What is the difference between 2 and 3 pronged outlets and do I need 3?

August 15, 2006 4:02 PM

What is the difference between 2 and 3 prong outlets and do I need 3?

I am renting an apartments with a friend and have the first choice of rooms. One room is better than the other but I noticed that it only has 2 pronged outlets. The other room (right across the hall) has three pronged outlets. I really want to take the one with 2 pronged outlets but am not sure what practically speaking that means to me. I am going to be plugging in computer(s), an air conditioner etc. I have a surge protector but I believe that the SP has three prongs on it so I am going to have to get an adapter (from 3 to 2). What exactly does the third prong do? Do I need it for this situation?

(Also, someone suggested to me that even the three pronged outlets were just faceplates but not really grounded. It is a really old building so he could be right. Is it possible for the same apartment to have one room with grounded outlets and one without?)

posted by D Wiz to home & garden (36 comments total)

It's a ground connection. Don't ask me what that means in theory.
posted by PowerCat at 4:10 PM on August 15, 2006

Do not use an adapter to plug a 3-pronged plug into a 2-pronged outlet. When you do this, you are not grounding your appliance. This is dangerous, because if the appliance shorts out, the case on it will carry dangerous voltage, instead of this voltage travelling to the earth over the third prong.

You can use a GFCI to lessen this risk.
posted by odinsdream at 4:12 PM on August 15, 2006

It's a rented apartment. I'm pretty sure I can't do any work to it myself... Needs to go through the landlord...
posted by D Wiz at 4:16 PM on August 15, 2006

odinsdream, my understanding is that the 3-to-2-prong adapters with the hoop-like appendage can actually ground the appliance IF the mounting screw for the outlet passes through the metal hoop when it's tightened.
posted by infinitewindow at 4:18 PM on August 15, 2006

Appliances with a metal case, such as a toaster oven, will generally have the third prong (ground). This means, among other things, that if an internal malfunction or damage results in a live wire touching the case, you won't get fried when you touch the oven's metal exterior. Same goes if you're trying to kill your wife by dropping the oven into her bath.

Some things use the ground for functional reasons (eg some radio equipment needs a ground as part of the antenna), but that's pretty uncommon. It's basically extra safety.

I'd suggest taking the room you like and running a 3-prong extension cord to a 3-prong socket elsewhere in the house, so you can run your computer, and use the 2 prong socket for 2-prong appliances only. Or have the 2 prong socket rewired with a 3-prong - the ground cable might be in the box behind the socket, just not hooked up. If it isn't already there, then it's a bigger job, since the extra wire needs to be put into the wall.
posted by -harlequin- at 4:19 PM on August 15, 2006

Thanks for the great explanation harlequin.
posted by PowerCat at 4:25 PM on August 15, 2006

infinitewindow; stuff is grounded when there is a physical connection between the third prong and the ground, as in, outside ... dirt. The screw in your faceplate cover probably goes into a plastic socket, which is useless in grounding.

If you want to actually ground something that isn't already grounded, you need to physically wire that third prong to something that leads out of the house and goes into the ground. In places with fully galvanized or copper water lines, you can safely wire this third prong to any plumbing. If you have PVC, you cannot do this.
posted by odinsdream at 4:26 PM on August 15, 2006

IF the mounting screw for the outlet passes through the metal hoop when it's tightened.

AND if the Box is grounded ( a green wire from the 3 wire feed will be screwed to the box somewhere in there. Take off the plate and use a flash light to find it). No green wire means no ground.
posted by sgobbare at 4:30 PM on August 15, 2006

OdinsDream is mostly right. Actually, most boxes are made of metal, and that's what the screw goes into. But if the box is floating, it isn't a ground.

There does need to be a connection to real earth ground, and it needs to be heavy enough to carry many amps. Otherwise when you need it most it's going to blow like a fuse, maybe starting a fire, and probably not protecting you against electrocution. (And of course if it's not there in the first place it can't protect you.)

