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Why must ground & neutral be seperate in subpanel?



 
 
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  #11  
Old November 28th 06, 12:01 AM posted to alt.home.repair
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Default Why must ground & neutral be seperate in subpanel?


I believe that GFIs are often recommended in cases where there
is no ground for safety. They do not need a ground AFAIN.


They need the ground wire.


When a ground is available at an outlet with a GFCI it is preferred
(and required) that it be installed,

However, where there is no ground the NEC still recognized that a GFCI
can still provide protection by detecting an imbalance in currents
between the neutral and hot wire. In this case, the user must be
informed (with a small sticker on the outlet) that the GFCI is
ungrounded.

Beachcomber

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  #12  
Old November 28th 06, 12:09 AM posted to alt.home.repair
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Default Why must ground & neutral be seperate in subpanel?

Charles Schuler wrote:
"Bob F" wrote in message


I believe that GFIs are often recommended in cases where there
is no ground for safety. They do not need a ground AFAIN.



They need the ground wire.


Actually, they don't.

See section 210-7(d) in the NEC, and section 26-700(9) in the CEC.

GFCIs are a legal substitute for a grounded outlet in an existing
installation where there is no ground available in the outlet box.

Chris
  #13  
Old November 28th 06, 12:49 AM posted to alt.home.repair
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Posts: 376
Default Why must ground & neutral be seperate in subpanel?

All household grounds must meet only at a common point for reasons
similar to why ground loops create hum in a stereo system. Neutral,
equipment ground, and earth ground that connect at a common point (main
breaker box safety ground) avoids 'ground loops' and other adverse or
surprise currents.

To explain same using a different perspective, first, all wires are
electrically different at both ends. That difference increases as more
current flows. To better explain this, we express that difference as a
separation or electrical distance.

A three prong appliance is powered from wires that are distant from
the breaker box (because they carry current). A separate safety
(equipment) ground wire connects directly (shorter) to breaker box
safety ground because it carries no current. Appliance connected
electrically shorter to breaker box means greater human safety.

Again, if safety ground and neutral wire were connected anywhere
(other than in breaker box), then that safety ground wire would be
electrically farther from breaker box (because it carries current).
Another perspective that explains why NEC demands separate neutral and
ground wires.

Another situation: suppose neutral and safety ground wire were both
carrying current. Suddenly that common wire breaks. What happens to
appliance connected to third prong safety ground? It suddenly becomes
electrically hot - directly connected to black hot wire. AND no safety
ground exists to protect human and trip circuit breaker. We want
neutral wire separated from safety ground so that any neutral wire
break always leaves appliance still connected directly to breaker box
safety ground and not connected to a neutral wire that is no longer
connected to breaker box. Just another reason why those two wires
always remain separate.

Home has its own single point safety ground inside breaker box.
Power wires connect that system to another system that has its own
single point ground - pole transformer. Pole transformer connects
primary (high voltage) ground, secondary neutral, and earth ground to a
common point. Lightning strike to primary (high voltage) wire simply
gets conducted safely to earth at transformer which is but one reason
why that primary wire can be highest on pole.

Meanwhile, household single point ground in breaker box is one ground
system centered at a single point. Transformer has its own single
point ground system. How far apart are those two grounds? As current
increases on neutral wire (transformer to house), then both grounds
become electrically more separated. Again, using a perspective of
electrical distance to explain a concept.

wrote:
I installed a subpanel when I switched from an electric stove to gas. I
used the 40A 220V breaker that formerly served the stove to power the
sub panel. .... The grounds and neutrals all share the common bus bar
in the sub panel. Everything has worked fine for years now. Can
someone explain why I read that ground and neutral are to be isolated
in the sub panel? Please don't answer because of the NEC since
that does not explain why. What is the risk of my current situation?


  #14  
Old November 28th 06, 01:17 AM posted to alt.home.repair
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Default Why must ground & neutral be seperate in subpanel?

wrote in message
ups.com...
I installed a subpanel when I switched from an electric stove to gas. I
used the 40A 220V breaker that formerly served the stove to power the
sub panel. The cable is #4 with two conductors and a ground. I have 6-
15 amp breakers in the panel providing branch circuits for my kitchen
and other areas of my house. The grounds and neutrals all share the
common bus bar in the sub panel. Everything has worked fine for years
now. Can someone explain why I read that ground and neutral are to be
isolated in the sub panel? Please don't answer because of the NEC since
that does not explain why. What is the risk of my current situation?

Thanks,
Joe


So, what is the proper way to connect a 220v sub-panel that has a single
bus bar for neutral and ground to a main panel with the neutral bar bonded
to the ground bar?


  #15  
Old November 28th 06, 01:35 AM posted to alt.home.repair
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Posts: 1,963
Default Why must ground & neutral be seperate in subpanel?

