120vac household; NEED N.E.C. RESOLUTION 4

[charlesDL]

Help ... I'm unable to resolve this issue: When replacing 2-prong outlets with 3-prong, the consensus is that you have to run a seperate gnd conductor or install gfci ... The main panel has a neutral bus that is shorted to the ground bus (pretty standard). Since the neutral is "electrically" the same point as the ground, why can't a jumper be installed at the outlet from neutral to the ground post?? This is a 2-wire household with a ground bus in the panel!! Installing this jumper would be putting neutral and ground at the same potential at the outlet just as it is in the panel. CAN SOMEONE HELP, PLEASE?? What is the N.E.C reference?

 

[jghrist]

The neutral and the ground are not at the same potential because of voltage drop in the branch circuit.

Relevant NEC 2002 articles are 250.130(C) and 406.3(D).

 

[Senselessticker]

Note that the neutral is (or is intended to be)a current carrying conductor for such a receptacle. Splicing neutral and ground at the receptacle will create a "ground loop". The return current will have two paths of return to two different places (VERY BAD). Grounded conductors are only intended to carry current under fault conditions. NEC article 250 covers all kinds of grounding, but I think there is some basic circuit theory that needs to be clarified. I'd suggest calling an electrician before someone gets hurt.

 

[JBender]

A ground and a neutral are at nearly the same potential, but they serve different purposes. They only place the two can be connected is at the main panel (and must be connected).
The difference between them is that a neutral is the wire that carries the current from the hot back to the main panel. A ground conductor should have no current, it is to assure that the metal components of the outlet are referenced to ground potential.
If you did as you suggest, imagine this scenario: Some day the neutral of this circuit gets broken somewhere after the outlet it powers. Also imagine there is a space heater plugged in...it shuts off because the circuit is no longer completed. So you now have an outlet that appears to be off, though it is anything but. The space heater is completing the connection between hot and neutral but neutral has nowhere to go. The neutral will now be at 120 Volts! And in your case as will be the outlet box, screws on the face...etc!

 

[weh3]

Ground fault protection senses imbalance between the hot and the "neutral" (the up-to-date terminology is "grounded circuit conductor" or GCC). Ground fault currents at equipment must be caused to flow in the equipment grounding conductor (EGC) only and not the GCC. If you connect EGC and GCC at the outlet, then ground fault currents would flow in the GCC, then the current through the GFP would be in balance and would not trip the GFP.

Single point grounding is VERY IMPORTANT.

William

 

[dpc]

Just put in the gfci receptacle and call it a job. The NEC requirements are there for a reason. Green wire and white wires are connected at one point - the service entrance panel. If you start bonding them at every receptacle, you may end up with substantial neutral current in the green wire and this causes a lot of other problems, as others have noted.

 

[rbulsara]

dont have time to write long explanation at this time..

But in brief:

You can not ground neutral at more than one point (the source) per NEC. The ground conductor is NOT supposed to carry current in normal operation while the neutral does! By connecting neutral and ground at any other point then the service will make ground condcutor (and any thing bonded to it like the equipment enclosure etc.) carrying (sharing) the neutral current which is not acceptable. Ground condcutor shall only carry the current during a line to phase fault.

NEC does not mandate a separate equipment grounding condcutor. You can also use a metal conduit or armour of a cable for equipment ground but it has to be a continuous path back to the source. A separate conductor is the safest way.

I also agree with dpc.

 

[rbulsara]

i meant..."line (or phase) to ground fault.." in the last sentence of my first paragraph..

 

[jghrist]

I agree with the conclusions of the various posters, but not the reasoning. The original poster does not propose connecting the grounding conductor to the neutral. In his situation, there is no grounding conductor.

The problem that I see is not ground loops but the fact that by connecting the ground prong to the neutral, you will be putting a voltage to ground (from voltage drop) on the grounded parts of the connected equipment. This could cause some local arcing to something that is really grounded. In most cases, the voltage will be low and there probably won't be a problem, but it is not allowed by the NEC and the NEC does provide other solutions.

If the idea is to provide a ground reference for sensitive electronic equipment, then you might be introducing troublesome transients by connecting to the neutral. If the idea is to just provide a safety ground, then the GFCI receptacle is a better solution.

 

[weh3]

jghrist,

You're right about charlesDL's real question. The neutral and ground are identically the same potential only at the service entrance.