That must be a pretty damned old building, because true 3-wire electrical outlets have been required by the building code since I was a kid.
posted by Steven C. Den Beste at 4:40 PM on August 15, 2006

It isn't possible to create a 3-wire outlet just by replacing the face plate. But it might be that someone rewired the box to replace the old 2-wire outlet with 3-wire outlets -- and didn't actually connect anything to the third wire. If so it violates code, but that's been known to happen.
posted by Steven C. Den Beste at 4:47 PM on August 15, 2006

D Wiz, where are you? UK? Canada? Australia? Outer Mongolia? These things vary depending on the location. In the US the building code varies by city and state.
posted by Steven C. Den Beste at 4:56 PM on August 15, 2006

You can buy a small tester at a hardware store that will verify wiring on three prong outlets. The shorter of the slots is Hot, the longer Neutral, and the D-shaped one is Ground.

The little tester will verify with lights that proper wiring of the three lines and whether the ground is floating.

In the US, the hot lead is nominally 120 V AC, RMS (about 170 V AC Peak. Neutral is nominally at ground potential, but since it carries the return currents, a small voltage is measured there, proportional to the distance back to your distribution box and the amount of current flowing. In other words, it is not at ground.

What all this means is that if you have two devices plugged into two different outlets AND the outlets are miswired (or faulty) AND their outer conductive parts are touching these wires, AND you simultaneously come into contact with the two, you can apply a voltage across your own sweet self and current will flow. No big deal if it's just between your thumb and forefinger, but between your ears or opposing hands and you could have a fatality.

The scenarios seem unlikely, but I lost a distant uncle this way, sort of.

Most modern appliances are double insulated, and since vacuum tubes aren't all that popular any more, it's rare to see a chassis wired hot, but old tube radios and TVs.... it was fairly common.

Best thing to do is hire an electrician to run a three wire run back to the fuse box. Second best thing... hire an electrician to verify the wiring of the two wire outlets (and check the three wire ones).

It's not illegal to have two wire outlets... but in most municipalities these days, any repairs have to upgrade the circuits to three wire.
posted by FauxScot at 5:01 PM on August 15, 2006

(Also, someone suggested to me that even the three pronged outlets were just faceplates but not really grounded. It is a really old building so he could be right.

It wouldn't be surprising. I've noticed that two-prong outlets are a little more expensive (go figure).

Is it possible for the same apartment to have one room with grounded outlets and one without?)

Possible? Sure. If there had been some electric work done on the place, or an addition, then the new work probably would have been done under current code, leaving the old stuff in place. But I think the ungrounded 3-prong outlet is more likely.

If you're really curious, you can buy an outlet tester for about $20 (?) that will show whether the outlet is grounded, whether hot and neutral are reversed, etc.
posted by adamrice at 5:05 PM on August 15, 2006

Thanks for all the responses...

From what I understand, if I use a 3 to 2 prong adapter then I am risking electrocution and/or a fire? Why/How does this happen? I don't fully understand from the explanations above. Can it damage my computer too or will the surge protector take care of that (if I am plugging a surge protector into a 3 to 2 adapter).

BTW for those who wanted to know I am in the USA.
posted by D Wiz at 6:14 PM on August 15, 2006

Appliances with a metal case, such as a toaster oven, will generally have the third prong (ground). This means, among other things, that if an internal malfunction or damage results in a live wire touching the case, you won't get fried when you touch the oven's metal exterior. Same goes if you're trying to kill your wife by dropping the oven into her bath.

Some things use the ground for functional reasons (eg some radio equipment needs a ground as part of the antenna), but that's pretty uncommon. It's basically extra safety.

This is why you should not use a 3 to 2 prong adapter.
posted by stew560 at 6:28 PM on August 15, 2006

You'll need to learn more about grounding in general if you want to better understand the process beyond "warning - may catch fire or kill you."

To start, you can read about electrical grounding:

"Grounding is primarily used for safety to prevent electric shock or fires caused by a voltage potential between the earth and a conductor such as an appliance cabinet or chassis. Grounding is often used to conduct lightning strikes harmlessly to earth rather than starting fires and damaging equipment. It is also used to control electrical noise in computer, audio and video, and communications circuits. This illustrates that an electrical ground should have an appropriate current-carrying capability in order to serve as an adequate zero-voltage reference level."

