Any Electricians

I have a question about measuring AC voltage from an outlet and what does this mean: 120v office outlet

Fluke meter positive lead to neutral and negative lead to ground reads 280v.

I have an outlet that burnt out and measuring the outlet:
1- positive to hot and negative to ground 117v
2- positive to hot and negative to neutral 117v
3- positive to neutral and negative to ground 280v.

Any help is appreciated....Thanks in a advance:twothumbs

Diagnosis by internet is worth what you pay for it.

When measuring AC, it makes no difference which lead goes on which test point. So assuming your meter is set on AC voltage, clearly your Neutral is running a considerable voltage above ground.

This is not good, it could be a faulty transformer, faulty transformer ground connection, or a crossed wire between neutral and a "hot" phase somewhere. You need the services of a real electrician on-site to solve this.

FlashKat said:

I have a question about measuring AC voltage from an outlet and what does this mean: 120v office outlet

Fluke meter positive lead to neutral and negative lead to ground reads 280v.

I have an outlet that burnt out and measuring the outlet:
1- positive to hot and negative to ground 117v
2- positive to hot and negative to neutral 117v
3- positive to neutral and negative to ground 280v.

Any help is appreciated....Thanks in a advance:twothumbs

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FlashKat,

Wow! NOT GOOD! Something is wrong. You should get 110 to 120 VAC from Line/hot to Neutral, and Line/Hot to Ground, but less than 1 or 2 VAC from Neutral to Ground. Pracitcally there is usually some small voltage difference between neutral and ground, but it shouldn't be much at all.

The only explanations are the following:

1. Your meter is bad.

2. What you are calling "hot is actually neutral, what you are calling "neutral" is actually Line, and what you are calling Ground is actually -Line. In other words, in a 220 VAC situation there are three wires: Red, Black, and white. The white is neutral, which is close to ground potential. The black is 120 and the red is also 120, but the red is 180 degrees out of phase with the black, such that the AC potential across them is 220 VAC. In a normally wired building, the black is LINE/HOT and the white is NEUTRAL, and the bare wire is ground. In an outlet, the smaller blade goes into the LINE, and the wider blade goes into NEUTRAL, and the round pin goes into GROUND.

If indeed your meter is good, and you are measuring 200+ VAC anywhere from anything to anything, then THROW THE BREAKER OR PULL THE FUSE AND CALL AN ELECTRICIAN ASAP.

The only time you might measure more than 120 VAC might be is between a pair of hots on different outlets. As described by js, this is due to the way most electric systems are set up. The line coming into the breaker box is usually three wires-one neutral, and the other two hot but out of phase 180°. These wires come from a 240 VAC center-tap transformer. The neutral wire is connected to the center tap. Therefore, you get 120 VAC between either hot wire and neutral, but 240 VAC between both hot wires. The purpose for this system should be obvious-you can put a pair of breakers in and run the two hots in order to get a 240 VAC outlet. However, you can sometimes also get 240 VAC between the hots of different outlets in the same room if the outlets happen to go to opposite sides of the breaker box. Under no circumstances should you be measuring 280 VAC at all, much less between neutral and ground. At best you might read ~250 VAC between at pair of hots in different outlets under the circumstances I just mentioned. And even if the situation js mentioned in 2) is what is happening, then you should be reading roughly 2 times 117 VAC, or about 234 VAC, not 280 VAC.

I think perhaps there is a strong source of EMI nearby that is throwing your meter off. Try putting a small load like a nightlight bulb between neutral and ground. If it instaflashes then you definitely have some serious problems. If it glows regularly you still do, but not as severe. If it doesn't light at all try a lower voltage bulb like 12V or 6V. If that still doesn't light then you have no significant voltage between neutral and ground. That means something else, probably EMI, is causing the 280V reading.

To save money on wire it is sometimes done where 2 circuits are wired with a shared neutral through various outlets. This is generally not a problem except for the obvious danger of having more than 1 circuit through any box you can't make it safe to work in there by flipping only 1 circuit breaker... But if you have a bad connection anywhere in the chain then you can get low or high voltages and burn stuff out if say the neutral were to get loose somewhere. It is also possible that someone working in there has accidently connected wires up from different circuits in totally wrong ways.

In any case, if you've verified that there is a problem then you need to call in someone to help chase down the problem. It may be wires connected together incorrectly anywhere along the chain and it may end up being difficult to figure out exactly where it has gone wrong. Or, if you happen to know that the handyman replaced the outlet yesterday it might be easy to find Get some help in after you've tested it with another meter just to rule that out.

I also forgot to mention that one thing I like about using armored cable (or metal conduit) and metal boxes (as required by NYC electrical code because of rats) is that ground is taken care of automatically, provided the cable jacket is solidly connected to the box. This means that a situation like you described pretty much can't happen. I would personally wire a house with armored cable and metal boxes whether required by code or not. The extra robustness alone is worth the extra aggravation fighting with the armored cable.

