|
Posted by Wayne Whitney on September 15, 2008, 4:12 pm
> The whole grounding thing has me a little confused, here's what I
> think I understand as correct.......(I hope I'm using the correct
> name for all of these things.)
It is confusing. Here's a distinction that helped my understanding
considerably:
"Bonding" of metal parts that should never become energized is done
with the EGC (Equipment Grounding Conductor), which is connected
ultimately to the service neutral. This ensures that in the event
such parts accidentally become energized, the OCPD (overcurrent
protection device) will open. An earth connection can not perform
this function for low voltage (120V-480V) systems, as the earth has
too much impedance.
The entire bonding system is "earthed" to reduce the effect of
accidental overvoltage, either from lightning or POCO (Power Company)
error. This also eliminates any touch potential between the bonded
metal parts and the earth.
> The grounding conductor from the main panel to grounding electrode
> must be continuous, no splices.
The GEC (grounding electrode conductor) from the main panel to the
first grounding electrode must be continuous, no splices. Additional
electrodes can be connected via a bonding jumper to that electrode, or
to the main panel.
Note that main panel here means the first OCPD after the POCO service.
If you have an outside disconnect with a breaker, and an interior load
center, then the outside disconnect is the "main panel", and the
interior load center should be treated as a subpanel.
> The grounding conductor & neutral are only connected at main
> panel.....neutral & ground busses remain isolated at sub panels.
Right, every subpanel feeder needs to have a separate neutral and EGC
in the feeder, and the neutral bar is isolated from the case of the
subpanel.
> An underground metal water pipe (>20') is the preferred grounding
> electrode, if one is available.
An underground metal water pipe is a good grounding electrode,
although there are issues with it. For use as an electrode, you must
connect to it within 5' of its entrance into the building. The
preferred grounding electrode nowadays is the CEE (Concrete Encased
Electrode), i.e. a copper wire running to the rebar in your concrete
foundation. For new construction, you are required to provide this
electrode.
> Grounding rods would be supplemental (?) & merely recommended in
> this case (?) as a backup in case the water line is replaced?
All grounding electrodes present are required to be connected to the
grounding system. A metal water supply is required to have a
supplemental ground rod, in case the water service is replaced with a
nonmetallic service in the future. And a single ground rod requires a
second ground rod unless you can prove that the ground impedance of
the single rod is less than 25 ohms. In practice this measurement is
much harder to do than driving a second ground rod, so if you are
required to have a ground rod, you drive two.
> So ideally the grounding conductor would run from the main panel to
> each of the grounding electrodes (water pipe & ground rods) without
> being cut?
It does not have to, whatever is convenient will work as long as all
grounding electrodes are connected, and there is at least one
unspliced GEC from the "main panel" to one grounding electrode.
> How about the sub panels?
Subpanels should basically be ignored for this discussion. You do
have the option to connect a "supplemental" grounding electrode to a
subpanel, but that is not defined.
Also, if you have a grounding electrode that is much closer to a
subpanel than it is to the main panel, it would be tempting to have
that EGC do double duty as the bonding jumper for that electrode.
Whether this is permissible under the NEC is not clear to me, it is
debatable.
> My old grounding system was a # 4 or # 6 connected to the galvanized
> piping at approximate midpoint of the house.....about 25' from water
> service entrance. But now all the galv is gone.
As mentioned above, you need to connect within 5' of the water service
entrance.
> My plan is to finish the grounding system properly by running a # 4
> from main panel to water service entrance,
#4 Cu is a good choice as long as your service entrance conductors are
3/0 Cu or smaller [or 4/0 Al or smaller]. You can run the #4 Cu as a
bare wire as long as it is not exposed to "severe physical damage".
> My idea was to split nut a grounding conductor from the sub panel
> onto the "main grounding conductor" along its run from main panel.
> Of course, not cutting the #4 "main grounding conductor" at this
> connection, just merely split nutting onto it. This would bond the
> sub panel ground back to the main panel without having to run an
> separate conductor back to the main panel from the sub.
The subpanel should already have a 4 wire feeder including an EGC, so
it should already be bonded to the main panel via the EGC. As I
mentioned it is a grey area whether you can use the subpanel feeder
EGC as a bonding jumper for the grounding electrode. I suggest
avoiding that issue and just running a separate bonding jumper from
the water service entrance to the main panel.
Hope this helps.
Cheers, Wayne
| 
Yahoo! 
Google 
Windows Live 
del.icio.us 
digg 
Netscape 
XML Sitemap