That bit about noise is something I neglected to mention, and hasn't been brought up yet. Aside from the safety issues, you'll probably notice buzzing in your speakers if they aren't properly grounded along with the equipment they're plugged in to.
posted by odinsdream at 6:39 PM on August 15, 2006

Ok... So I went back to the apartment and found in the 2 pronged room one of the outlets already has a 3 to 2 with the loop around the screw as someone here said to do... So can I assume that if that was already done when I moved in that there is a grounding wire attached to the whole box and that it is safe to plug 3 pronged items into it? Can I just do the same thing to the other two outlets in the room? (ie. buy a 3 to 2 and unscrew the faceplate and put the loop around the screw and screw it back in) Also, can I now plug a 6 outlet surge protector into the 3 to 2 (which is grounded it seems) and plug all my stuff into there?
posted by D Wiz at 7:42 PM on August 15, 2006

D Wiz, where are you? UK? Canada? Australia? Outer Mongolia? These things vary depending on the location. In the US the building code varies by city and state.

Steven, Australia is 100.0% three prong. This two prong idea sounds quite dangerous to me. I would like my electricals grounded.
posted by wilful at 7:56 PM on August 15, 2006

D Wiz, as has already been pointed out, attaching the little adapter to the screw is not grounding. Grounding is accomplished by hard-wiring that part to the plumbing or to the mains switchbox in, say, the basement.

You need to actually open up the outlet and investigate to even begin to understand whether this has been accomplished, and you really should invest in an electronic circuit tester to be sure.
posted by odinsdream at 8:26 PM on August 15, 2006

First thing to do is ask the landlord to install a grounded outlet in the room that doesn't have them. This is a very quick and inexpensive task. I'd expect a good landlord to agree.
posted by winston at 8:29 PM on August 15, 2006

I think that wikipedia description sucks..

Here is what the electrical wiring FAQ has to say:

What's the purpose of the ground prong on an outlet, then?

Apart from their use in electronics, which we won't comment on, and for certain fluorescent lights (they won't turn on without a good ground connection), they're intended to guard against insulation failures within the device. Generally, the case of the appliance is connected to the ground lead. If there's an insulation failure that shorts the hot lead to the case, the ground lead conducts the electricity away safely (and possibly trips the circuit breaker in the process). If the case is not grounded and such a short occurs, the case is live -- and if you touch it while you're grounded, you'll get zapped. Of course, if the circuit is GFCI-protected, it will be a very tiny zap -- which is why you can use GFCIs to replace ungrounded outlets (both NEC and CEC).

There are some appliances that should *never* be grounded. In particular, that applies to toasters and anything else with exposed conductors. Consider: if you touch the heating electrode in a toaster, and you're not grounded, nothing will happen. If you're slightly grounded, you'll get a small shock; the resistance will be too high. But if the case were grounded, and you were holding it, you'd be the perfect path to ground... [emphasis added]

If you don't understand the bolded section (or anything else, really) I'm sure we can clarify further.

Third prong ground is not intended as a fire protection measure, as far as I can tell. However, I'm sure you could imagine a convoluted scenario where the existence of ground could help prevent a fire..

The "use in electronics" mentioned in the quote above is surge suppression and electromagnetic interference (EMI) filtering. The FAQ has a section that addresses this:

Surges, spikes, zaps, grounding and your electronics

Theoretically, the power coming into your house is a perfect AC sine wave. It is usually quite close. But occasionally, it won't be. Lightning strikes and other events will affect the power. These usually fall into two general categories: very high voltage spikes (often into 1000s of volts, but usually only a few microseconds in length) or surges (longer duration, but usually much lower voltage).

Most of your electrical equipment, motors, transformer-operated electronics, lights, etc., won't even notice these one-shot events. However, certain types of solid-state electronics, particularly computers with switching power supplies and MOS semiconductors, can be damaged by these occurances. For example, a spike can "punch a hole" through an insulating layer in a MOS device (such as that several hundred dollar 386 CPU), thereby destroying it.

The traditional approach to protecting your electronics is to use "surge suppressors" or "line filters". These are usually devices that you plug in between the outlet and your electronics.

