Is it really so bad to plug an air conditioner into a 2-prong outlet with an adapter?

July 30, 2006 3:57 PM

Is it really so bad to plug an air conditioner into a 2-prong outlet with an adapter?

The manual says don't do it. My optimal AC location mandates it. And I'm stuck in the middle. What's the danger? Alternately, I could use an extension cord (we're talking about 25') which is also, canonically, a no-no. What's the danger of that? Which is worse?

posted by TonyRobots to home & garden (30 answers total)

get the outlet grounded. it won't cost much. if you're renting, your landlord may be obligated to do it for you.

don't mess with AC. it means business.
posted by fourcheesemac at 4:12 PM on July 30, 2006

It's fine, right up until the moment it isn't.

The danger of running it from an ungrounded outlet is that, in the case of a breakdown of the electrical insulation to the frame, you get line voltage on the frame of the unit, with no reliable way of blowing the circuit protection device, typically a fuse or circuit panel breaker. So the risk profile on that would like:
OK>OK>OK>OK>OK...maybe OK a long time>OK>OK>DEAD!

An extension cord is not recommended because the length of the run, and the wire size of most extension cords creates low voltage for the appliance, which can cause overheating, and there is a fire danger, too, from the extension cord itself, if its wire gauge is too small:
OK>OK>OK>OK>OK...maybe OK a long time>OK>OK>FIRE!

"Which is worse?" Do you fear dying more by electrocution or fire?

Get the outlet changed. You can do it yourself if there is a ground leg in the box, for $3 and 10 minutes of your time. Or an electrician can do it, for $50 t0 $100, unless he has to pull grounded cable from the service box, in which case, it will cost more.

But if I absolutely couldn't do that I might get an extra heavy duty 3 conductor 15 amp outdoor extension cord (one of those safety orange or yellow jobs with a single outlet on the end) and go that way. If I were a prayin' man, that is.
posted by paulsc at 4:17 PM on July 30, 2006 [1 favorite]

Probably nothing bad will happen, but probably nothing bad will happen if you never wear your seatbelt.

I'd think a properly grounded extension cord, rated for the air conditioner load, and set up safely would be a bit better, but I'm not an expert.

Alternatively, power outlet cases and faceplates are supposed to be grounded, which leads to the proper, safe use of the grounding adapters which hardly anyone practices: You remove the faceplate screw in the center, and replace it but through the hole in the adapter.

But if your wiring is bad, the faceplate may not be grounded, and you get nothing. You'd need equipment to tell if the case is grounded.
posted by TheOnlyCoolTim at 4:18 PM on July 30, 2006

The kind of ground lug adapters TheOnlyCoolTim mentioned are OK for light loads, if you install them as he explained, but I've never seen one rated for 15 amp loads, like most motor driven appliances, such as air conditioners.
posted by paulsc at 4:22 PM on July 30, 2006

The danger of not having ground is that an internal fault may make the case live, and electrocute you.

The danger of an extension cord is that the cord may overheat and melt causing a fire hazard. The gauge of an extension cord is typically less than that of household wiring. Also, the cord can develop 'weak' spots from flexing and from people stepping on it. Finally, the plug on a cord is sometimes built more cheaply which can lead to overheating.

It is often suggested that you put a ground fault outlet where you need ground but don't have it. While it may be good enough (I'm not sure), it is not at all equivalent. Alternatively you could use a very heavy gauge extension cord (12 or 14 gauge), in good condition, and carefully tucked out of the way of traffic. Perhaps even mounted up on the wall with cable clips.

In any case, make absolutely sure the circuit you are plugging the air conditioner into can handle the added current safely.
posted by Chuckles at 4:31 PM on July 30, 2006

We own an older (1965) home with all ungrounded recepticles, and a few weeks ago had GFI recepticles installed in 14 locations so they would accept 3-prong plugs without using an adapter. GFI's are okay for small loads (alarm clock, telephone, computer) but I wouldn't trust them for AC.
posted by davcoo at 4:32 PM on July 30, 2006

They do make extension cords that are capable of handling the load an AC requires. If there's an outlet in the room that can handle it, that's probably the easiest and cheapest thing to do.

Not having the AC grounded could be really really bad. They ARE exposed to elements, including rain. In case something happens you NEED it to be grounded, or it could kill you or burn your house down. Or both....

