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Home > Archive > Electrical Engineering > February 2007 > Grounding/Bonding in a subpanel
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Grounding/Bonding in a subpanel
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| Randell Tarin 2007-02-24, 8:25 pm |
| SETUP: 200 AMP Main Panel connected to a 100 AMP sub-panel via metallic
nipple. A 75 AMP breaker in the main panel feeds the sub via #4
conductor. The Neutral is isolated from the ground in the sub. Bare
grounding conductors are attached to the subpanel through an equipment
bar attached to the metallic case of the sub.
Can someone explain to me why the NEUTRAL and GROUND must be isolated in
the sub-panel since they are already bonded in the Main panel. Since
the ground in the SUB is derived from the link of the two metallic
enclosures, isn't the NEUTRAL in the SUB already bonded to the ground???
What happens if the NEUTRAL AND GROUND are also bonded in the subpanel?
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| Ryan Evans 2007-02-24, 8:25 pm |
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If N & G are bonded in the subpanel then the grounding conductor(s) become
current carrying conductor(s).
This could cause magnetic field problems.
RE
"Randell Tarin" <thetarins@earthlink.net> wrote in message
news:9b4Eh.6212$Jl.2121@newsread3.news.pas.earthlink.net...
> SETUP: 200 AMP Main Panel connected to a 100 AMP sub-panel via metallic
> nipple. A 75 AMP breaker in the main panel feeds the sub via #4 conductor.
> The Neutral is isolated from the ground in the sub. Bare grounding
> conductors are attached to the subpanel through an equipment bar attached
> to the metallic case of the sub.
>
> Can someone explain to me why the NEUTRAL and GROUND must be isolated in
> the sub-panel since they are already bonded in the Main panel. Since the
> ground in the SUB is derived from the link of the two metallic enclosures,
> isn't the NEUTRAL in the SUB already bonded to the ground???
>
> What happens if the NEUTRAL AND GROUND are also bonded in the subpanel?
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| Randell Tarin 2007-02-24, 8:25 pm |
| O.K., but to repeat:
Since the ground in the SUB is derived from the link of the two metallic
enclosures, isn't the NEUTRAL in the SUB already bonded to the ground
since they are bonded in the MAIN?
Ryan Evans wrote:
> If N & G are bonded in the subpanel then the grounding conductor(s) become
> current carrying conductor(s).
>
> This could cause magnetic field problems.
>
> RE
>
> "Randell Tarin" <thetarins@earthlink.net> wrote in message
> news:9b4Eh.6212$Jl.2121@newsread3.news.pas.earthlink.net...
>
>
>
>
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| Ben Miller 2007-02-24, 9:25 pm |
| "Randell Tarin" <thetarins@earthlink.net> wrote in message
news:9b4Eh.6212$Jl.2121@newsread3.news.pas.earthlink.net...
> SETUP: 200 AMP Main Panel connected to a 100 AMP sub-panel via metallic
> nipple. A 75 AMP breaker in the main panel feeds the sub via #4 conductor.
> The Neutral is isolated from the ground in the sub. Bare grounding
> conductors are attached to the subpanel through an equipment bar attached
> to the metallic case of the sub.
>
> Can someone explain to me why the NEUTRAL and GROUND must be isolated in
> the sub-panel since they are already bonded in the Main panel. Since the
> ground in the SUB is derived from the link of the two metallic enclosures,
> isn't the NEUTRAL in the SUB already bonded to the ground???
>
> What happens if the NEUTRAL AND GROUND are also bonded in the subpanel?
Neutral and ground are only bonded at the service entrance. If they are
connected together elsewhere in the system, then the grounding conductors
will carry some of the neutral return current. This can cause elevated
potential on the housings of equipment, appliances, counduit, etc which
creates a possible electric shock hazard. In the event of an open neutral at
the supply, those exposed conductive surfaces could reach lethal voltage
levels.
Ben Miller
--
Benjamin D. Miller, PE
B. MILLER ENGINEERING
www.bmillerengineering.com
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| Randell Tarin wrote:
> O.K., but to repeat:
>
> Since the ground in the SUB is derived from the link of the two metallic
> enclosures, isn't the NEUTRAL in the SUB already bonded to the ground
> since they are bonded in the MAIN?
>
Yes it is bonded to ground back at the service. Neutral and ground are
bonded at the service so a failure that connects hot to metal frames,
conduits, ... will have a solid path back to the source transformer (via
ground - service neutral,)resulting in high current, resulting in rapid
breaker trip.
The service neutral-ground connection is also earthed to limit the
voltage from earth to frames, conduits, ... and minimize the voltage
from earth to the hot and neutral wires.
But bonding downstream puts ground conductors in parallel with the
neutral, which puts neutral current on the ground wires as both Ben and
Ryan said.
I would view the major reason not to bond donwstream the same as Ben.
Another reason is neutral current on the ground system will put
electrical 'noise' on the ground system which can cause problems with
electronic equipment.
--
bud--
[color=darkred]
> Ryan Evans wrote:
>
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| Randell Tarin 2007-02-25, 9:25 am |
| Bud-- wrote:[color=darkred]
> Randell Tarin wrote:
>
>
> Yes it is bonded to ground back at the service. Neutral and ground are
> bonded at the service so a failure that connects hot to metal frames,
> conduits, ... will have a solid path back to the source transformer (via
> ground - service neutral,)resulting in high current, resulting in rapid
> breaker trip.
