A code change for sub-panels in separate buildings where the feeder does not contain a grounding conductor.
2009 IRC E3607.3.2 Grounded conductor, existing premises.
This section shall apply only to existing premises wiring systems.
Where an equipment grounding conductor is not run with the
supply conductors to the building or structure, there are no continuous
metallic paths bonded to the grounding system in both
buildings or structures involved, and ground-fault protection of
equipment has not been installed on the supply side of the
feeder(s), the grounded conductor run with the supply to the
buildings 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.
Where used for grounding in accordance with this provision,
the grounded conductor shall be not smaller than the larger of:
- That required by Section E3704.3.
- That required by Section E3908.12.
IRC Commentary
Where the feeder that supplies the separate building is
run without an equipment grounding conductor, the
rules of this sub-section apply. Note that this section
applies only to existing wiring systems. If, for example,
new wiring was to be installed to supply a building that
did not have power run to it, the code intends to require
the installation to comply with Section E3607.3.1. In
previous editions of the code, this section applied to
new and existing installations and this was considered
to be an unnecessarily risky practice. Recall that the
code recently underwent similar revisions regarding
the grounding of electric clothes dryers and ranges,
where the long-standing practice of grounding the appliance
frames to the grounded (neutral) conductor
was banned, except for existing installations.
The preferred method is covered in Section
E3607.3.1 and for new installations, there is no excuse
for not complying with the preferred safer method, thus
this section (E3607.3.2) was revised to apply only to
existing installations. Let’s assume that the panelboard
at the workshop discussed in the previous section
is served by an existing underground feeder cable
consisting of three conductors: two ungrounded and
one neutral conductor of the appropriate sizes. The
neutral or grounded conductor of the feeder is
grounded again at the separate building by a grounding
electrode conductor connected to the separate
building grounding electrode(s).
The panelboard cabinet of the separate building may have
one terminal bus for the grounded conductors and another for the
grounding conductors; if so, they must be bonded together
or bonded to the cabinet by a main bonding
jumper. Or there may be only one terminal bus to connect
all grounded and grounding conductors. In this
case, the bonding screw supplied for the purpose of
bonding the neutral terminal strip to the metal cabinet
is installed, and this bonding screw (with a green
head) is considered a main bonding jumper. The
grounding electrode is also bonded to the equipment-
grounding terminal, which is bonded to the metal
cabinet.
An important rule that applies where only a
three-conductor feeder is run to the separate building
and the feeder grounded conductor is bonded to the
grounding electrode system at the separate building is
that there must be no common continuous metallic
paths bonded to the grounding electrode system in
both buildings. Bonding at both ends of such objects
as a metal water pipe or gas pipe that was run from the
house to the separate building could result in a parallel
path for neutral current as well as for fault current. If
the neutral were broken or became disconnected, the
return current could be through the metal pipe, which
would cause a dangerous potential on noncurrent carrying
metal surfaces (see Commentary Figure
E3607.3.2).
Related post: http://www.nachi.org/forum/f19/grounding-electrodes-outbuildings-56128/#post719537