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Old 01-24-2009, 12:50 PM   #1
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Code Clarification


hey hey everyone, i was reading some incidences involving homeowners getting shocked modifying their electrical system, and came across code that i am confused about.

Code: Neutrals and grounding should not be bonded at a sub-panel.

If the neutral wire from the main panel is cut in this case, the grounds on the sub-panel then has current, which can be a hazard in two ways, if ground is touched or touching the charged equipment that it connects to.

If an exposed hot wire is touching the sub-panel, causing a hot to ground short, the hazards remain the same as above.

Now my set up is this, I have an older townhouse, where at the Main Service Panel, there is NOT a ground bus, therefore the grounds of all the circuits converge and is bonded to the neutral bus. This setup therefore, contains all the hazards of the above scenario with the sub-panel. So is this against code?

And if so, can i just install a ground bus in my Main Service Panel, with all the other necessary things to create an independent grounding system?

Thanks!!

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Old 01-24-2009, 01:45 PM   #2
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Code Clarification


Your theories are all wrong. If the neutral opens there is still NO current flowing on the grounds. The current does NOT flow backwards from the main panel.

If a hot touches the panel it will trip the feeder breaker at the main panel because the fault current will flow back to the main panel on that equipment grounding conductor.

The main panel/disconnect is the ONLY place the neutral is bonded to ground.

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Old 01-24-2009, 02:19 PM   #3
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Code Clarification


when i did this place's wiring, i HAD to install a ground bus in the box in the house as per code, (it had none) it got bonded outside at the box on the pole.

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Old 01-24-2009, 02:33 PM   #4
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Code Clarification


I made this awhile ago

A ground rod is installed at the "main panel" for lightning/overvoltage reasons. Not clearing faults. The ground is bonded with the neutral to clear faults and for lightning/overvoltages. If a hot touches the ground, it will flow back to the "main panel" and into the neutral wire back to the transformer.

The grounds and neutrals are connected ONLY at the first disconnect, in your house this would be the "main panel".
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Old 01-24-2009, 03:00 PM   #5
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Code Clarification


It is less likely that the neutral at the main panel will become severed, as it is much bigger wire. Same hazards, just less likely. If your panel is the first disconnecting means for the system, then the grounds and neutrals can land on the same bar.
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Old 01-24-2009, 04:04 PM   #6
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Thanks to all for the prompt replies, but i am getting different perspectives so i'll just post the article i read.

http://www.inspect-ny.com/electric/E....htm#reviewers

Speedy: The neutral at the sub-panel was cut, as stated in the article, and current flowed into the ground to the equipment that was connected to the sub. My question to you is......the code we are discussing MUST serve some safety purpose for certain fault situations.......what are they??

InPhase: You understand what i mean right??
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Old 01-24-2009, 04:36 PM   #7
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Quote:
Originally Posted by GoyimPersuasion View Post
Speedy: The neutral at the sub-panel was cut, as stated in the article, and current flowed into the ground to the equipment that was connected to the sub. My question to you is......the code we are discussing MUST serve some safety purpose for certain fault situations.......what are they??
Once again, in a properly installed sub-panel, the current will not flow on the ground.
The panel in that article was NOT installed properly.
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Old 01-24-2009, 04:58 PM   #8
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Speedy: Haha, we think differently. You didn't answer my question. But i'll try again. You are exactly right!!........the sub-panel was installed incorrectly. WHY??.......because the ground was connected to the neutral bar, therefore, when the neutral to the sub's neutral bar was cut, the current flowed through the device, its neutral to the neutral bar, through the GROUNDS (connected to the neutral bus) and to equipment. THIS SETUP WITH THE GROUND CONNECTED TO THE NEUTRAL BUS IS EXACTLY THE SAME AS WITH THE MAIN PANEL............so why, once again like in the OP, is not hazardous in the MAIN PANEL???
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Old 01-24-2009, 05:12 PM   #9
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Quote:
Originally Posted by GoyimPersuasion View Post
Speedy: Haha, we think differently. You didn't answer my question. But i'll try again. You are exactly right!!........the sub-panel was installed incorrectly. WHY??.......because the ground was connected to the neutral bar, therefore, when the neutral to the sub's neutral bar was cut, the current flowed through the device, its neutral to the neutral bar, through the GROUNDS (connected to the neutral bus) and to equipment. THIS SETUP WITH THE GROUND CONNECTED TO THE NEUTRAL BUS IS EXACTLY THE SAME AS WITH THE MAIN PANEL............so why, once again like in the OP, is not hazardous in the MAIN PANEL???
I understand what you are saying. But, there must ultimately be a point where the ground and neutral are connected, otherwise the ground system wouldn't work. So no matter what, you can always ask the question: "What happens if that neutral opens?"

