How to wire an electrical 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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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 ground 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.
See SAFETY for ELECTRICAL INSPECTORS
Our photo shows a conventional grounded three-prong electrical receptacle - the round hole is the ground connection - at the left end of the picture closest to my thumb. 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.
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 outlet 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 circuit 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.
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.
Details are at REVERSED POLARITY ELECTRICAL DEVICES / CIRCUITS.
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.
Watch out: do not ignore the NEC requirement to label the GFCI receptacle on a two-wire (un-grounded) circuit as
"GFCI Protected" and "No Equipment Ground."
Because some devices can be damaged if older style cord plugs are reversed on an un-grounded circuit and because some devices may depend on electrical grounding for safety, n my OPINION a better approach would be to put a GFCI breaker in the panel serving the circuit and to use only 2-prong electrical outlets on a two-wire circuit. That adds GFCI protection while at the same time it prevents fooling people into thinking that the circuit has a ground.
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.
Thanks to NECreader who commented on 2016/06/13:
This article should state that NEC 406.3 permits GFCI receptacles to replace two prong ungrounded outlets:
(3) Non–Grounding-Type Receptacles. Where attachment to an equipment grounding conductor does not
exist in the receptacle enclosure, the installation shall comply with (D)(3)(a), (D)(3)(b), or (D)(3)(c).
(a) A non–grounding-type receptacle(s) shall be permitted to be replaced with another non–grounding-type receptacle(s).
(b) A non–grounding-type receptacle(s) shall be permitted to be replaced with a ground-fault circuit interrupter type of
receptacle(s). These receptacles shall be marked "No Equipment Ground." An equipment grounding conductor shall not be
connected from the ground-fault circuit interrupter-type receptacle to any outlet supplied from the ground-fault circuit-interrupter
(c) A non–grounding-type receptacle(s) shall be permitted to be replaced with a grounding-type receptacle(s) where supplied
through a ground-fault circuit interrupter. Grounding-type receptacles supplied through the groundfault circuit interrupter shall be marked "GFCI Protected" and "No Equipment Ground." An equipment grounding conductor shall not be connected between the grounding type receptacles.
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
Thanks 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 would be unsafe.
In other words, if the circuit wiring into the junction boxes in which you ask about converting from 2-prong to 3-prong receptacles 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 receptacle'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 ground 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 vac 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 unreliable 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 designed for nor intended for secure electrical contact to serve as a grounding conductor.
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 electrode 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.
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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