3-wire feeds to existing detached buildings

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:

  1. That required by Section E3704.3.
  2. 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
. 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

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

Related post: http://www.nachi.org/forum/f19/grounding-electrodes-outbuildings-56128/#post719537

Unfortunately, it can be impossible for an inspector to determine if there is a connection between buildings. For this reason, it is prudent for a home inspector to “defer for further evaluation” any three-wire feed to a detached building, where the neutral has been grounded.

One other note, this was code compliant for the 2005 NEC and earlier versions. The 2008 NEC has removed the use of a 3 wire feeder in this instance.


RM was saying that prior to the 08 NEC that a 3 wire feeder was allowed as long as no other metallic paths between buildings existed. the panel was treated the same as service equipment with the neutrals and grounds bonded together. The 08 now requires a 4 wire feeder regardless.

No it doesn’t. Did you read the code quote and commentary above? Existing buildings with 3-wire feeds are acceptable provided they are properly grounded at the building and that the cabinet is properly bonded and provided there is no other continuous, bonded, metallic path between both buildings.

What if the existing building was built under the 2008 NEC? The exception to 250.32 would not apply unless the feeder was installed under an earlier code version. The 2008 rule is that you must install an EGC with the feeder unless the feeder is existing under a code version prior to the 2008 NEC.

I read this part. Bold and underline by me.

The change doesn’t make you change the feeder already in existance. A new feeder would need to comply with the 4 wire requirement/


Jeff, do you have any recommend language you’d like to share?

Mine below: please comment.

“While the main house was equipped with a Grounding Electrode System (GES), no GES (usually a ground rod) was apparent at the detached XXX. Today’s commonly accepted electrical standards require a GES at all detached buildings. We recommend service by a licensed and competent electrician to install a GES at the detached structure. Note that if the feeder wire is only a 3-wire conductor the electrician should: 1) ensure that the grounding and grounded conductor buses are bonded together or bonded to the cabinet by a main bonding jumper; and, 2) also ensure that there are no common continuous metallic paths bonded to the GES at both the house and the detached XXX.”

As far as I know, the GES has always been required for detached buildings. However, there are several different conditions that may exist, so it depends on what I find. Here are some examples;

Thanks Jeff. Simple and to the point.

Jeff, how would a sub-panel even function without a neutral feeder? (Is that what you’re saying?) I’ve seen plenty of 3-wire feeds w/o grounds, but never a missing neutral.

The EGC can (will) carry the load that would normally be carried on the neutral.

It is all about bonding…

More importantly is bonding the neutral at the panel in the outbuilding so that there is no possibility of any potential between the incoming neutral and other local grounds within the building.