Roughly speaking, surge suppressors work by detecting overvoltages, and shorting them out. Think of them as voltage limiters. Line filters usually use frequency-dependent circuits (inductors, capacitors etc.) to "tune out" undesirable spikes - preventing them from reaching your electronics.

So, you should consider using suppressors or filters on your sensitive equipment.

These devices come in a very wide price range. From a couple of dollars to several hundred. We believe that you can protect your equipment from the vast majority of power problems by selecting devices in the $20-50 range.

A word about grounding: most suppressors and EFI filters require real grounds. Any that don't are next to useless.

For example, most surge suppressors use MOVs (metal oxide varistors) to "clamp" overvoltages. Yes, you can have a suppressor that only has a MOV between neutral and hot to combat differential-mode voltage excursions, but that isn't enough. You need common-mode protection too. Good suppressors should have 3 MOVs, one between each pair of wires. Which means you should have a good solid ground. Eg: a solidly connected 14ga wire back to the panel. Not rusty BX armour or galvanized pipe with condensation turning the copper connection green.

Without a ground, a surge or spike is free to "lift" your entire electronics system well away from ground. Which is ideal for blowing out interface electronics for printer ports etc. Secondly, static electricity is one of the major enemies of electronics. Having good frame grounds is one way of protecting against static zaps.

If you're in the situation of wanting to install computer equipment on two wire groundless circuits take note:

Adding a GFCI outlet to the circuit makes the circuit safe for you. But it doesn't make it safe for your equipment - you need a ground to make surge suppressors or line filters effective.

Personally, I don't think surge suppression is particularly critical. It will depend a lot on how the power is where you live.

The reason computers all use third prong ground.. They have to pack a lot of electronics in that little power supply box, double insulating would waste a lot of space and add a lot of cost. Also, ground may help them meet EMI standards with fewer/cheaper components. In the end though, I expect it is mostly about "Having good frame grounds is one way of protecting against static zaps." and tradition. You do know about computers and static electricity, right?

Finally, the 'solution' to your problem, from the electrical wiring FAQ again:

How do I convert two prong receptacles to three prong?

Older homes frequently have two-prong receptacles instead of the more modern three. These receptacles have no safety ground, and the cabling usually has no ground wire. Neither the NEC or CEC permits installing new 2 prong receptacles anymore.

There are several different approaches to solving this:

1. If the wiring is done through conduit or BX, and the conduit is continuous back to the panel, you can connect the third prong of a new receptacle to the receptacle box. NEC mainly - CEC frowns on this practice.
2. If there is a metallic cold water pipe going nearby, and it's electrically continuous to the main house ground point, you can run a conductor to it from the third prong. You MUST NOT assume that the pipe is continuous, unless you can visually check the entire length and/or test it. Testing grounds is tricky - see "Testing Grounds" section.
3. Run a ground conductor back to the main panel.
4. Easiest: install a GFCI receptacle. The ground lug should not be connected to anything, but the GFCI protection itself will serve instead. The GFCI will also protect downstream (possibly also two prong outlets). If you do this to protect downstream outlets, the grounds must not be connected together. Since it wouldn't be connected to a real ground, a wiring fault could energize the cases of 3 prong devices connected to other outlets. Be sure, though, that there aren't indirect ground plug connections, such as via the sheath on BX cable.

The CEC permits you to replace a two prong receptacle with a three prong if you fill the U ground with a non-conducting goop. Like caulking compound. This is not permitted in the NEC.

The NEC requires that three prong receptacles without ground that are protected by GFCI must be labelled as such.

I'm kind of torn about GFCI being equivalent to third prong ground - the protection is quite different - but who am I to argue..

In the end, if you don't want to bug your landlord about the electrical, at least get a GFCI power strip. Yes, I know it is still three prong.
posted by Chuckles at 8:59 PM on August 15, 2006 [1 favorite]

No, you can't make that assumption. If you want to know for sure if things are grounded then spend ten bucks and get a tester. You can then check both the three pronged outlets and the adapters for ground.

The cable that comes into the back of the outlet box has either two or three wires. Two wires is old school. The third in new(er) installations is the ground.

If the cable coming into the box has only two wires there is no ground. You can still install a three pronged receptacle into such a box - it fits and will provide power - but it's not grounded. You could also install a two pronged receptacle and an adapter which will look nice - but there is still no ground.