'Course, if you're dead, you won't care about the house burning down ;-)
posted by jaded at 4:35 PM on July 30, 2006

Don't worry about it, too much. If you do worry, put a GFI plug in-between. That takes care of the issue of you getting shorted to ground through the unit issue. You can find these at Home Desperate, in the outdoor extension cord area, advertised for use in outdoor outlets for electric leaf blowers and the like.
posted by caddis at 5:00 PM on July 30, 2006

For the record, it is worth checking out how GFCI works. Also worth reading the electrical wiring FAQ.

The problem with using ground fault outlets as an alternative to ground is complicated..

GFCI is normally used in locations were people may be in incidental contact with ground already. This can be copper/iron pipe, the chassis of a stove, wet earth, or whatever. In these cases, if you become exposed to live, the interrupter will trip before harm can come to you.

This situation isn't really the case in a living room, for example, where contact with ground is unlikely (there is the sheath of the television cable, which would be enough..). On one hand, the need for the third prong ground in that kind of location is greatly reduced - electrocution would require you to complete a circuit to ground, which would often require two failed devices. However, the benefit of GFCI is also greatly reduced - those two devices are probably plugged into the same GFCI, so it would not detect the problem.

For the air conditioner, there are other potential paths to ground. The metal frame of the window, the aluminium siding, or the wet wood/brick siding, etc. So..

GFI's are okay for small loads (alarm clock, telephone, computer) but I wouldn't trust them for AC.

Sorry, but this isn't correct.. For one, most small loads only require two prong plugs. More generally though, GFCI does a very specific thing (see how stuff works link above), and the size of the load doesn't have an effect.
posted by Chuckles at 5:00 PM on July 30, 2006 [1 favorite]

for what it is worth, I think Chuckles is an electrician, correct me if I am wrong Chuckles.
posted by caddis at 5:18 PM on July 30, 2006

I'm a renter, and landlord cooperation is out of the question. He is selling the building, and a freak, and is not going to help. I can't wait for the new owners to take care of this -- we're talking about 105 degrees next week.

What's a GFI plug?

I fear fire more than electricution. Electricution requires me to touch something, and I'm rarely home, while fire can happen any time. But, of course I would get a serious extension cord, rated for more than the AC is drawing. (It's an <9 amp draw btw.) br>
So should I just string a big orange cord to the good outlet? Chuckles, I quite appreciate the information in your answer, but I'm too dumb to be able to apply it to my situation. (Yes, I read both links.)

I live in an old NYC building. What are the chances that connecting the screw through an adapter on a 2-prong outlet actually grounds anything?
posted by TonyRobots at 5:43 PM on July 30, 2006

No, I am an electrical engineer though. Mostly electronics and control systems, not power systems and safety standards. I find it really funny how nobody ever clicks through to user's profile pages :P

In a way that is bad - a little knowledge is dangerous, and all that (which isn't a notion I completely agree with, but it isn't complete bunk either). There are a lot of interacting areas coming into play, and safety standards have been written based on extensive field experience as well as fundamentals. Fundamentals alone are not nearly enough when you are trying to compare something that happens once in 10,000 installed years versus once in 100,000 (installed years? MTBF, if you like, or whatever you want to call it).

How to know when you can tell somebody with little electrical knowledge "yes that is safe" or "no it isn't safe" is a very hard problem :)


But, back to the problem at hand.. There are a couple of funny things going on with this exact question - the idea that GFCI can replace a grounded outlet, and the idea that extension cords and power bars are only for temporary use. The GFCI thing is probably reasonable - as I've mentioned the details are a bit funny - regardless, it is becoming officially accepted (see the electrical wiring FAQ). The notion that extension cords and power bars are for temporary use only is officially entrenched, even though they are used in permanent fixtures 24 hours a day all over the place.
posted by Chuckles at 5:49 PM on July 30, 2006

One example of a GFCI plug. A GFCI or GFI outlet can be had at Home Desperate for less than ten bucks, but must be wired into the wall. This thing plugs into an existing outlet. Frankly, I think you can find one for far less money than this list price. For what it is worth, I would skip it myself. You are not likely to get smoked by your AC and these things will not prevent too many fires. They occur in your wiring more often than in your appliances.

