Why Service Panels don't need isolated neutral busses?

Originally Posted By: Aaron Rosenbaum
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Just curious if anyone knew why service panels don’t have isolated ground and neutral busses (they have seperate busses, but they are bonded), but subpanels do???..

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Originally Posted By: jmyers
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Aaron,


The buss bars in the sub panels are isolated to prevent a condition known as a ground loop.

Joe Myers

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Originally Posted By: gbeaumont
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Hi Joe, I thought a ground loop was when you were landing an aircraft, and one wing tip hit the ground and span the plane around, I’ll be so glad when you have a copy of NEC 2002 on disc


Gerry

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Gerry Beaumont
NACHI Education Committee
e-mail : education@nachi.org
NACHI phone 484-429-5466

Inspection Depot Education
gbeaumont@inspectiondepot.com

"Education is a journey, not a destination"

Originally Posted By: Aaron Rosenbaum
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jmyers wrote:

Aaron, The buss bars in the sub panels are isolated to prevent a condition known as a ground loop. Joe Myers

Kind of like in a car audio setup (power wires on one sideof the car.... RCAs on the other)

Gotcha.... Thanks 

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Originally Posted By: jmyers
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Gerry,


You are funny......Looking! 

Aaron,

The idea behind the ground is to send electricity safely to earth (that is where it wants to go to begin with), not to send it all over HE**'s creation before it gets there. Remember, electricity takes the path of least resistence which is what you are providing with the ground path. Thus all sub panels should have isolated ground/neutral buss bars with each panel having a seperate ground wire and rod.

Ground loops as referred to in electronic equipment are the same ground loops that are in electrical panels. The major difference between the two is that while it is present in electronic equipment it causes unwanted humming or buzzing. When you are talking about the ground loop in a panel you are talking about a dangerous situation that causes electricity to cycle through the panel instead of sending it to the ground rod (the path of least resistence).

I hope that was easy enough for you to follow, explaining this kind of stuff is not my strong point. 

Joe Myers

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Originally Posted By: Aaron Rosenbaum
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Thanks for all the help I really appreciate it…

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Originally Posted By: jmyers
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Aaron,


I stand corrected, sub panels do not need seperate ground wires and rods unless they are in a different building from the main service panel. Sorry! 

Joe Myers

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Originally Posted By: gbeaumont
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Hey Joe


If you had NEC on CD rom you wold have known that



Still busting you

Your Pal

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Gerry Beaumont
NACHI Education Committee
e-mail : education@nachi.org
NACHI phone 484-429-5466

Inspection Depot Education
gbeaumont@inspectiondepot.com

"Education is a journey, not a destination"

Originally Posted By: jmyers
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Gerry,


Yeh, I should have it but it most have gotten lost in the mail! 

Joe Myers

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Originally Posted By: Dennis Bozek
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Yo Joe


A subpanel regardless of where it is located must have a bond back to the main panel via a ground wire, which would be connected to a separate ground buss. The neutral buss would not be bonded to the enclosure and be allowed to float. In some instances, a ground rod is used to ground the subpanel if it is located in another building.

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This information has been edited and reviewed for errors by your favorite resident sparky.

Originally Posted By: Dennis Bozek
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oops

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This information has been edited and reviewed for errors by your favorite resident sparky.

Originally Posted By: jmyers
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Thanks Dennis.


Joe Myers

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Originally Posted By: Dennis Bozek
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Your welcome Joe

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This information has been edited and reviewed for errors by your favorite resident sparky.

Originally Posted By: Bob Badger
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OK I know this is a little old but it is an important topic and is one of the most misapplied, misunderstood topics we could touch on.

Quote:

The idea behind the ground is to send electricity safely to earth (that is where it wants to go to begin with), not to send it all over HE**'s creation before it gets there.

That is just not true of utility supplied power, lightning yes but lets stick with the power we use daily.

Electricity "wants" to go back to it's source (even that is not exactly correct but correct enough for our purpose) 

There are electric systems in use in hospitals called isolated systems that have no potential to ground. meaning that you could literally be laying down in a puddle and hold on to one of the phase conductors and not receive a shock.(Not all circuits in a hospital are like this usually reserved for operating rooms)

There are also systems in industrial facility's called ungrounded deltas that while will give a person a good blast will not short out if a "hot" conductor touches grounded equipment.

The key issue with those system is that none of the circuit conductors are bonded (attached) to the earth.

Now all services suppling dwelling units in the USA are from a bonded (grounded) source.

At the POCO (power company) transformer the neutral conductor is immediately attached to the ground at the pole or pad.

At the service panel this is done again, the Neutral is firmly connected to the grounding electrodes (water pipe, ground rods, footing grounds) and also to the panels ground bar and enclosure.

Now say you have a fault from a hot conductor to a ground conductor inside the house. The fault current will flow through the ground wire back to the panel then go into the neutral conductor back to the source (the POCO transformer)

Now here is where it gets tough, at both the POCO transformer and at the house we have the neutral attached to the earth.

Some of the fault current will go out the ground rod at the house and into the ground rod at the POCO transformer using the earth as a conductor.

Quote:

electricity takes the path of least resistance

True but not the whole truth, electricity takes all paths available, dividing the current flow in proportion to the resistance of each path.

As an example if you have a bucket of water with one 2" hole in it and one 1" hole in it, the water will flow out both holes just at different rates.

Now you are outside standing on the ground and you come in contact with a live wire you now complete the circuit back to the POCO transformer using the earth as a conductor.

With that out of the way back to the original question, Why are are service panels the only place inside the house that the neutral and grounds meet.