William

 

[rbulsara]

weh3 and jghrist..you may want to re-review what you said..

the original poster asks "why can't a jumper be installed at the outlet from neutral to the ground post??"

Well, you cannot per NEC or per any instructions that may come with such a device or by any engineering principles!!

Why do you say ground loop is not a problem? unless you think the "ground post" is not grounded.

 

[rbulsara]

NEC reference:

2002 Edition: Article 250.130 (C). See the fine print note too leading to GFCI receptacles 406.3.(D)(3). These articles are dedicated for replacing/wiring non-grounding type recetacles.

There is no option or discussion of connectig ground termina to neutral.

 

[weh3]

rbulsara,

Gently! We all agree. CharlesDL's contention is that "the neutral is "electrically" the same point as the ground", but this is incorrect, because a current can flow in the neutral to create a potential difference between the outlet neutral connection and the panel neutral bus. EGC is prohibited from being a current-carrying conductor for exactly this reason--the potential must be ground potential all along its length.

We all agree that what charlesDL proposes is not OK. At first, I thought that by "ground post" he meant the panel ground bus, but on reflection, I believe he means the green screw on the outlet or on the outlet box. He is asking why he can't just put a jumper between the white and green connections within the outlet box, because they are ultimately connected to the same point within the panel. The ground post would not be grounded in this case, that is, connected to the service ground by the EGC.

William

 

[stevenal]

If the ground prong is connected to neutral instead of ground, an open neutral situation will cause a normally grounded case of the cord connected equipment to become live.

 

[mc5w]

That and that some electricians were connecting both the neutral conductor and the equipment ground to the same terminal at a service panel out of laziness. There have been instances where the terminal later lossened causing the equipment ground to become hot to some extent. Hence, the rule that each neutral wire MUST be terminated 1 wire per terminal unless both neutrals are the same circuit.

You are still allowed to connect 2 equipment grounding wires to the same terminal if the terminal is rated for that.

 

[MrHandy]

I believe in this scenario, if this were safe practice, all electrical recepticles would have only two terminals. One for hot and one for (in this hypothetical situation) "neutral-ground". This is AC remember. Direct current is a whole different story.
I have recently been asked to rewire a building that was originally wired in the 1940's. This method that this post refers to is common in each and every receptacle. The apartment is empty as of now and the electrical service is completely shut off(meter pulled). I do not know how it went this many years without any accidents but one thing is for sure...the usage was astronomical due to the constant current running to "ground".

 

[jghrist]

MrHandy,
Why would there be constant current running to "ground"?

 

[mc5w]

There are some problems with using a GFCI instead of a real equipment ground:

1. You are using the human body as the equipment ground. GFCIs DO NOT stop you from getting shocked. They only stop the shock from killing you. Likewise, AFCIs do not prevent electrical fires - they only stop the fire from getting bigger.

2. As mentioned above, surge protectors do not work so well unless there is a genuine equipment ground.

3. Some items such as engine block heaters on truck and older refrigerators have enough normal leakage that they will have a nasty tingle voltage if not grounded. Same can be said about 480 volt motors even when brand new.

For the amount or work that it takes to run just an equipment grounding wire you can just as easily run new romex to new receptacles and blank off the old ones. For receptacles that are embedded in wood frame walls putting in a bunch of new outlets is actually less labor.

Sure, you can run a #4 solid copper ground wire her and there in the basement ceiling and split bolt #12 solid copper to it, but you are ALWAYS better off to stick in new romex if your house is too old to have equipment grounds.

 

[bigbillnky]

A couple of things I would like to mention. First, replacing ungrounded receptacles to GFCI's is an acceptable and legal practice. Second, the real problem of placing a jumper between the ground and the neutral can create a current path(without a fault) thru any equipment plugged into the receptacle if the equipment becomes a better path to the panel for return currents.

Bigbillnky,C.E.F.....(Chief Electrical Flunky)

 

[tommom]

NEC 250.6 (2002) states that the grounding of electrical systems shall be installed and arranged in a manner that will prevent objectionable current over the grounding conductors or grounding paths.

Grounding paths or conductors are only meant to carry current in the event of a fault. They are not meant to continuously carry current.

The problem with the proposed installation is that "ground" at outlet would not be at the same potential as the ground in the rest of the house, say the water piping. Somebody could get killed with that installation, if the difference in voltage was enough, or if a ground fault occurred while a person was touching an electrical device.