Minor detail guys but the two hots are not out of phase 180 degrees. Actually they represent each end of a single 240V transformer secondary winding, which is center-tapped. That center tap ties directly to earth ground, which is then tied common with the neutral at the main breaker panel. Otherwise you guys seem to be on the money with your advice.

In a commerical building with three-phase power it can get more complicated. 208V 3-ph is fairly common with a wye-connected transformer secondary, which gives 208V leg to leg, and 120V leg to center. Instead of two hots as in a residence, you have three hots.

In neither of the above scenarios do we have 280V under any circumstances. Something is wrong with the meter or something is VERY wrong with the wiring. Forget the meter and CALL AN ELECTRICIAN.

I just remembered that sometimes the lighting circuits in commercial buildings are 277 VAC. That's about the only time you should ever see 280 on a meter. That might be the problem-someone connected the ground to the hot wire of a 277 VAC lighting circuit.

jtr1962 said:

I also forgot to mention that one thing I like about using armored cable (or metal conduit) and metal boxes (as required by NYC electrical code because of rats) is that ground is taken care of automatically, provided the cable jacket is solidly connected to the box. This means that a situation like you described pretty much can't happen. I would personally wire a house with armored cable and metal boxes whether required by code or not. The extra robustness alone is worth the extra aggravation fighting with the armored cable.

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I second that! BX isn't that much more expensive than Romex. I think it might amount to about $500 in supplies for a typical house. Of course, . . . unless you're doing it yourself the labor costs could skyrocket.

As for 220VAC and phases, the two hots are indeed out of phase with each other by 180 degrees. Or you can think of them as two series 120 VAC sources, at 0 degrees relative phase, but oppositive polarities connected to the neutral. This thread explains it pretty well, especially the embedded subject link refered to.

And yes, as jtr mentioned, you should never read 280 VAC.

jtr1962 said:

I just remembered that sometimes the lighting circuits in commercial buildings are 277 VAC. That's about the only time you should ever see 280 on a meter. That might be the problem-someone connected the ground to the hot wire of a 277 VAC lighting circuit.

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Since he reports 117V hot to ground, and 117V hot to neutral, I don't see how there could be anything but 0 or 234 volts when measuring ground to neutral.

jtr1962 said:

I just remembered that sometimes the lighting circuits in commercial buildings are 277 VAC. That's about the only time you should ever see 280 on a meter. That might be the problem-someone connected the ground to the hot wire of a 277 VAC lighting circuit.

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Yeah. Actually, the lights in my office are 277 VAC, which I was told is "half of 480 VAC." Hmmmm.

soffiler said:

Since he reports 117V hot to ground, and 117V hot to neutral, I don't see how there could be anything but 0 or 234 volts when measuring ground to neutral.

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Yes. Quite.

Reading up on this a bit, many commercial buildings now use 480V three phase. There is 480VAC between each of the three phases, and 277VAC between each phase and neutral. The three phases together often power heavy loads like electric motors for AC or elevators. The individual phases are usually used for things like lighting. The same building also has 208V three phase service to supply the standard 120VAC outlets. This service will have 208VAC between phases, and 120VAC between each phase and neutral. In both cases the neutrals are usually connected to earth (i.e. grounded)

Now consider a situation where the neutral is connected to the neutral on the 208V service, the hot is connected to one of the phases, but the ground is connected to one of the phases of the 480V service (i.e. it is at 277VAC). You will have 120VAC between the hot and neutral. You will have a voltage between hot and ground. This voltage will basically be the difference between 277V and 120V complicated by the fact that the voltages may not necessarily be the same phase. Therefore, it is entirely possible to be reading 117V RMS and not (277-120) or 157V because of the phase difference. And you will of course get ~280V between ground and neutral because ground is connected to one of the phases of the 480V service. I think this is what happened, and it needs to be fixed immediately since many appliances ground the metal case to the outlet ground. This means that a metal appliance case could be at 277V relative to ground! DO NOT USE THAT OUTLET UNTIL IT IS FIXED! And this looks like it's a problem that will require a competent electrician to trace to the source.

When you say the outlet was burnt out what did you mean? There are burn marks around the outlet or a device plugged into it wasn't getting power?

Elmer's white should do 'er.