>
> The service neutral-ground connection is also earthed to limit the
> voltage from earth to frames, conduits, ... and minimize the voltage
> from earth to the hot and neutral wires.
>
> But bonding downstream puts ground conductors in parallel with the
> neutral, which puts neutral current on the ground wires as both Ben and
> Ryan said.
>
> I would view the major reason not to bond donwstream the same as Ben.
> Another reason is neutral current on the ground system will put
> electrical 'noise' on the ground system which can cause problems with
> electronic equipment.
>
> --
> bud--
>
>
Thank you Bud. I get it now. It was just somewhat counter-intuative.
For me it was a matter of understanding the actual function of the GROUND.
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| Long Ranger 2007-02-25, 1:25 pm |
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"Randell Tarin" <thetarins@earthlink.net> wrote in message
news:9b4Eh.6212$Jl.2121@newsread3.news.pas.earthlink.net...
> SETUP: 200 AMP Main Panel connected to a 100 AMP sub-panel via metallic
> nipple. A 75 AMP breaker in the main panel feeds the sub via #4 conductor.
> The Neutral is isolated from the ground in the sub. Bare grounding
> conductors are attached to the subpanel through an equipment bar attached
> to the metallic case of the sub.
>
> Can someone explain to me why the NEUTRAL and GROUND must be isolated in
> the sub-panel since they are already bonded in the Main panel. Since the
> ground in the SUB is derived from the link of the two metallic enclosures,
> isn't the NEUTRAL in the SUB already bonded to the ground???
>
> What happens if the NEUTRAL AND GROUND are also bonded in the subpanel?
You don't want your ground to carry the neutral current in the event of a
loss of the neutral connection. Two reasons: 1. The grounds aren't supposed
to carry current for obvious reasons, and 2. Your ground is likely a #8
copper in this case and it isn't sized to carry the current that might
possibly come from that sub-panel. It might help to imagine a regular duplex
receptacle. You wouldn't connect the ground and the neutral at the
receptacle, would you? Same thing, larger scale in your sub-panel.
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| Anusol Man 2007-02-25, 1:25 pm |
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"Randell Tarin" <thetarins@earthlink.net> wrote in message
news:9b4Eh.6212$Jl.2121@newsread3.news.pas.earthlink.net...
> SETUP: 200 AMP Main Panel connected to a 100 AMP sub-panel via metallic
> nipple. A 75 AMP breaker in the main panel feeds the sub via #4 conductor.
> The Neutral is isolated from the ground in the sub. Bare grounding
> conductors are attached to the subpanel through an equipment bar attached
> to the metallic case of the sub.
>
> Can someone explain to me why the NEUTRAL and GROUND must be isolated in
> the sub-panel since they are already bonded in the Main panel. Since the
> ground in the SUB is derived from the link of the two metallic enclosures,
> isn't the NEUTRAL in the SUB already bonded to the ground???
>
> What happens if the NEUTRAL AND GROUND are also bonded in the subpanel?
| |
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| Randell Tarin wrote:
> Bud-- wrote:
>
>
>
> Thank you Bud. I get it now. It was just somewhat counter-intuative.
> For me it was a matter of understanding the actual function of the GROUND.
The grounding chapter in the NEC is probably the most confusing of the
commonly used ones. I saw a video from Mike Holt where he talked about
"earthing" and "bonding". "Grounding" implies earthing, which largely is
not what ground wires do. IMHO "grounding" is almost hoplessly
confusing. I have grown to really like "earthing", which is used in the
UK, and clearly describes a part of the "grounding" function. The 2008
NEC is supposed to revise the language a little, but probably not as
much as I would like to see.
--
bud--
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| Matthew Beasley 2007-02-26, 1:25 pm |
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"Randell Tarin" <thetarins@earthlink.net> wrote in message
news:9b4Eh.6212$Jl.2121@newsread3.news.pas.earthlink.net...
> SETUP: 200 AMP Main Panel connected to a 100 AMP sub-panel via metallic
> nipple. A 75 AMP breaker in the main panel feeds the sub via #4 conductor.
> The Neutral is isolated from the ground in the sub. Bare grounding
> conductors are attached to the subpanel through an equipment bar attached
> to the metallic case of the sub.
>
> Can someone explain to me why the NEUTRAL and GROUND must be isolated in
> the sub-panel since they are already bonded in the Main panel. Since the
> ground in the SUB is derived from the link of the two metallic enclosures,
> isn't the NEUTRAL in the SUB already bonded to the ground???
>
> What happens if the NEUTRAL AND GROUND are also bonded in the subpanel?
Others have already pointed out the code requires them to be separate.
They have also mentioned that bonding them together will cause a portion of
the neutral current to return via the ground wire.
As a practical matter, your panels are very close to each other and likely
won't cause a problem. But consider what happens if the panels are further
apart: The neutral current can develop a significant voltage drop between
the panels. If both panels are bonded to ground, there will be a voltage
difference between grounds in the house. This can cause current to flow in
other paths that were never intended to have a current flow, like the
building structure, phone, sat/cable TV, computer network cable, etc..
Worst case is if a fault occurs - the 100's or 1000's of amps of fault
current can cause 10's of volts difference with very low source impedance.
That could cause a significant current through the stray paths, causing
damage to the other conductors or even a fire.
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