On Mon, 27 Nov 2006 15:42:02 -0800, "Bob F"
wrote:


"Charles Schuler" wrote in message
The neutral (white) is a return ... it carries the same current as the hot
wire (black). The ground wire is a non current-carrying safety wire

(often
bare copper). The purpose of the ground wire is to reduce voltages in the
case of lightning or an accident (wires falling across other wires outside
of your home and raising the voltage with respect to ground to a dangerous
level). The ground wire only conducts current in the case of a fault.
Ground fault circuit interrupters need the ground wire to detect such

faults
and open the circuit when they occur.


I believe that GFIs are often recommended in cases where there
is no ground for safety. They do not need a ground AFAIN.

Bob


A GFCI does not need a ground. Then some people get the strange idea
that it PROVIDES one. It does not.
--
28 days until the winter solstice celebration

Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy
  #16  
Old November 28th 06, 01:44 AM posted to alt.home.repair
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Posts: 6,380
Default Why must ground & neutral be seperate in subpanel?

In article , "The Streets" wrote:

So, what is the proper way to connect a 220v sub-panel that has a single
bus bar for neutral and ground to a main panel with the neutral bar bonded
to the ground bar?


There isn't one.

To make a Code-compliant connection, you must install a second bar so that you
can separate the neutral and ground conductors for the various circuits to
separate busses. The neutral bus must be electrically insulated from the
ground bus and from the panel chassis, and the ground bus must *not* be
insulated from the chassis.

*Also* you must connect the subpanel to the main panel using *four*
conductors, e.g. black, red, white, and bare (or green). White goes from the
neutral bus bar in the main panel to the neutral bus bar in the subpanel. Bare
(or green) goes from the ground bus bar in the main panel to the ground bus
bar in the subpanel. Black and red go from the two lugs on the circuit breaker
in the main panel which feeds the sub, to the lugs on the main breaker in the
subpanel.

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.
  #17  
Old November 28th 06, 02:48 AM posted to alt.home.repair
Bob
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Default Why must ground & neutral be seperate in subpanel?


Beachcomber wrote:


What about the transfromer at the pole? Is the neutral center-tap in
the North American System bonded to the transformer enclosure? Is
this point often connected to a ground wire running down the pole and
into the earth?

Also, why in the US systems is the top wire on the pole the hot wire
(for the transformer primary) and the neutral is usually several feet
below this? Is this arrangement not more prone to lightning damage?

That was the old way - keeping the hot wire as far from people/animals
as possible. The vast majority of new installations will put the
ground wire on top.

But what about the rural Canadian systems where there is only a hot
wire with no ground at all? Are they any more susceptable to lightning
than a US hot top wire system?

  #19  
Old November 28th 06, 03:39 AM posted to alt.home.repair
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Posts: 1,963
Default Why must ground & neutral be seperate in subpanel?

On Mon, 27 Nov 2006 21:52:35 -0500, krw wrote:

In article ,
says...
On Mon, 27 Nov 2006 15:42:02 -0800, "Bob F"
wrote:


I believe that GFIs are often recommended in cases where there
is no ground for safety. They do not need a ground AFAIN.

Bob


A GFCI does not need a ground. Then some people get the strange idea
that it PROVIDES one. It does not.


No one said GFCIs did.


I said that because people have. Just not in THIS thread, but in this
group.

BobF said they are "recommended in cases
where there is no ground". They are. What's your beef?


I was saying that only because some people think they provide ground.
--
28 days until the winter solstice celebration

Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy
  #20  
Old November 28th 06, 03:01 PM posted to alt.home.repair
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Posts: 4
Default Why must ground & neutral be seperate in subpanel?


Doug Miller wrote:
In article , "The Streets" wrote:

So, what is the proper way to connect a 220v sub-panel that has a single
bus bar for neutral and ground to a main panel with the neutral bar bonded
to the ground bar?


There isn't one.

To make a Code-compliant connection, you must install a second bar so that you
can separate the neutral and ground conductors for the various circuits to
separate busses. The neutral bus must be electrically insulated from the
ground bus and from the panel chassis, and the ground bus must *not* be
insulated from the chassis.

*Also* you must connect the subpanel to the main panel using *four*
conductors, e.g. black, red, white, and bare (or green). White goes from the
neutral bus bar in the main panel to the neutral bus bar in the subpanel. Bare
(or green) goes from the ground bus bar in the main panel to the ground bus
bar in the subpanel. Black and red go from the two lugs on the circuit breaker
in the main panel which feeds the sub, to the lugs on the main breaker in the
subpanel.

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.


Thanks everyone, especially Doug for this post which spells it out
clearly. Please comment on this proposed "fix":
I run another wire back to the main panel (I'll probably use some 12/2
with ground) and attach all three conductors to the ground bar in the
main panel. Then, at the sub panel, I will connect all grounds to the
new cable but not to the neutral bus bar. Now all grounds will be
grounded back at the main panel, and the neutral in the sub will be
isolated from the grounds and from the sub panel chassis. (as long as I
remove the grounding screw from the neutral bus bar)
Thanks again for your responses.
Joe

 




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