It is all about not creating parallel paths for neutral current and keeping the neutral at the same electrical potential to earth.

Lets say you have a house with both a service panel and a sub panel both of which have the neutral and ground bonded.

If this sub panel is heavily loaded the neutral conductor will have voltage drop due to resistance, this voltage drop changes the potential between the neutral and ground.

So now you are in the kitchen working and you touch the grounded plumbing fixtures at the same time you grab the refrigerator.

The plumbing fixtures will be a 0 volts to ground but the refrigerator case will not be at 0 volts to ground as the voltage drop on the neutral has created a difference of potential and you end up getting a shock.

There are also problems with parallel paths for neutral current and I can explain that if anyone is interested.

By the way all separate buildings with a panel or more than one branch circuit require a grounding electrode.

So a small shed with two circuits should have a ground rod attached to the grounding conductors.

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Bob (AKA iwire)
ECN Discussion Forums
Mike Holt Code Forum

Originally Posted By: Bob Badger
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OK I just noticed the question was Why service panels do not need isolated neutral bars.


I answered why sub panels need isolated neutrals.

If you read above you may have figured it out already.

If we did not connect the neutrals to the grounds at the service panel we would not have what is called an adequate fault clearing path.

Lets say the neutrals and grounds where kept separate at the main panel and somewhere in the house a 15 amp 120 volt plug circuit has a fault from the hot conductor to the grounding conductor.

The fault current would flow through the ground wire and out the ground rod "trying" to get back to the POCO transformer.

And it will do that BUT the combined resistance of the ground rod at the house, the earth itself and the POCO ground will be to high to trip the breaker.

It is hard to say what the total resistance would be it changes depending on how wet the soil is.

But lets use 25 ohms as that is what the NEC would like for a ground rod (25 ohms or less)

120 volts / 25 ohms = 4.8 amps of current flow.

This will not even warm a 15 amp breaker never mind trip it.

By attaching the neutrals to the grounds at the service panel you create a low resistance path back to the transformer.

Ground rods are for two things High voltage utility company faults and lightning.

They have nothing to do with the day to day operation of a the house wiring.

If one of the POCO high voltage lines breaks and falls across the overhead service drop to the house it may try to energize the bare neutral ground conductor on that overhead feed.

Your house wiring will not like 13,800 volts, if this happens the ground rod will help.

13,800 volts / 25 ohms = 552 amps which will be about 50 times more current then the POCOs high voltage fuse, this will cause the fuse to open ASAP hopefully before the romex bursts into flame. 

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Bob (AKA iwire)
ECN Discussion Forums
Mike Holt Code Forum

Originally Posted By: roconnor
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Bob Badger wrote:

As an example if you have a bucket of water with one 2" hole in it and one 1" hole in it, the water will flow out both holes just at different rates.

I like that analogy ... 

Good explanations ... I just hope we didn't lose some of you guys. That also explains why a remote appliance (e.g. dryer or water heater in a detached garage) MUST be on a circuit with a ground wire back to the service panel (where the ground and neutral wires are connected).

The ground can NOT just be a driven ground rod, because as Bob explained the 4.8A of current from a fault just isn't enough to trip a breaker ...

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Robert O'Connor, PE
Eagle Engineering ?
Eagle Eye Inspections ?
NACHI Education Committee

I am absolutely amazed sometimes by how much thought goes into doing things wrong

Originally Posted By: Bob Badger
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I have not figured out a way to explain any of that clearly and concisely.


The shortest answers I can come up with.

Why do we bond the neutral to ground at a service panel?

To create a low resistance fault clearing (breaker tripping) path.

Why must the neutrals and grounds always be separate after leaving the service panel?

To prevent parallel paths for neutral current and to keep the grounding conductor at the same potential of the earth.

Why can't a ground rod be driven for a circuit that does not have a ground conductor?

To much resistance in the earth and rods to trip a breaker or melt a fuse.

This is a tough subject and many good electricians eyes glaze over if they try to think about it.

This does not mean I am special, I am just interested in the subject so I have made a point to learn all I can about it, I still have much to learn.

I will say this it is much easier to explain with some diagrams where you can follow the current flow.

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Bob (AKA iwire)
ECN Discussion Forums
Mike Holt Code Forum

Originally Posted By: jremas
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Outstanding, Superb and well needed on this forum. Thank you Bob B.

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Jeff Remas
REMAS Inspections, Inc.
Northeastern PA & the Poconos
www.NEPAinspector.com

570-362-1598

Originally Posted By: Mike Parks
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Rob


"The ground can NOT just be a driven ground rod, because as Bob explained the 4.8A of current from a fault just isn't enough to trip a breaker ... "

The electrode is not for fault protection.

Mike P.

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Originally Posted By: roconnor
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Bob … I think NFPA is partially to blame for some people not understanding the grounding requirements due to the terminology they use in the NEC.


Per the NEC ... we have grounded conductor, grounding conductor, grounding system, grounding electrode conductor, ground rod, grounding electrode, grounded system, grounding electrode system, bla bla bla ... all meaning different things with two different basic purposes, which makes peoples head spin if they think about it too much ... 

You can "ground" something with a "grounding electrode conductor", but it's not the same as connecting the " grounding conductor" to the "grounded conductor" ... DOUBLE ARGH even though I understand what they are saying ...

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Robert O'Connor, PE
Eagle Engineering ?
Eagle Eye Inspections ?
NACHI Education Committee

I am absolutely amazed sometimes by how much thought goes into doing things wrong