How to wire an eletrical receptacle ("outlet" or "wall plug") when there are just two wires (hot and neutral) but no ground wire.
This article explains that when there is no safe grounding conductor or "ground wire" at an electrical receptacle location you need to choose the proper receptacle type and make the proper wire connections for safety.
This article series describes how to choose, locate, and wire an electrical receptacle in a home. Electrical receptacles (also called electrical outlets or "plugs" or "sockets") are simple devices that are easy to install, but there are details to get right if you want to be safe.
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Step 1: Recognize that the electrical circuit has just two wires and no electrical grounding conductor
In Carson Dunlop Associates' sketch at left the wire circuits shown at upper right and lower right are both two-wire electrical circuits where no grond wire is present. At right in the photo is the type of electrical receptacle to use on two-wire (no ground) circuits.
[Click to enlarge any image]
If no ground wire or ground path is provided, it is improper and unsafe to install a grounding (3-prong) electrical receptacle on that circuit.
Watch out: as you see in the two illustrations at the left of our sketch, a circuit with a ground wire will present a bare or green-insulated wire and there will be three wires (or more) present. The flexible metallic conduit exterior of BX cable, for example, is not a safe, usable pathway for electrical grounding.
We use the proper term electrical receptacle to describe the "wall plug" or "wall outlet" into which you will insert a two-prong or three prong plug to connect an appliance, lamp, etc. Technically in the electrical code, an "outlet" is any place in where you provide a junction box and electrical wires to which something can be connected: a light fixture or an electrical receptacle, for example.
Before doing any work on the switch, the power source must be turned off by setting a circuit breaker to OFF or removing a fuse.
Step 2: buy the right type of electrical receptacle
At right in the photo is an ungrounded electrical receptacle. This is the right device to install if no ground is present on the electrical circuit.
You don't want to "fool" a building occupant into thinking that a ground is present when there is not one, so you don't install a receptacle that has that third ground opening in its face.
Some older two-wire circuits which are covered with a flexible metal jacket ("BX" or "armored cable" wire) may provide a ground path by means of the cable jacket itself.We don't rely on it, and in event of certain short circuits it's unsafe: the exposed metal sheathing of the wire becomes live, risking a shock.
Step 3: Wire the un-grounded electrical receptacle
So where do the wires go: to which screws on the electrical receptacle (shown just above) do we connect the black wire, white wire when there is no ground wire?
On a conventional 120-volt "two pronged" electrical outlet that accepts grounded plugs (two prongs plus the rounded center ground connector prong), your circuit will have three wires:
The illustration at left shows the typical wiring of an electrical outlet or "receptacle", courtesy of Carson Dunlop Associates.
But the typical wiring instructions for receptacles include a ground wire that may not be present on your circuit - as we explained just above.
Keep in mind that while a two-wire circuit may be permitted and "legal" in some jurisdictions it is not as safe as an electrical circuit (and receptacle) that has a grounding conductor.
Let's at least not make the un-grounded and two-wire circuit / electrical outelt even more dangerous by installing the wrong receptacle type. Installing a receptacle that includes a third opening for the wall plug's ground connector is dangerous if the ciruit is not really grounded. Such as "false ground" means a false sense of safety that is not present.
Watch out: Electrical components in a building can easily cause an electrical shock, burn, or even death.
Click any image to see an enlarged, detailed view of electrical wiring details for "plugs" or electrical receptacles.
The electrical receptacle must be properly screwed to or mounted in the junction box, and the extra length connecting wires carefully pushed back into the junction box so as to avoid crimping, damage, etc.
An electrical receptacle cover plate must be installed over the finished receptacle. We like plastic cover plates better than metal as they reduce the chances of a cover plate becoming electrically "live" and thus unsafe.
Avoid These Unsafe Practices When Wiring a 2-Wire (no-ground) Receptacle Circuit
The hot and neutral wires must be connected to the proper terminals on the electrical receptacle. The "hot" or "live" black wire (or red wire) is connected to the brass-colored screw terminal on the electrical receptacle, and the "neutral" white wire is connected to the silver-colored screw terminal on the electrical receptacle.
Carson Dunlop Associates' sketch points out that the white wire, i.e. the neutral wire, will be connected through the receptacle's internal parts to the wide slot on the receptacle face in order to assure that the neutral wire side of an appliance being plugged-in there is properly connected.
Watch out: Reversed polarity on an electrical outlet is dangerous. If you accidentally reverse these wires the device you plug in to the receptacle may "work" but it is unsafe and risks a short circuit, shock, or fire.
Some appliances and some electronic equipment may be damaged if left connected to a reversed-polarity electrical circuit.
Generally, if installed on a 2-wire circuit that has no electrical ground conductor, a GFCI electrical receptacle will protect against a hot to neutral short or a hot to ground short at the receptacle but its internal test circuit cannot be used - that is, you can't easily test to know know that the receptacle is working.