Sorry Chuckles. I am an engineer too, at least by schooling. I still think the electricians, the guys who do this every day, have it all over us theoretical types.
posted by caddis at 6:04 PM on July 30, 2006

Extension cords have Amperage ratings. If you get an extension cord rated for 15 Amps, and you plug it into an outlet which is connected to a 15 Amp breaker (as most outlets are, some few are 20Amp or 10Amp), then you really should be fine with a 9 Amp AC unit.

If you get an extension cord with 14 Gauge wire, the main difference between the extension cord and the wiring in the wall is that the extension cord is stranded wire instead of solid. This will derate the extension cord a little, but it keeps the cord flexible. As I said, the extension cord will have a rating on it. If you stay well away from reaching that rating, then don't worry so much. Check the cord's rating, make sure it is well over what you need, and plug nothing else but the AC into it.

If you want a sanity check, then run the AC for a while (an hour or two) while you are around monitoring the situation, then feel the cord with your hand. Is it hot? If it's warm, you should get a beefier cord. Otherwise, don't worry. If you damage the cord (you like to practice throwing knives around, dont you???) then replace it right away.

Oh, and the extension cord can be tripped over. I actually think this is the main reason why extension cords are frowned upon (well, ok, that and people buying thin little cords and overloading them...). I think you can evaluate your risk of tripping without help ;-)

You can use the extension cord until you get a new landlord, then see if you can get the outlet installed closer to the AC.
posted by PJensen at 6:28 PM on July 30, 2006

So should I just string a big orange cord to the good outlet? ... I quite appreciate the information in your answer, but I'm too dumb to be able to apply it to my situation.

I really hate giving people excuses not to think.. :P If you can't glean enough information from the various resources to have confidence in your own decision, you should seek help from somebody in person!

However, I really do sympathies with the problem :)

What's a GFI plug?

Err.. GFI and GFCI are the same thing.. Are there parts of the how stuff works article you don't understand?

I fear fire more than electricution. Electricution requires me to touch something, and I'm rarely home, while fire can happen any time.

I can't really agree with this thinking.. Electrocution takes a fraction of a second, but it might take several hours of operation to build up enough heat to start a fire (hell, it could take several years of wear, followed by hours of operation, etc..). They are very different types of hazard, and I don't think we should be comparing the riskiness of one or the other (I'm sure paulsc didn't mean to imply that it was a choice)..

I live in an old NYC building. What are the chances that connecting the screw through an adapter on a 2-prong outlet actually grounds anything?

Even if the screw is grounded, I wouldn't do it. The connection has to be able to handle more than the current that the item is fused for, or your screw and wire will just melt and you may as well have done nothing. Anyway, you can't be talking about chances when wondering if a screw is grounded (hopefully you didn't mean it this way).. You are either going to test it and know, or you don't rely on it.



How to decide..

For the record, paulsc's answer is great. I'm slightly less worried about a long extension cord than he is, though.

What you really need to know is everything else going on with the circuits you are considering..

A circuit that is already running more than one medium load (TV, couple of light bulbs, computer) or any heavy loads (toaster, kettle, iron, really large TV, really high power computer + monitor), or a circuit that is fused lightly (or has thin / old wiring), is no good for your air-conditioner regardless of the questions you initially asked.

Have you ever blown a fuse/breaker on either of those circuits? Which outlet is fused higher?

I might even open up the outlets and inspect the state of the wiring..
posted by Chuckles at 6:46 PM on July 30, 2006

Sorry, but this isn't correct..
I appreciate the corrected information, Chuckles. Will ask our electrician why he told us otherwise the next time we need him.
posted by davcoo at 6:48 PM on July 30, 2006

Will ask our electrician why he told us otherwise the next time we need him.

I think PJensen may have the answer to why..

If you get an extension cord rated for 15 Amps, and you plug it into an outlet which is connected to a 15 Amp breaker (as most outlets are, some few are 20Amp or 10Amp),

The outlets may be rated low, and he wants you to steer clear of the big loads.
posted by Chuckles at 7:20 PM on July 30, 2006

One issue with a large load, a fault might fry the outlet. Our pool motor, not an insubstantial load, is plugged into a GFI outlet. Once, foolishly, I thought to turn it off by pressing the test button, rather than walk over to the motor and switch it off. It fried the outlet and I had to put a new one in. The sound of its death was also a bit scary. Careful with big loads.
posted by caddis at 7:23 PM on July 30, 2006

A GFCI outlet or a ground wire run to the outlet (and a new outlet) is the only way to do this up to code.