If you have a three wire cable coming into the box the ground wire is usually attached to the metal box and the box is then grounded.

If you then install a two pronged receptacle into the grounded box, the outlet will be connected to the ground at the mounting points. The idea behind the adapter is that the grounding wire(loop) will be connected to the ground through the screw which passes through the faceplate and into the body of the outlet where it connects to a metal plate which extends the length of the receptacle right to the mounting points which are connected to the box which is grounded.

There are a couple of problems here. In some crappy/old receptacles there will be no metal in the screw hole which is actually connected to the mount points and to the ground. Sometimes the screw used is the one that came with the faceplate and it is covered in enamel paint to match the colour of the faceplate. If that's the case it may be a very poor conductor and you won't have a good or any connection to the ground. And if the ground wire was not connected to the box properly or if there is no ground wire at all then your adapter is useless.

In the best case scenario you have a three wire cable coming into the box, the ground is connected to the metal box and directly to the three pronged receptacle. And of course the ground wire also has to lead back to actual "ground."

It's not possible to simply look at your set up from the outside and say, "Yep, there's an adapter and it looks like it's hooked up right so we're OK." The easiest way to be sure is get a little ground tester.

Opening up the outlets to investigate on your own is not generally a good idea.
posted by shoesfullofdust at 9:14 PM on August 15, 2006

I agree that for safety, you want a plug circuit tester. Putting the faceplate screw through the ground loop on the 2-to-3 adapter can work if the box is grounded via conduit, but YOU HAVE TO TEST IT.

My house was built in 1949 and I've had to systematically replace sockets by drilling into the box, tapping threads, putting in a ground wire, and connecting it all to a new receptacle.

Again, I suggest getting a tester.
posted by Argyle at 10:24 PM on August 15, 2006

I just bought an old home(circa 1935) so i sympathize completely. We just had an electrician friend install new outlets and leave the ground disconnected. I'm watching this thread for a better idea.
posted by j_gd00 at 11:20 PM on August 15, 2006

j_gd00, what was the point of the new outlets? If they're 3-prong outlets, you're misleading future buyers (and current guests) of the house into thinking the outlets are grounded. If your state requires home inspection as part of selling a house, you're going to have to rewire it as soon as the inspector sees the outlets aren't grounded. Even if there's no home inspection, you'll be on the hook if there is some future accident or fire involving those outlets.
posted by Kirth Gerson at 8:13 AM on August 16, 2006

We just had an electrician friend install new outlets and leave the ground disconnected

I am surprised that a licensed electrician would blatantly violate code like that and at the same time expose his friends to safety/liability issues.

There is a lot of good information about grounding in this thread.
posted by TedW at 8:33 AM on August 16, 2006

A cautionary tale: I was getting shocks from my computer/music setup. Thanks to some helpful AskMeFi advice, I got the landlord to bring in an electrician, who discovered that they had never bothered to connect the ground to the outlets. He was able to wire it up correctly without having to open up the walls, and the problem disappeared.
posted by fuzz at 10:23 AM on August 16, 2006

OK... so I bought the tester and a couple of 2 to 3 prong adapters. As I mentioned before there is a 2 to 3 already installed with the loop around the screw in one of the outlets in the room. I stuck the tester in it and it lit up and the readout lights said hot/neu reverse... Will this be a problem? I then took one of the 2 to 3 adapters and WITHOUT the loop around the screw tried it and it lit up to say open ground. So I guess the box is grounded and if I put the loop around the screw it should be grounded properly. I'll try it later tonight. In the meantime, will the hot/neu reverse be a problem? Practically speaking, what does that mean?
posted by D Wiz at 1:24 PM on August 16, 2006

Open ground apparently means ungrounded, so don't leap to any conclusion that anything is grounded. You need to find out what readout of the tester means "this outlet is grounded properly", and then only use the outlet with a three-prong plug if your tester gives you that result.