Ground fault protection - GFCI's: The NEC also requires that only special ground fault circuit interrupter (GFCI) protected outlets can be installed in certain hazardous locations like kitchens, baths, garages, outdoors. A GFCI-protected electrical receptacle includes circuitry that turns the electric power off at the outlet quickly should a ground-fault (electricity flowing to earth, such as through your hand and down a water pipe) be detected. 
Arc fault protection - AFCI's: Beginning in 2002 the NEC also required arc fault protection for electrical outlets for bedrooms. 
AFCI's are similar to GFCI's discussed above, but they include an additional level of protection against fire by detecting small electrical arcing at a connection - a condition that can lead to overheating and fire.
As you can see from this US CPSC photo, you can add Arc fault protection to a home circuit by installing a special circuit breaker in the electrical panel.
By this means you can provide arc fault protection and thus improved fire safety for all electrical outlets on the circuit - for example in the building's bedrooms.
Other Proposals for Adding a Ground to a 2-Wire Electrical Circuit
I was reading your sections on grounding on older home as I am currently in the process of having several two-prong outlets upgraded. In my research, several people suggested it would be possible to ground to the box using a grounding screw.
I didn't go that route but was looking to see what your site had to say about the practice. I was disappointed to find out it wasn't even mentioned. Is there anything you can say, is it a safe practice? - Thanks. K.B. 8/5/13
Thaks so much for the question - in the article above and elsewhere I have mentioned the problem of the missing ground wire on a two-wire circuit and also the problem of unreliable ground connections through the receptacle mounting strap screw - but your question helps me see that I must not have made the point clear and easy enough to find.
When you are replacing electrical receptacles ("outlets") in an existing two-wire (hot, neutral, no ground) circuit that is in good physical condition, the only recommended and code-approved solution (short of re-wiring) that I have found is to install new two-slot (no ground prong opening) electrical receptacles in the box. [I add that if the existing two-wire circuit is a knob-and-tube installation, it is also forbidden to extend or add devices (such as more electrical receptacle outlets or lighting outlets) to that circuit.]
I take it from your message that you already understand the danger of just placing a 3-prong receptacle into an ungrounded box, offering a "faux" ground that woudl be unsafe.
Why We Don't Just Connect the 3-Prong Receptacle Ground Screw to the Metal Junction Box
In other words, if the circuit wiring into the junction boxes in which you ask about converting from 2-prong to 3-prong receptcles does not include a ground wire, do not install 3-prong outlets and DO NOT rely on just grounding the box to a new 3-prong receptcle's ground screw.
Indeed, mechanically you can sometimes create a detectable ground via such a connection even when there is no ground wire if the box is metal and the incoming wiring is metal-clad BX cable. But the groudn pathway back to the panel in that case is unsafe and unreliable for sat least these reasons:
1. Hot wire short: the exterior metal BX cable becomes electrically live in the event of a short circuit - a condition that could shock anyone happening to touch that cable exterior anywhere along its pathway, and a condition that in some cases could even start a building fire
2. Neutral wire short: similarly unsafe but more subtle is a short between the neutral wire and ground anywhere in the circuit. In this case the circuit appears to continue to "work" properly, in that lights light or a device is powered when plugged-in; but the BX exterior sheathing will be carrying the return circuit all of the time that the circuit is in use - potentially shocking someone, and again unreliable as I explain in the next point.
Working on an older home in which someone had done this I encountered exactly this situation - it's not just "theory". Turning off power to a circuit on which I was working, I tested to see that the hot wire was "dead" before touching anything. My helper, working in the same room, plugged in our shop vac to begin some cleanup, connecting the shop vack to a nearby receptacle that had a different hot wire entering it.
The box and BX cable I was working on became "live" (and shocking) when she turned on the vacuum cleaner! The BX cables from several circuits had some metal contact points in common and the neutral circuit was flowing through the BX not through the proper neutral wire (or part of the current was thus flowing).
3. Even if you didn't care about shocking someone or starting a fire, the "apparent ground" path in this case is unlreliable because it passes through a great may often loose connections (metal clips that connect each segment of BX sheathing to each electrical box) - connectors that are not designe dfor nor intended for secure electrical contact to serve as a grounding conductor.
Adding individual, properly-wired new grounding conductors, electrodes, etc. to an existing 2-wire circuit?
If on the other hand, you are considering providing anew, separate local grounding conductor and local grounding electrode to which you connect a metal electrical box, other than a ground wire that passes all the way back to the main panel, that approach might be technically possible.
We need to research the code details further about inconsistent system grounding; an example that comes to mind is the code requirement that a separate branch panel in a detached garage is often connected both to a local grounding elecrode at the garage and back to the system ground bus (and through it to the main building grounding electrode) back in the main panel.
In general I'm nervous about any home-brew wiring solutions that, even if they seem to "work", may be unreliable, may be confusing to an electrician working on the building in the future, and can certainly add confusion to troubleshooting.