The grounding in AC is a safety feature that you will rarely need, but appreciate when needed.

GCFI is 100% equivalent, from what I've read: it provides the same protection a different way. That is why they are the only code-approved way to plug three-prong devices into an outlet with no eq. ground. A 15A GCFI outlet can accept any 15A appliance you throw at it. (GFCI was developed for miners in S. Africa using high-current tools standing in water. It works, and works well, even with shit-loads of current. Assuming all eq. is up to code and working, you can get in the bathtub, throw in a toaster plugged into GFCI, and get nothing but a minor jolt. Don't do that).

And yes, you can use an extension cord, quite easily, It just requires a rudimentary understanding of EM and some common sense. Which means 95% of the world can't use one safely (hence the end-of-the-world warnings about using one with high current appliances). The reality is that the proper gauge cord of a short enough length will work quite safely. It's just up to you to figure out what that is, and you accept all the risk.
posted by teece at 8:16 PM on July 30, 2006

I thought I should add: the two-to-three prong adapters are rated for 120V 15A (I've got one in my hand right now). They have to be, as that's what can pass through them. Any 15A appliance could plug into one with no problem -- that's what they're for. They just bypass a safety feature that is seen as better not to bypass by the electrical code people.

The reality is that you could use one for your AC unit for years and years and never have a problem. High current is not important for one of those 2-3 adapters. The trouble is something different. If you use one on an .5A clock that goes bad, and the case electrocutes you, you say "ouch." If you use one on a 12A AC unit that goes bad and that case electrocutes you, it might stop your heart. But it was not the 2-3 adapter that caused the problem, and nor would it likely ever. It's a malfunctioning AC unit that would be the problem. The adapter would just bypass it's built in safety mechanism -- it would not cause any other problems.

But neither is at all likely to happen. Remember, these risks are considered across millions of homes over many decades.

(And getting a 15A, one-outlet extension cord of 3 to 6 feet will be absolutely fine, and anyone saying otherwise is being way too cautious. I ran an AC unit that way for years, (despite the manual saying God would smote me if I did), and the extension cord never so much as even got mildly warm). A 15A cord means it can carry 15A without lighting your house on fire. It's just stupid to pretend otherwise. Any increase in risk is infinitesimal.
posted by teece at 8:34 PM on July 30, 2006

If you use one on an .5A clock that goes bad, and the case electrocutes you

Wouldn't you start drawing significantly more current, though? (Isn't that where the whole concept of fuses comes from? You'd be forming a path to ground, and drawing ever-increasing current until something, somewhere burned up.)

I've always perceived 2-to-3 prong adapters as being very cheaply made. This isn't based on anything scientific at all, but I've never had a lot of faith in them for much.

It's just stupid to pretend otherwise

I'd stop short of saying it's stupid. As others have pointed out, extension cords are much more prone to becoming frayed, worn, bent, etc. The risk of using an extension cord is greater than in-wall wiring. It's certainly not wildly reckless or anything, I'm just saying that the people cautioning against using them aren't paranoid wackos. But yes, it does work. I'm running my AC off of a 6' cord rated for 15 amps as we speak. teece's summary of the issue is spot-on: they're quite safe if you know what you're doing.
posted by fogster at 10:22 PM on July 30, 2006

teece, the electrocution hazard of the clock is equal to the AC unit. What matters is how much juice your body draws from the circuit, not how much the device does.
posted by caddis at 10:55 PM on July 30, 2006

"I thought I should add: the two-to-three prong adapters are rated for 120V 15A (I've got one in my hand right now). They have to be, as that's what can pass through them. Any 15A appliance could plug into one with no problem -- that's what they're for. They just bypass a safety feature that is seen as better not to bypass by the electrical code people."
posted by teece at 11:34 PM EST on July 30

"I've always perceived 2-to-3 prong adapters as being very cheaply made. This isn't based on anything scientific at all, but I've never had a lot of faith in them for much."
posted by fogster at 1:22 AM EST on July 31

Here's a link that proves you're both right. You can see in the illustration, if you look carefully near the blade holes, that this one is indeed rated for 125VAC 15A, as teece says, and at $0.90 it is cheap as hell.

