By Mike Holt for EC&M Magazine
Q1. Neutral and Ground Connections
At service equipment the NEC permits the grounded (neutral) conductor, the equipment
enclosure, and the grounding electrode conductor to all terminate together. Is it permissible
to land the equipment grounding conductors on the neutral bus bar with the grounded (neutral)
conductors, or am I required to set a ground bar for the equipment grounding conductors?
A1: Equipment grounding conductors can terminate on the same terminal bar with the grounded
(neutral) conductors, but this is only permitted if the panelboard is rated service equipment
and it is used as service equipment in accordance with 230.71(A). However, each grounded
(neutral) conductor must terminate within the panelboard in an individual terminal [408.21].
Article 100: Service Equipment. The
necessary equipment, usually consisting of a circuit breaker(s) or switch(es) and fuse(s)
and their accessories, connected to the load end of service conductors to a building or
other structure, or an otherwise designated area, and intended to constitute the main
control and cutoff of the supply.
Q2. Generator Grounding 250.34
I have a question regarding small 5 kW generator. Let us say that the generator
supplies only equipment mounted on the generator, cord-and-plug-connected equipment through
receptacles mounted on the generator, or both. Should the generator have a ground rod
connection? There is a point on the frame of the generator marked "ground" so
it appears that it may be intended, or at least an option.
A2. No, a ground rod is not needed, not required, and would serve no purpose, see 250.34(A).
So why does the manufacture provide the ground lug on the generator? Because they do not
know any better.
Q3. Transformer Grounding 250.30
I have recently moved to a new project were some of the Journeyman electricians
believe that the neutral and grounds should be connected together in a panel that is being
fed from a 480V to 120/208V transformer. I have always made this neutral-to-ground connection
at the transformer, and kept the grounds and neutrals separated. Which is more correct?
A3. A neutral-to-ground connection shall be made at one location on the secondary of the
transformer at any point from the source to the first system disconnecting means or overcurrent
Caution: A bonding jumper connection at both the source and the first disconnecting must
be avoided because doing so would create a parallel path where dangerous neutral current
could flow over the grounding conductors or grounding paths in violation of 250.6.
Q4. Grounding Remote Building and Structures
NEC 250.32 allows the grounded (neutral) circuit conductor supplying a second
building to be bonded to the equipment grounding conductor at the remote building. grounding
electrode system of the second building. If the two buildings are electrically connected
via a conductive path, such as the shielding of a coax cable or water pipe, this would
provide an alternate path for neutral current to flow during normal operation (like resistors
in parallel). Is this a trivial concern? Do you know of any problems that can arise from
such a situation such as GFI relays in the main service? All those whom I have discussed
this with have dismissed it as trivial. They site residential houses connected via a common
water line. I would like to hear your comments.
A4. The practice of bonding the grounded (neutral) conductor to the equipment enclosure
is only permitted by 250.32(B)(2) where
(1) an equipment grounding conductor is not run with the supply to the building or structure,
(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,
(4) the size of the grounded conductor shall not be smaller than the larger of:
That required by 220.22 (maximum unbalanced neutral load), or
That required by 250.122 (equipment grounding conductor size).
When an equipment grounding conductor is not run to a separate building or structure,
the grounded (neutral) conductor must be used to provide the effective ground-fault current
path required to clear any ground-faults (line-to-case faults) in addition to carrying
any unbalanced neutral current [250.4(A)(3)].
CAUTION The use of the grounded (neutral) conductor for equipment bonding is permitted
by the NEC. This practice should only be done after careful consideration and a review
of the potential safety hazards. Even if the initial installation will not result in an
unacceptable parallel path for neutral current flow, there remains the possibility that
a future installation of, say, metal piping between the separate buildings or structures,
could reverse that situation.
Q5. Grounding Patient Care Areas 517.13(A)
Wiring in patient care areas must be provided with a ground path for fault current
by the installation of the circuit conductors in a metal raceway system, or a cable armor
or sheath assembly that qualifies as an equipment grounding return path in accordance
with 250.118. Can flexible metal conduit or surface metal raceways be used for branch
circuits in these areas?
A5. Flexible metal conduit in lengths not exceeding 6 ft can be use for branch circuit
wiring in patient care areas of a health care facility, if the circuit conductors contained
in the raceway are protected by overcurrent devices rated at 20 amperes or less. Surface
metal raceways are also considered suitable for grounding unless there were some marking,
UL information that would prohibit its use as a sole grounding path.
Mike Holt's Comment: If you have any comments
or feedback, please let me know, Mike@MikeHolt.com.