Bottom line on Updating Receptacles on a 2-wire Circuit
Please let me know if I've misunderstood your question or the nature of your proposed 2-wire circuit grounding solution.
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Frequently Asked Questions (FAQs)
Question: converting 2-prong to 3-prong grounded electrical receptacles on a two-wire system
I'm changing out 2 prong recepticles to two prong + ground recepticles on a two wire system. The boxes are metal. Is it acceptable to screw a bare wire to the box with a self tapping screw and hook to the ground screw? Do I need to check to make sure the box is grounded, if so, what is the best method? - Ben 10/9/12
Watch out: no, what you propose is improper and unsafe even though it would "appear" to work. To add a grounding conductor or "ground wire" to a two-wire circuit you need to add a physical wire. The dangers of using the existing metal box and BX or armored cable for the ground path (I know that it's tempting) are several:
That path, which relies on numerous parts that can often be loose, is unreliable: the path is from the new grounded receptacvle's internal grounding connector through a rivet to a metal strap on the body of the recptacle, through mounting screws through the receptacle's mounting ears, through a threaded hole in the metal receptacle box, through a metal BX connector which itself relies on at least four parts to be secured to the metal box, through a set screw from that connector into the armored cable, and thorugh god knows how many more sets of these parts down the entire remainder of the electrical circuit.
That path also is not intended to carry current, and should it do so in an emergency, someone touching the BX exterior could be electrocuted.
Finally, the receptacle manufacturer provides a ground screw on the receptacle that is intended to be connected by copper wire to a ground wire, parts and codes being ignored if you try using the box and its wire as the ground path.
On an ungrounded electrical circvuit you should install only two-prong, ungrounded receptacles - that is code compliant and that also lets users know that there is no reliable electrical ground present.
Question: what do I do with a third white wire attached to the side of the old receptacle when I'm moving them
I have changed several receptacles in my 1994 home, but this time when removing the receptacle from the wall I see a third white wire attached to the side of the old receptacle. Can you explain to me what this third neutral wire is? - DW 6/12/12
DW, it is just too dangerous to claim to know what wiring connections someone has made in a building that is unseen and untested.
It would be common for an additional white wire connected to an electrical receptacle to be carrying the neutral line to another receptacle downstream. In other words the incoming neutral is connected to one terminal and the outgoing neutral is connected to a second screw that is electrically common with the first.
Check for a second hot wire also present in the same box.
Question: what do I do with the screws to which no wire is connected on a conventional "plug" (wall receptacle)?
At the end of a circuit, I'm only using 2 of the 4 screws on a conventional plug. What should I do with the 2 unused screws? Should they be screwed all the way in? Or left partially unscrewed? Or does it matter? - Chris Rasko 7/8/12
Don't remove the screws - it's not necessary, they are deliberately hard to remove completely, and they could be needed in some future wiring change.
Watch out: in some older installations the metal junction box is so tight that an extended, un-used wiring screw may be rather close to contacting the metal sides of the box. This can be particularly dangerous (risk of a short circuit) if down the road the receptacle becomes a bit loose in the box. I would screw the un-used screws all the way in to reduce this risk. I've also seen cautious electricians wrap the whole receptacle sides with electrical tape before pushing it back into a small box. One wonders if at that point, if we're that worried, we ought not to be installing a larger junction box.
Question: AmI allowed to add one more outlet onto an existing string?
I have an existing outlet being used for lamps I wanna run one more outlet shares from the hot on is it okay? - PHantum 113 8/1/12
If the circuit is overloaded already, no.
If the circuit is knob and tube wiring, no - we don't extend knob and tube.
Question: I'm installing a box extension but the top screw won't hold - it's stripped
I started installing a box-extender on a receptacle in my kitchen because I'm tiling my backsplash and need to raise the outlet above the tile. However, the top screw connecting the outlet and box wouldn't hold. I spent way too much time bent double under my cabinets trying to get it to bite, but when I finally gave up and pulled it out it was stripped at the tip (which was as far as it'd go in). I'm sorry to bother you with triviality, but I'm new to home renos and don't know what to do. Advice? - Julia 1/29/13
Reply: step by step tips for replacing a stripped electrical receptacle or switch box mounting scrrew or screw opening
Iif the problem is the screw itself is stripped, simply purchase a replacement screw or a handfull of them from your elecrical supplier. These screws are a standard thread and length, but longer versions are available at any hardware store.
For the case you describe, if the stripped problem is the mounting hole you'll need to either enlarge and tap the hole for tne next size larger screw, or purcase a clip-on adapter that slips over the stripped ear through which the original hole passed.
Taking care to move electrical wires out of the way of your drill bit, in a metal electrical box you can drill out the 6/32 screw opening to tap and accept an 8/32 screw.
For photos and step by step details on how to repair stripped electrical outlet mounting screws, see OUTLET BOX SCREW REPAIR.
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