We don't want to see you hurt yourself. Don't make any assumptions about safety, because this isn't something you can afford to guess wrong about.
posted by Mr. Gunn at 4:20 PM on August 16, 2006

Well, if the tester indicated it was grounded with the adapter, then the box apparently is grounded, probably through conduit or armored cable. How well grounded is unknown, but it is better than nothing. If hot and neutral are reversed, just remove the adapter, turn it upside down and plug it into the other outlet of the pair on the receptacle so you can still connect the grounding screw. I'm assuming that the wide and narrow blades on the adapter don't prevent you from doing this.

Most electrical devices do not care about reversed neutral and hot, except that often the device's switch is intended to be on the hot side to prevent any current going downstream in the event of a fault in the device. This could be a minor safety issue.

This may be heresy, but we lived without grounded outlets for over 50 years until they became required by the electrical code in the early 60's. Yes, if a fault in the device occurs you could be in danger, but when was the last time you or anyone you know had an electrical fault in a device that electrified the housing? Could it be as rare as being struck by lightning?
posted by JackFlash at 5:19 PM on August 16, 2006

Not all that rare. I can remember about three devices I had that gave me the tingles, back when I lived with ungrounded outlets. I may be wrong, but I think it would be more of a risk with ungrounded outlets that have hot and neutral reversed.

D Wiz, if you know where the fusebox (or circuit-breaker box) is, you can disable the electricity to the outlet and then swap the two wires to it. Use the tester before you take the cover plate off the outlet to make sure the power is off.
posted by Kirth Gerson at 4:08 AM on August 17, 2006

Jackflash... I can't turn it upside down, the prongs won't let me. It only fits in one way. I don't know much about electricity. That's why I am asking...

Kirth... I may just try that...
posted by D Wiz at 7:42 AM on August 17, 2006

Which wires do I switch to undo the hot/neu switch? There are four wires in the box. One red, one black, and two whites and they are hooked up to screws. Red and black on one side and two whites on the other...
posted by D Wiz at 12:01 PM on August 17, 2006

Try swapping the white ones to the screws on the other side of the outlet, and the red and black ones to where the white ones were. One of the white ones and one of the other ones go on to a different outlet or switch. The polarity of that device will not be affected by what you do to this outlet.

After you swap the wires, restore power, reconnect the pigtail in the two-to-three adapter, and see if your tester says all OK. If it gives you a different error code, it's time to find an electrician.

There is a diagram on this PDF. Scroll down to where it says "Receptacles," just above the U-13 page marker.
posted by Kirth Gerson at 1:13 PM on August 17, 2006

It should be reiterated: Open Ground means Not Grounded.
posted by odinsdream at 4:24 PM on August 17, 2006

Tingles from non-earthed equipment..
.. are common, it does not necessarily indicate a fault. I still have some questions about it myself, but here is an article on the topic, I'm shocked, why am I getting a tingle?

Basically, even though the device is double insulated and the exposed inputs and outputs are electrically isolated, there is still some leakage current to the AC line. When you touch something that is earthed, like the shield of the television cable, and exposed metal (especially input/output terminals) you will get a shock. It may be so minor that you don't even feel it.. The topic has come up here a couple of times: Eject the core, A DVD player, a TV, composite cables, and a spark.

How to switch the wires.
You've indicated that there are at least two white, one black, and one red wire in the outlets electrical box, but we need to know how those wires combine to form the cables that lead to that box. Technically red should be something else, in the context of an outlet you would expect it to be the other phase.

Odds are there are two cables, one with black, white, and possibly a bare copper ground wire, the other with red, white and again possibly a ground. You can just barely make out what I'm talking about in this drawing:

There are two cables, one entering from the top and the other from the bottom. The two whites, one from each cable, are joined to a white pigtail by a wire nut, the white pigtail is connected to the outlet. This is repeated for the other colours.

That pic comes from this guide to replacing an outlet, which is brief but adequate.

Here is a drawing of just an outlet.

You can see that the left side has two silver terminal screws, these are for neutral (white), and the right has two brass/copper terminal screws for live (typically black). Screw colours on your old outlet may not match.. What you need to do is swap sides so that the neutrals are going to the correct pair of screws, etc.. (If the red did indicate the other phase, one of the "break out fins", as indicated in the second drawing, would be gone).
posted by Chuckles at 4:52 PM on August 17, 2006