PGates
Member
- Location
- United States
Hi all! I live and work in a remote mountain village with a private hydro electric dam with Pelton Water Wheels that produce about 200KW of power that supplies the whole village. We have been working to upgrade our whole infrastructure from overhead power to underground power. I work with an IEEE engineer who came from the Power Distribution and Utility side.
We are in this odd place where we are our own Utility "sort of". Fortunately we have a good working relationship with the state Electrical Inspector
The soils up here are very rocky and sandy and during the summer quite dry.
When we were planning our new underground system there was lots of
conversation about not having "good grounding" and whether or not our system
was "safe"as a result.
Since we had to put in conduit around, across and through the village for
our new underground distribution system (the village consists of 25
buildings including 14 Chalets, 6 dorm/lodges, a Hotel with large commercial
kitchen and various outbuildings), we put in a ground grid in
the form of a #2 bare copper conductor buried in the trenches above the
conduit. In all we have a little over 5000 feet of this conductor in the
trenches, cad-welded together and brought to each of the 13 outside
pad-mounted transformer vaults and 3 switchgear vaults. I tried early on to
drive ground rods but was unsuccessful due to all the large rocks here in
the mountains. I consulted the AHJ and he agreed to allow this new ground
grid instead. We have not brought this new ground grid to the individual
buildings as yet, though we could do so either by running it with the
service entrance conductors or else by digging it up and cad-welding an
extension to the location of the existing ground rods or metal water pipes
and then up to the main disconnect or service panel.
Since our ground grid is present at all of the transformer vaults that is
where we'd like to establish the Neutral and bond the XO to ground.
All our conduits are 3 inch schedule 80 PVC. Our MV is 2400 volts and the
secondary will be 120/208 3-phase wye.
So that is the background. Where we are at now is the conduits are in and we
are getting ready to pull new MV cables and then set the xfmrs and begin
cutting the buildings over one at a time.
I'd like to give you an example of a couple of the buildings:
In our Administrative Building, ABC = 2 parallel 500 Kcmil Aluminum, the Neutral is (based on
250.24 (C) 2014 NEC Table 250.102(C)1 ) would be 4/0 Aluminum.
in the Hotel, we have 3 parallel feeds each with ABC = 250Kcmil AL , so the Neutral would be 3/0 AL?
One of the questions is do we need another grounding or bonding conductor in
these conduits? Why or why not?
Because of our "New and Improved Grounding Grid of bare #2 copper" that we
installed in all the trenches, we would establish the neutral at the
transformer and bond it to the grounding grid there using a listed pressure
connector approved for Al/Cu connection in the transformer vault. We would
then float the Neutral at the Main Disconnect or Service Panel in the
building since we already BONDED the Neutral at the xfmr, right?
Which still leaves the question of the "Grounding conductor" (Would this be
the Supply Side Bonding Jumper from 250.30 A(2) AND/OR the GEC as in
250.24 E?) -- sized from 250.66 ? Or something else?
Re the GEC and the GES... It would be nice since the soils in the village
are so poor to also have a connection from the Service Panel grounding bus
to the new grounding grid which is present at the xfmr, and then we could
bond that to the existing other ground rods and pipes etc. If we do this,
the ground grid is all #2 copper, so our connection to it would also be #2
copper, right?
We do get some pretty substantial lightning storms up here...
I am an 01 licensed electrician in WA and I'm using the 2014 code book.
We are in this odd place where we are our own Utility "sort of". Fortunately we have a good working relationship with the state Electrical Inspector
The soils up here are very rocky and sandy and during the summer quite dry.
When we were planning our new underground system there was lots of
conversation about not having "good grounding" and whether or not our system
was "safe"as a result.
Since we had to put in conduit around, across and through the village for
our new underground distribution system (the village consists of 25
buildings including 14 Chalets, 6 dorm/lodges, a Hotel with large commercial
kitchen and various outbuildings), we put in a ground grid in
the form of a #2 bare copper conductor buried in the trenches above the
conduit. In all we have a little over 5000 feet of this conductor in the
trenches, cad-welded together and brought to each of the 13 outside
pad-mounted transformer vaults and 3 switchgear vaults. I tried early on to
drive ground rods but was unsuccessful due to all the large rocks here in
the mountains. I consulted the AHJ and he agreed to allow this new ground
grid instead. We have not brought this new ground grid to the individual
buildings as yet, though we could do so either by running it with the
service entrance conductors or else by digging it up and cad-welding an
extension to the location of the existing ground rods or metal water pipes
and then up to the main disconnect or service panel.
Since our ground grid is present at all of the transformer vaults that is
where we'd like to establish the Neutral and bond the XO to ground.
All our conduits are 3 inch schedule 80 PVC. Our MV is 2400 volts and the
secondary will be 120/208 3-phase wye.
So that is the background. Where we are at now is the conduits are in and we
are getting ready to pull new MV cables and then set the xfmrs and begin
cutting the buildings over one at a time.
I'd like to give you an example of a couple of the buildings:
In our Administrative Building, ABC = 2 parallel 500 Kcmil Aluminum, the Neutral is (based on
250.24 (C) 2014 NEC Table 250.102(C)1 ) would be 4/0 Aluminum.
in the Hotel, we have 3 parallel feeds each with ABC = 250Kcmil AL , so the Neutral would be 3/0 AL?
One of the questions is do we need another grounding or bonding conductor in
these conduits? Why or why not?
Because of our "New and Improved Grounding Grid of bare #2 copper" that we
installed in all the trenches, we would establish the neutral at the
transformer and bond it to the grounding grid there using a listed pressure
connector approved for Al/Cu connection in the transformer vault. We would
then float the Neutral at the Main Disconnect or Service Panel in the
building since we already BONDED the Neutral at the xfmr, right?
Which still leaves the question of the "Grounding conductor" (Would this be
the Supply Side Bonding Jumper from 250.30 A(2) AND/OR the GEC as in
250.24 E?) -- sized from 250.66 ? Or something else?
Re the GEC and the GES... It would be nice since the soils in the village
are so poor to also have a connection from the Service Panel grounding bus
to the new grounding grid which is present at the xfmr, and then we could
bond that to the existing other ground rods and pipes etc. If we do this,
the ground grid is all #2 copper, so our connection to it would also be #2
copper, right?
We do get some pretty substantial lightning storms up here...
I am an 01 licensed electrician in WA and I'm using the 2014 code book.