The equipment ground works by providing a low resistance path for short circuit current to complete its circuit. It has to be connected to the neutral at some point. By limiting the location to a single point, called the Main Bonding Jumper, we limit the potential hazard. Can the service neutral become open? Sure. I've seen it alot. But what's more likely to be correctly installed? The main service, or a cobbled together subpanel that a homeowner rigged up?
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Old 01-24-2009, 06:33 PM   #10
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The specific code is 250.24 A (5). I can't word the reason for this any simplier than Inphase277 has explained. Also just to add.... when a service neutral opens your going to know it real quick.
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Old 01-24-2009, 06:35 PM   #11
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I assure you that you must float the neutral (isolated grounds from neutrals) in a sub panel. The NEC code requires it, and anyone that says otherwise lacks a basic understanding of what's going on. Bonding takes place at the point of service only, and that is where the main disconnect is located.

Not having the grounds and neutrals isolated at a subpanel is an unsafe condition 100% of the time, no exceptions.
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Old 01-24-2009, 07:10 PM   #12
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Quote:
Originally Posted by Stubbie View Post
The specific code is 250.24 A (5). I can't word the reason for this any simplier than Inphase277 has explained. Also just to add.... when a service neutral opens your going to know it real quick.
I'll add my agreement here as well.

Like I said in post #2, the current does not flow backwards. If the service neutral opens the current will always seek it's source, which is the POCO's transformer outside. Open neutral fault current will not make everything in the house live with voltage. Other bad things will happen, but that is not one of them.
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Old 01-24-2009, 07:13 PM   #13
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Quote:
Originally Posted by thekctermite View Post
Not having the grounds and neutrals isolated at a subpanel is an unsafe condition 100% of the time, no exceptions.
Welllllll, 100% is a bit extreme.

Unless installed under the codes of the 2008 NEC, we are still allowed to run a 3-wire feeder to a remote structure if certain conditions are met.
In this case the neutral IS bonded to ground and the sub-panel is treated just like a main panel. See NEC 250.32
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Old 01-24-2009, 08:39 PM   #14
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I'd just like to add that there is nothing unsafe about 3 wire feeders neutral and ground bonded at a sub-panel or 4 wire feeders neutral and ground unbonded. Done correctly they are both safe. The preference has become a 4 wire feeder. 3 wire feeders are less liked because of the addition of another conductive path being ran to the building with the sub-panel after the sub-panel has been installed. This provides for an alternative path for a neutral return to the service equipment. If you would have a neutral open event the grounding of the other metallic path may allow neutral current to flow on it back to the source and you would never know your actual neutral had opened. Worse you may get a fire if the phoney neutral isn't capable of carrying the current load or it may kill you under the right cicumstances. The second issue is you will lose your ground fault path if the neutal of the feeder opens and breakers likely will not trip.
But as Speedy has said there is nothing unsafe about either if they are installed correctly.
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Old 01-27-2009, 11:55 AM   #15
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Phase - "Cobbled together sub-panel that a homeowner.........".......thats exactly why i am making certain to understand not ONLY the Codes, but their significance. That means understanding that if a specific code was violated, what are the hazards behind it.

I had two initial confusions. One being what Phase already clearly explained, and was my deduction from the beginning. Second was why some setups had a grounding conductor to an electrode and some did not. Than answer laid in this 12 part article about "Grounding vs Bonding" by Mike Holt.

http://ecmweb.com/mag/electric_groun...x.html?smte=wl

As stated in the article, overcurrent of above 600V, from lightning, high voltage lines, line surges etc requires a grounding (earthing) conductor. And since, the earth is NOT a low impedence path, and Code requires one for a fault current path, grounding to an electrode cannot soley be used for such purposes, and require the use of the Main Bonding Jumper to direct conduction from equipment back to the source.

However, as a response to Speedy........
Quote:
Originally Posted by Speedy Petey View Post
If the service neutral opens the current will always seek it's source, which is the POCO's transformer outside.
and anyone please jump in, how does the current seek it source (transformer) if Service Neutral was cut?? Through what path??

Sorry if i have many question, but i want to have a comprehensive understanding, on the residential side of the trade at lease. I am a Civil Engineering student, with no electrical background at all........but i am technical, with IT practice experience. I am working to expand my current electrical system.........and don't want to DIE.....haha.

Thanks to all.

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Last edited by GoyimPersuasion; 01-27-2009 at 12:04 PM.
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