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CAUTIOUSHOMEOWNER 11-23-2008 08:21 PM

Subpanel grounding
 
hello gents!
I have read and read every possible article about subpanels , but something still escapes me, here is the scenario and thank you in advance for your guidance.

I have a detached building that the previous owner , I feel meant to have a a shop, judging by the height of the receptacles. It is now being used as sleeping quarters (very occasionally), no bath, no water, only phone line and cable conductors running to building .

There is a 220V subpanel (with breakers) inside this building, where neutrals and grounds are on the same bus bar, I will be moving the grounds to its own bar which I already purchased.

When I move the grounds to their own bar which would be bonded to the panel, in case of a ground fault, where does the current go if the "tub" is not bonded to anything??????????? does it just disipate ???????

This building is only 4 feet away from the main panel located outside the main house. Did the last owner miss something such as grounding the tub somehow???, do I need to install a ground rod for this subpanel, or run a grounding conductor to the main panel - HELP PLEASE !!!!!!!!!

jerryh3 11-23-2008 08:24 PM

There should be a seperate grounding conductor run between the buildings and a ground rod driven at the detached building. You are correct in that the ground and nuetral should be seperated. How many wires are run to the sub panel? I assume you're refering to the breaker box when you say "tub."

rgsgww 11-23-2008 08:27 PM

In a typical sub panel setup, there are 4 wires, 2 hots and a neutral (PLUS GROUND)

The ground wire goes back to the main panel neutral/ground busbar.

The reason they are isolated at the sub is that in case the neutral was interrupted, the ground would NOT take any current because it is NOT a current carrying conductor.

A ground rod is installed for potential balancing purposes,etc. It IS required.

Billy_Bob 11-23-2008 08:37 PM

What do you mean by "tub"?

CAUTIOUSHOMEOWNER 11-23-2008 09:09 PM

I will have to run a separate ground , back to the main panel, there are 2 sweepts connecting main house to guest house, will i still need the ground bar??

CAUTIOUSHOMEOWNER 11-23-2008 09:10 PM

by "Tub" I meant the pan itself, the can wherein the busbars are installed, sorry if i used the wrong term

chris75 11-23-2008 09:47 PM

Quote:

Originally Posted by CAUTIOUSHOMEOWNER (Post 189228)


There is a 220V subpanel (with breakers) inside this building, where neutrals and grounds are on the same bus bar, I will be moving the grounds to its own bar which I already purchased.


When I move the grounds to their own bar which would be bonded to the panel, in case of a ground fault, where does the current go if the "tub" is not bonded to anything??????????? does it just disipate ???????

This building is only 4 feet away from the main panel located outside the main house. Did the last owner miss something such as grounding the tub somehow???, do I need to install a ground rod for this subpanel, or run a grounding conductor to the main panel - HELP PLEASE !!!!!!!!!


3 wire feeders were quite common, only until recently have they done away with it, so your current install is most likely correctly done.

rgsgww 11-23-2008 09:50 PM

Quote:

Originally Posted by chris75 (Post 189296)
3 wire feeders were quite common, only until recently have they done away with it, so your current install is most likely correctly done.


Isn't the 4 wire feed required when tv/communications cable is ran to the structure? Or is it not?

jerryh3 11-23-2008 09:51 PM

Quote:

Originally Posted by chris75 (Post 189296)
3 wire feeders were quite common, only until recently have they done away with it, so your current install is most likely correctly done.

Even with phone and cable running to the building?

Nestor_Kelebay 11-23-2008 10:36 PM

Quote:

Originally Posted by rgsgww (Post 189234)
In a typical sub panel setup, there are 4 wires, 2 hots and a neutral (PLUS GROUND)

The ground wire goes back to the main panel neutral/ground busbar.

The reason they are isolated at the sub is that in case the neutral was interrupted, the ground would NOT take any current because it is NOT a current carrying conductor.

A ground rod is installed for potential balancing purposes,etc. It IS required.

In my case, I have a small apartment block. I have my main electrical panel and a sub-panel for the electrical plugs in my parking lot. The whites and ground wires are separate at the sub-panel, but I cannot tell what happens in the main panel because it's designed in such a way that I cannot open that main panel without shutting off all the power to the whole building. I have never opened that main panel.

