Why can i install a service lateral in rigid steel, 3 wire
triplex to a dwelling, but not it's outbuildings?

The best way that I can answer your question is you  provide you
a copy of the rule that applies in the NEC then read the
commentary after the copy written by me as a reply. See the copy
of the rule that applies below;

240-32
(b) Grounded Systems. For a grounded system at the separate
building or structure, the connection to the grounding electrode
and grounding or bonding of equipment, structures, or frames
required to be grounded or bonded shall comply with either (1) or
(2).
(1) Equipment Grounding Conductor. An equipment grounding
conductor as described in Section 250-118 shall be run with the
supply conductors and connected to the building or structure
disconnecting means and to the grounding electrode(s). The
equipment grounding conductor shall be used for grounding or
bonding of equipment, structures, or frames required to be
grounded or bonded. Any installed grounded conductor shall not be
connected to the equipment grounding conductor or to the
grounding electrode(s).
(2) Grounded Conductor. Where (1) an equipment grounding
conductor is not run with the supply to the building or
structure, and (2) there are no continuous metallic paths bonded
to the grounding system in both buildings or structures involved,
and (3) ground-fault protection of equipment has not been
installed on the common ac service, the grounded circuit
conductor run with the supply to the building or structure shall
be connected to the building or structure disconnecting means and
to the grounding electrode(s) and shall be used for grounding or
bonding of equipment, structures, or frames required to be
grounded or bonded.


Be sure to notice part 2 or rule 2 in the last section above
where it say where there are no continuous metallic paths bonded
the thre grounding system in both buildings. This is where the
concern lies. The concern is what is called a difference of
potential. If you look out on a 7,200 volt primary line you can
see a bird sitting on that energized power line and that bird is
quite happy. This is because there is no difference of
potentiality between the high voltage line that bird is touchning
and anything else that bird is touching. Even though there is
7.200 volts present on that line that bird is touching there is
no other metal path that the bird is also touching that is of a
different potential than the energized wire. The electrical shock
can only occur if there is a place for the 7,200 volt line to
flow its power to. All is the same potential. Now if that bird
touched a different metal that has a path back to the center
neutral tap of the transomer or touched a path to mother earth
then that electricity has someplace to flow through that birds
body, most likely disitegrating that bird to feathers only. The
shock is not present because there is nowhere that the 7,200
volts to flow to. All is equal.

When you connect power to your dwelling from the Utility company
or a generator supplied by you, you have a connection that is a
separately derived power source separate from all other entities.

When you connect power from that dwelling that has power to the
detached building that is getting its power through the first
structure then you have what is called a common service.

If you have a non current carrying metallic path that is
connecting to both structures then you must ensure that both
buildings are at the same potential to limit exosure to shock.
When this metallic path connects both buildings as one building
then you must run the equipment grounding conductor with the
feeder from the first building to the second to ensure that same
potential between the two common buildings. This wiring method
would be following choice 1 of the two rules above of the NEC
including a four wire feeder between the two buildings and the
fourth wire of that feeder being an equipment grounding conductor
connecting all non current carrying metallic paths such as water
pipes as one entity totally between the two buildings. This panel
must be treated as a sub panel separating the white neutral bar
and the green or bare equipment grounding bar in the panel
located in that second building fed by a common service of the
two buildings.

If you have no non current carrying metallic paths connecting
between the two buildings such as metal water pipes, ect., then
you may wire a 3 wire feeder without an equipment grounding
conductor being run with that feeder as the fourth wire of that
feeder. YOu then must install a new grounding electrode system
just like you did in the main dwelling when you received power to
that main dwelling from a separtely derived source. This three
wire feeder with a new ground rod etc. installed in that second
building makes this panel installed in that detached structure to
be treated the same as the main service of the dwelling. YOu are
required to marry both the neutral and the equipment grounding
bars together as one entity. This connecting the neutral and
grounding bar together in the panel installed in the detached
structure because you have no non current carrying metallic
existing paths between the two buildings using the common service.

The main concern is the difference of potentiality that would be
amplified if not wired with the fourth wire if there is a metal
gas line or metal water line, or even a small grounding conductor
in your phone line installed between the two buildings. YOu would
not have this concern if this building was attached directly from
the Utility power supply, only if you are receiving power to that
detached structure from another building supplying power to both
buildings from the same power supply.

Hope this helps

Wg

Thank you Warren.
The prob i'm having is that i need to think conceptually,I like
to know why i install as i do. Not just walk around spittin'
codes,most electricians can do the code 'razel-dazel' but may
fall short of the true theory.
This is a potential Q that a customer could ask, so i am now
perpared better to reply per your post....thanx!

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