But I'm holding one in my hands that is only rated for 600W, 120VAC 5 AMP, shipped with a lamp, and those are pretty common, too. It's probably even more cheaply made than the one linked above, because it was free to me. I wouldn't use it, but I keep it to give away to someone I might not like in the future.

Check any device you intend to trust with your life carefully. And be willing to spend more than $1.
posted by paulsc at 11:11 PM on July 30, 2006

Here are a couple of articles on electrocution: the fatal current, and another with the same title. The amount of current required to stop your heart is very small.

Read the comments at the end of the first one.. Because of the way resistance changes with surface area and volume distribution, it is hard to compare one shock with another. Whether you get killed or not seems almost random..

That said, the shock from a .5A device will probably be the same as the shock from a 12A device (technically, it is possible that the fuse in the lower current device will blow fast enough to save you, but that seems unlikely).

I'd really like to learn more about electrocution, actually.. If anyone has good articles I would appreciate it.
posted by Chuckles at 11:34 PM on July 30, 2006

Bad link, paulsc.
posted by Chuckles at 11:35 PM on July 30, 2006

"Bad link, paulsc."
posted by Chuckles at 2:35 AM EST on July 31

Correction.
posted by paulsc at 12:10 AM on July 31, 2006

And getting a 15A, one-outlet extension cord of 3 to 6 feet will be absolutely fine, and anyone saying otherwise is being way too cautious.

I would need a 25" extension. Of course I'd get a 15A-rated cord (the AC draws ~9A). Is that simply too long to feel okay about?
posted by TonyRobots at 4:35 AM on July 31, 2006

I amy have missed it in all the discuaaion above, but if you end up using an extension cord to reach a three-prong receptacle, make sure it is properly installed first. In an older building with a slack landlord it is entirely possible that a previous renter just slapped a grounded receptacle in the box without actually grounding it, or reversed the hot and neutral wires.
posted by TedW at 5:56 AM on July 31, 2006

I would need a 25" extension. Of course I'd get a 15A-rated cord (the AC draws ~9A). Is that simply too long to feel okay about?

A 15Amp cord is a 15Amp cord. It will just need heavier guage wire to get that rating if it is longer. We're talking about the biggest (fattest) cord your local hardware store sells. I have a 50' cord rated at 15Amp that I use for some electric yard tools - purchased from Home Depot.

TedW brings up a good point. One of those cheap little plastic outlet checkers wouldn't be a bad thing to own. All the safety in the world wont help you if the wiring in the walls is bad. At least the simple checking device lets you do a quick-and-easy check of the wiring.

If the wiring is really bad, it can start a fire in the walls with nothing plugged in at all, so checking all the outlets goes beyond the original question, and into just general home safety. While you're at the hardware store, pick a little outlet checker up. It won't always catch bad connections, but it's a start.


Just to throw my $0.02 into the 2-3 prong outlet converters ("cheaters") debate, those work most of the time if you have a cold water pipe or something with a good ground connection to connect the ground blade to. If you are in a steel frame building, the building frame may work as well. The problem is, you probably don't have a good ground nearby, and you probably don't have a good meter to check the quality of the ground even if you did.

Nominally, the ground connection only has to be good enough to trip the breaker. Even a (relatively) thin wire would do that just fine. This gives you 90% of the protection. However, there is still a danger in an intermittant or weak fault, where the new ground wire is drawing less than the breaker's rated current, but enough to heat the wire. Your ground connection must therefore be able to sink up to the breakers rated current continuously.

Oh, and then there's the worry about using something like a cold water pipe for an alternative ground. Plumbers have been killed working on pipes because they disconnect an electrically hot pipe, and get zapped.

So... a 2-3 prong converter can work if you are very carefull about how you do it. You could even use a cheater and a GFI together. That way, if even a little current is placed on the alternative ground, the GFI will interrupt, and many of the above concerns don't come into play.


I suggest that an extension cord sure is easier, if a bit of an eyesore. It will get you through the heat wave without worries, and then you can start trying to get what you really want: an proper 3-prong plug installed by the (new) landlord.

Cheers.
posted by PJensen at 7:00 AM on July 31, 2006

teece, the electrocution hazard of the clock is equal to the AC unit. What matters is how much juice your body draws from the circuit, not how much the device does.

A good point I overlooked when I dashed that off, thanks.
posted by teece at 8:28 AM on July 31, 2006