

  • Turn off a breaker switch, check all the outlets on the circuit to make sure they had no power, then remove all the covers and receptacles on the circuit. Test ALL the wires again using a non contact tester (make sure it works) before removing the wires from the receptacles. All my receptacles were back-stabbed so I cut them with a diagonal cutter.

  • Strip the ends and cap every wire individually with a Wago Lever Nut or wire nut. Pull the wires out and separate them into quadrants to make identifying them easier.

  • Turn the power back on and carefully use the non contact tester to test each box to find a live wire (should be a black wire) which indicates the hot wire at the start of a branch. Be sure to remember which wire this is (can tape the wall to indicate position). This method to find the start of branches works because the circuit is completely broken because you capped all the wires separately. So if a wire is hot it is the start of a branch (most upstream receptacle). See tips below as there can be multiple branches on a circuit which can get confusing.

  • Turn power off. For each start of a branch receptacle install a GFCI with the hot black wire and corresponding white into the LINE of the GFCI and the other set of wires into the LOAD (remove tape on GFCI receptacle). Continue to test all wires with a non contact tester before working on them.

  • Install regular 3 prong outlets downstream using pigtails if the box has both LINE and LOAD lines as explained in the video (not sure if pigtails are necessary but they help prevent loss of power downstream if an individual receptacle has some kind of mechanical failure so I went with it). Once again, test all wires before working on them!
  • Turn power back on.
  • Use a receptacle tester to test the GFCI for power and correct wiring. May need to reset the GFCI first. When GFCI has power, test the 3 prong outlets downstream to ensure power and correct wiring. "Open Ground" is an expected outcome though it may also test "Correct" i.e. when you have metal cased wiring to a metal box like I do which is not the same as modern day grounding.
  • Use the GFCI receptacle test button and ensure that all downstream receptacles lose power when testing. This double checks to make sure all the regular 3 prong outlets downstream are in circuit with the GFCI. Reset GFCI to power back on.
  • Label all the receptacle covers. "No Equipment Ground" for the GFCI and "No Equipment Ground" and "GFCI Protected" for the regular 3 prong receptacles.
  • Repeat for all other breaker switch circuits and done!
  • Please see tips below. A few things confused me, costing me hours of troubleshooting

Took me many many hours, but I learned a lot, and everything works as intended so very happy with the results.

Tips:

  • Don't try to guess at anything! Be Safe! Think carefully about what you are doing and try to understand how it works. If it gets really confusing it’s probably time to hire a professional.

  • Be sure to test all the wires even the whites. I had one receptacle that had both the white and the black hot which indicated a problem upstream (I found bad wiring in a ceiling light upstream from the receptacle.)

  • The start of a branch is typically the closest receptacle to the Electrical Panel. But be sure to still test every single receptacle to see if there are additional branches. One thing that threw me for a loop is when I had 3 receptacles test "hot" at the same time. I thought each breaker switch was one circuit but didn't realize that the circuit can branch. In my case one circuit went from the breaker to a ceiling light, then branched 3 ways to the room upstairs so I had 3 start of branch receptacles each requiring it's own GFCI.
  • A few of my circuits branched in seemingly weird ways mostly from ceiling lights, but by testing each receptacle with all wires individually capped I was able to find all the most upstream receptacle in each branch to put a GFCI on each of them.
  • If the box only has 1 set of wires (1 black, 1 white) then it is the end of a branch. You should also still test this set of wires. I had a couple branches that only fed a single receptacle so it was the start (requiring a GFCI) as well as the end of the branch (so nothing downstream). If the box has 2 sets of wires then it has a LINE (power) and LOAD (power out). As a newbie to electrical, this helped me visualize the circuit better. Though branches coming from ceiling lights often confused the visualization. Thorough notes and diagramming also helped tremendously with understanding how things worked.

  • I will never paint over a receptacle cover again. They were a huge pain but needle nose pliers helped a bunch to rip the stuck covers off.

  • Ask for help when you get stuck. I posted my first post on this forum and got an answer in a few hours. I was about to give up the project thinking I was in way over my head, but got an answer that helped me figure everything out.
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