There is a 5/8 inch thick (or so) bare copper cable going into that main panel and it disappears into the ceiling of my electrical room. I have no idea where it comes out, and I've never seen either a 5/8 inch bare copper cable anywhere on my property nor a grounding rod anywhere.

The building was built in 1960 and my parking lot was gravel until 30 years ago when it was paved with asphalt. I'm thinking that the people that repaved the parking lot may have covered my grounding rod with asphalt. Or, even pulled it out when they were excavating in preparation for paving the parking lot and never put it back in.

Am I correct in assuming that the white neutral wires in my parking sub-panel are connected to that 5/8 inch diameter bare copper cable coming out of my main panel?

Is there any easy way of telling where that 5/8 inch diameter bare copper cable goes? Or, is there any way of telling whether my main panel is still properly grounded?

Can you please explain your statement:
"The reason they are isolated at the sub is that in case the neutral was interrupted, the ground would NOT take any current because it is NOT a current carrying conductor."
in plain English so that I can get my head more completely around it. Is it ONLY the ground wire from the sub-panel that goes to this ground/neutral busbar, or is it BOTH the ground wire and neutral wire from the sub-panel that go to the ground/neutral busbar in the main panel? Also, does the white wire in the main panel also get connected to that grounding rod? Finally, what is "potential balancing"?

Billy_Bob 11-24-2008 07:06 AM

Using a separate ground and neutral at the subpanel is for the same reasons you use a separate ground and neutral wire at an outlet.

Most of the time everything is wired and connected properly and appliances are in proper working order.

But sometimes things happen to wiring. A wire may come loose, someone may drill a hole in a wall and through a wire and break the connection, someone may run their lawnmower against the wall and break a ground wire, etc.

And sometimes you have these situations along with a malfunctioning appliance. The electrical codes try to keep things as safe as possible when these situations occur. They can't protect from every situation, but do a pretty good job keeping things safe with most common problems which crop up with wiring.

rgsgww 11-24-2008 07:31 AM

2 Attachment(s)
Quote:

Originally Posted by Nestor_Kelebay (Post 189318)
In my case, I have a small apartment block. I have my main electrical panel and a sub-panel for the electrical plugs in my parking lot. The whites and ground wires are separate at the sub-panel, but I cannot tell what happens in the main panel because it's designed in such a way that I cannot open that main panel without shutting off all the power to the whole building. I have never opened that main panel.

There is a 5/8 inch thick (or so) bare copper cable going into that main panel and it disappears into the ceiling of my electrical room. I have no idea where it comes out, and I've never seen either a 5/8 inch bare copper cable anywhere on my property nor a grounding rod anywhere.

The building was built in 1960 and my parking lot was gravel until 30 years ago when it was paved with asphalt. I'm thinking that the people that repaved the parking lot may have covered my grounding rod with asphalt. Or, even pulled it out when they were excavating in preparation for paving the parking lot and never put it back in.

Am I correct in assuming that the white neutral wires in my parking sub-panel are connected to that 5/8 inch diameter bare copper cable coming out of my main panel?

Is there any easy way of telling where that 5/8 inch diameter bare copper cable goes? Or, is there any way of telling whether my main panel is still properly grounded?

Can you please explain your statement:
"The reason they are isolated at the sub is that in case the neutral was interrupted, the ground would NOT take any current because it is NOT a current carrying conductor."
in plain English so that I can get my head more completely around it. Is it ONLY the ground wire from the sub-panel that goes to this ground/neutral busbar, or is it BOTH the ground wire and neutral wire from the sub-panel that go to the ground/neutral busbar in the main panel? Also, does the white wire in the main panel also get connected to that grounding rod? Finally, what is "potential balancing"?

That bare copper wire is most likely the main ground, does the building have a metal water main? There is not much you can do other than tracking the wire.

Look in the diagrams, the ground is never supposed to take normal current.

The earth has different ground potentials based on the type of soil, etc. If the potentials aren't balanced properly, different things (like cable tv lines, etc) could have high voltages. And in the case of a lightning event, you don't want the current to travel into the main panel to get to ground, you want it to get to ground as soon as possible to maintain minimum damage.