Many Thanks to EVERYONE!!!!!
A Technician from work replaced the outlet and everything is reading correctly now.:thumbsup:
What happened is that we had a Arrowhead water cooler plugged in and someone reported that the water cooler got extremely hot.
On the bottom outlet that the water cooler was plugged in looked okay, but the top plug had a burn mark around the hot (smaller blade) and it was blocked out like something either broke off or it got fused from a burnout where you could not get the plug or the meter lead in it. We do have heavy machinery and AC units throughout the building. The night before we had a carpet cleaning company come in and they may have damaged the outlet somehow.
I thought the meter was bad, but I double checked with a different Fluke meter and tested a different outlet.

jtr1962 said:

Reading up on this a bit, many commercial buildings now use 480V three phase. There is 480VAC between each of the three phases, and 277VAC between each phase and neutral. The three phases together often power heavy loads like electric motors for AC or elevators. The individual phases are usually used for things like lighting. The same building also has 208V three phase service to supply the standard 120VAC outlets. This service will have 208VAC between phases, and 120VAC between each phase and neutral. In both cases the neutrals are usually connected to earth (i.e. grounded)

Now consider a situation where the neutral is connected to the neutral on the 208V service, the hot is connected to one of the phases, but the ground is connected to one of the phases of the 480V service (i.e. it is at 277VAC). You will have 120VAC between the hot and neutral. You will have a voltage between hot and ground. This voltage will basically be the difference between 277V and 120V complicated by the fact that the voltages may not necessarily be the same phase. Therefore, it is entirely possible to be reading 117V RMS and not (277-120) or 157V because of the phase difference. And you will of course get ~280V between ground and neutral because ground is connected to one of the phases of the 480V service. I think this is what happened, and it needs to be fixed immediately since many appliances ground the metal case to the outlet ground. This means that a metal appliance case could be at 277V relative to ground! DO NOT USE THAT OUTLET UNTIL IT IS FIXED! And this looks like it's a problem that will require a competent electrician to trace to the source.

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I used Gorilla Glue :devil:
just kidding
P.S. js I am glad you chimed in

TedTheLed said:

Elmer's white should do 'er.

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js,

Thanks for detailing the explanation better. Your reference is exactly how I tested and measured everything. I thought I was going CRAZY when I saw this reading on my meter, but the meter and myself is not CRAZY. It was a outlet that shorted out.

js said:

FlashKat,

Wow! NOT GOOD! Something is wrong. You should get 110 to 120 VAC from Line/hot to Neutral, and Line/Hot to Ground, but less than 1 or 2 VAC from Neutral to Ground. Pracitcally there is usually some small voltage difference between neutral and ground, but it shouldn't be much at all.

The only explanations are the following:

1. Your meter is bad.

2. What you are calling "hot is actually neutral, what you are calling "neutral" is actually Line, and what you are calling Ground is actually -Line. In other words, in a 220 VAC situation there are three wires: Red, Black, and white. The white is neutral, which is close to ground potential. The black is 120 and the red is also 120, but the red is 180 degrees out of phase with the black, such that the AC potential across them is 220 VAC. In a normally wired building, the black is LINE/HOT and the white is NEUTRAL, and the bare wire is ground. In an outlet, the smaller blade goes into the LINE, and the wider blade goes into NEUTRAL, and the round pin goes into GROUND.

If indeed your meter is good, and you are measuring 200+ VAC anywhere from anything to anything, then THROW THE BREAKER OR PULL THE FUSE AND CALL AN ELECTRICIAN ASAP.

Click to expand...

Digital meters can come up with some really far off readings on AC. When I was rewiring my basement I was using my Fluke and had some really bizarre voltages showing up in a ceiling junction box. I suspected there was actually no current to go along with the voltages and found that was correct after putting a bulb across it - no light at all - just some stray voltage fooling the digital meter. Analog meters are sometimes best when you are getting strange readings from a digital.

Guys . . . it's not what glue you use for the outlet so much as HOW YOU CLAMP IT. I keep telling you'all that.

jtr,

So, in the case of the 480VAC three phase power with 480 VAC between each phase and 277 between each phase and neutral, would it be the following (relative to neutral)?:

Van is 277@0 degrees
Vbn is 277@-120 degrees
Vcn is 277@+120 degrees

So Cos(-120) = -.5, so Vab between phase A and phase B is Van + Vnb = Van - Vbn = 277(1) - ((-.5)(277) - j(-.866)(277))
= 277 + 138.5 + j239.9
= 415.5 + j239.9
= 480@30 degrees.

Holy crap! That's cool. I finally understand why the lights in my office are 277 VAC and why that is "half of 480 VAC".

You are THE MAN :bow: :bow: :bow:

So . . . :thinking: . . . let's see. You're saying that maybe the outlet is wired such that Line and Neutral are both from the 208VAC three phase power like normal, but the ground, instead of being ground is actually one of the phases of the 480VAC three phase power.

I'm following that so far. That would make L to N be 120VAC nominal, because that's the way that three phase system is set up. Check. That correlates with FlashKat's measurement #2 above.

But what about measurement #1, L to G? No matter how the 277 attached to the ground is phased relative to the 120 of the line, you can't get smaller than 157VAC, right? If they are phased such as to produce maximum cancelation of instantaneous voltages, you still can't do worse than subtracting 120 from 277, right? Or am I missing something?