3 prong outlet, 2 wires?

April 17, 2018 11:34 AM

I've checked two electric outlets in my new construction (~2000) apartment. An outlet tester shows both as properly wired, grounded, 3 prong outlets. Inside, however, both outlets are only connected to hot and neutral wires. Can someone tell me what is going on?

As far as I can tell there are only 2 wires coming into the box, but I could believe that there's a third, hidden ground wire someone I can't see that's grounding the whole junction box. If it's not that, I don't understand how any ground is happening or how the building could possibly be up to code (or why an electrician would run only 2 wires in a modern home).

posted by Phredward to Home & Garden (11 answers total)

Pictures?

Are you testing the outlet when it is screwed to the box? If so you are probably getting a bond from the box. This isn't correct; you should have a bond wire direct to the receptacle. And yes that has been code since the 60s.
posted by Mitheral at 11:39 AM on April 17, 2018 [2 favorites]

Yes, as Mitheral said (if I'm reading that properly) - if you have have steel junction boxes and steel conduit the entire system can be used as a ground. I believe that's not uncommon, and in those situations you would not have a third wire in the metal conduit. This would only be true if you had metal boxes / metal conduit, but if you're in a new construction apartment building that likely follows commercial codes that would not surprise me.

As to the outlets, that may be down to your local code requirements. You *may* have 'self-grounding receptacles' that use the yoke screws as a legal ground , and they *may* be code legal in your area, and if so they *may* be installed properly, but that's a lot of maybes for a random person on the internet to comment on.
posted by true at 12:06 PM on April 17, 2018 [2 favorites]

true: "I believe that's not uncommon, and in those situations you would not have a third wire in the metal conduit. "

Note that normally (barring some weird AHJ amendment) you'd still have a green (or bare) wire going between the box and the receptacle.
posted by Mitheral at 12:24 PM on April 17, 2018

I have seen plenty of boxes where the tin plated ground wire is wrapped around the clamp screw and is difficult to see if you're not looking for it. You still should have a ground wire from the box to the green screw on the outlet. Home depot has ground clips that make this pretty easy.
posted by H21 at 12:32 PM on April 17, 2018

You can also look around your water meter to see if there is a ground wire that bypasses the unit.

That's a sign that the electrical system is using the water pipes as an earth ground. There will be a connection somewhere near the breaker box.
posted by JoeZydeco at 12:40 PM on April 17, 2018

Response by poster: Thanks everyone who answered so far! Here are some pictures I have taken.

You will probably be unsurprised to learn that the tester shows open ground when the outlet is out of the box, and normal (ie yes ground) when screwed into the box. There is no wire connected to the ground screw on the outlet (and wasn't on the other outlet I checked either). This is in New York City.

Is there anything else I can do / check to see what's going on without calling an electrician? (Tools available: outlet tester, voltmeter, kill-a-watt.) Does anyone think that this is an issue that an electrician should look at right away (as opposed to ask about the next time some other issues comes up)?
posted by Phredward at 1:13 PM on April 17, 2018

That's a GFCI receptacle, which in some cases is ok to install without a ground.
posted by bondcliff at 1:29 PM on April 17, 2018 [4 favorites]

Best answer: If I'm not mistaken that looks like an entry clamp for armored (BX) cable in your last picture, in which case it's likely correct that you're relying on a connection to that armor jacketing for a ground connection. That would explain why it's grounded when installed but not when removed, as others above have pointed out.
posted by range at 1:41 PM on April 17, 2018 [1 favorite]

Best answer: It may actually be code approved according to the national electrical code. See here , specifically 250.146(b). (Mitheral, what do you think? Assuming that is a self-grounding outlet, it would seem that a bonding jumper / green wire isn't required by the NEC, which I *believe* applies to NYC).

Regardless - to the OP, it kind of depends on what you're after here. If you want to know if the outlet is grounded, it is when it's connected. If you want to know if it's code approved, the only way you'll know with 100% certainty is to contact an electrician with knowledge of your local codes and who can carefully look at the installation. I personally do not think you need it looked at ASAP as long as all the outlets show grounded when they are attached to the box, but I've seen far far worse in houses I've lived in, and have built up a pretty high tolerance to this kind of stuff.
posted by true at 6:44 PM on April 17, 2018

Best answer: true: "Mitheral, what do you think? Assuming that is a self-grounding outlet, it would seem that a bonding jumper / green wire isn't required by the NEC, which I *believe* applies to NYC"

I'm Canadian and not quite up on hair split NEC requirements like that, especially as applied in 2000 (which would have been using a still older version of code with who knows what local admentments). It may have been legal when installed. As you said it looks like rule 2 allows it (if the receptacle is self grounding) but I won't say for sure it is because it is dangerous to look at rules in isolation. EG: I can't remember whether bonding through the BX armour was legal in 2000, I don't think it is now and don't have a NEC handy to check.

GFCIs are allowed to be installed un bonded in the case where no bond exists in the box (IE: wiring installed before bonds were required). They aren't generally allowed to be installed in unbonded boxes in new construction.

At any rate if your tester shows all good when the receptacle is installed I wouldn't worry about the install. It's not the way I'd do it ¹ (and the rule above doesn't exist in the CEC) but it isn't dangerous if you don't leave your receptacle hanging out of the wall.

¹I'm pretty conservative on electrical safety. I don't like things that depend on end users not making a mistake. This outlet will become unbonded if a non self bonding receptacle is ever used as a replacement or if the self bonding screw gets replaced with a drywall screw (happens _ALL_ the time in residential electrical when people pull out receptacles to paint/wall paper/tile and then lose the proper screws) leading to a dangerous condition. Other electricians validly may and often do think differently.
posted by Mitheral at 7:31 PM on April 17, 2018

PS: the layperson can substitute "ground" everywhere I wrote "bond" in my comment above.
posted by Mitheral at 7:37 PM on April 17, 2018 [1 favorite]