Stubbie 11-24-2008 10:57 AM

I would like to add a bit of clarification . The NEC, though concerned about neutral or objectionable current using the equipment ground of the sub-panel feeder for a return path to the source, the major concern was not an open neutral. Why they required the neutral and ground to be separated with 4 wire feeders to detached building was to prevent objectionable (neutral) current from flowing on the equipment ground of the feeder and the neutral of the feeder (parallel paths) if the bonding did take place. Both situations are certainly concerns regardless.

Grounding has nothing to do with where lightning may go given a closer path.
Grounding electrodes which would be required at a detached building fed with a feeder are for lightning type events and utility power surges. The grounding electrode system is an installation to help protect property.

Lightning will travel wherever it wants on every conductor and every ground means including the equipment ground connecting the house panel to the sub-panel.. It is hoped that if the event is not of too much magnitude in voltage and amperage (not a direct strike) that the grounding electrode system will provide a sink for which most of the damaging voltage and current will quickly decipitate to earth.

It also important to understand that the equipment ground is for human safety and the grounding electrode system is for protection of property. The equipment ground is not an intentional path to earth but a intentional path to the center tap of the source of power for dwelling and building.

Nestor_Kelebay 11-24-2008 07:29 PM

Ok, to see if my understanding is right, in Rgsgww's diagrams, the upper diagram shows an improperly wired subpanel because it has the neutral and ground wires connected together within the subpanel. The proper wiring would have the neutral and ground wires separate at the sub-panel and both connected to the neutral/ground bus bar in the main panel.

And, the reason for this is that the neutral wire can contain "objectionable current" which the NEC doesn't want flowing in the grounding wire between the main panel and sub-panel.

Q#1: Is this "objectionable" current the result of impedance loads that result in the voltage and current sine waves not cancelling each other out in the neutral wire so that the neutral wire carries some voltage and current?

Q#2: Would the green ground/neutral bus bar in RgSgww's main panel normally have a ground wire connected to it going to a grounding rod hammered into the ground on the property somewhere?

Q#3: What is the reason the NEC doesn't want to see those objectionable currents (which are unavoidable in the neutral wire) ALSO flowing in the grounding wire going to the main panel?

Is it because those objectionable voltages and currents can be fairly high, and someone might get a shock if they touch something that's grounded, like a metal electrical box?

Wherein exactly lay the problem in allowing that "objectionable" current from travelling along both the neutral and ground wires to the neutral/ground bus bar in the main panel if they're connected at the main panel to the same bus bar anyhoo?

InPhase277 11-25-2008 05:37 AM

Quote:

Originally Posted by Nestor_Kelebay (Post 189735)
Ok, to see if my understanding is right, in Rgsgww's diagrams, the upper diagram shows an improperly wired subpanel because it has the neutral and ground wires connected together within the subpanel. The proper wiring would have the neutral and ground wires separate at the sub-panel and both connected to the neutral/ground bus bar in the main panel.

And, the reason for this is that the neutral wire can contain "objectionable current" which the NEC doesn't want flowing in the grounding wire between the main panel and sub-panel.

Good so far.

Quote:

Q#1: Is this "objectionable" current the result of impedance loads that result in the voltage and current sine waves not cancelling each other out in the neutral wire so that the neutral wire carries some voltage and current?
Not necessarily. Objectionable current could simply be the normal current of a circuit diverted onto the equipment grounding conductor accidentally.

Quote:

Q#2: Would the green ground/neutral bus bar in RgSgww's main panel normally have a ground wire connected to it going to a grounding rod hammered into the ground on the property somewhere?
A rod or any of a number of other grounding electrodes, yes.

Quote:

Q#3: What is the reason the NEC doesn't want to see those objectionable currents (which are unavoidable in the neutral wire) ALSO flowing in the grounding wire going to the main panel?

Is it because those objectionable voltages and currents can be fairly high, and someone might get a shock if they touch something that's grounded, like a metal electrical box?
Yes. Electricity takes every path available to it to complete a circuit. By keeping the number of paths under control, we keep know where the current is or should be. If we allow it to take multiple paths, we'd get some current in the neutral, some in the ground wire, some in the earth itself, some along other wiring paths, like coax shields, some along water pipes etc. It is this lack of control and predictability that makes it "objectionable" and dangerous.


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