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Cast Your Vote
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250.142
Use of Grounded Neutral Conductor for Equipment Grounding (Bonding)
What's
Wrong
Here?
Code Q&A
Code Quiz
EC&M Code
Change Conferences
Let's Go Racing! Win
a Free Road America Race Weekend for Two
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Top 50 NEC Rules
250.142
Use of Grounded Neutral Conductor for Equipment Grounding
(Bonding)
By Mike Holt
To remove dangerous voltage on metal parts from a
ground
fault, the metal parts of electrical raceways, cables, enclosures, and
equipment must be bonded to an effective ground-fault current path in
accordance with 250.4(A)(3).
Supply side of service equipment. A grounded neutral
conductor
can be used as the effective ground-fault current path for metal parts
of equipment, raceways, and other enclosures. Because an equipment
grounding (bonding) conductor isn't run from the utility to electrical
services, the grounded neutral service conductor can serve as the
effective ground-fault current path to the utility power source. The
effective ground-fault current path for service equipment is provided
by
the installation of the main bonding jumper at service equipment in
accordance with 250.24(B) [250.28].
Where no equipment grounding (bonding) conductor is run to a
separate
building or structure disconnect, the grounded neutral conductor can
serve as the effective ground-fault current path to the source. This is
accomplished by bonding the grounded neutral conductor to the equipment
grounding (bonding) conductor at the separate building or structure
building disconnecting means in accordance with 250.32(B)(2).
Caution: Using the grounded neutral conductor as effective
ground-fault current path poses potentially dangerous consequences and
should only be done after careful consideration. The safest practice is
to install an equipment grounding (bonding) conductor with the feeder
conductors to the building or structure to serve as the effective
ground-fault current path, as provided by 250.32(B)(1).
On a separately derived system, the effective ground-fault current
path is established when the system bonding jumper is installed between
the metal enclosure of the separately derived system and the grounded
neutral terminal in accordance with 250.30(A)(1). Failure to install
the
system bonding jumper as required by 250.30(A)(1) will create a
condition where dangerous touch voltage from a ground fault will remain
on the metal parts of electrical equipment.
Load side equipment. To prevent dangerous voltage on metal
parts, the grounded neutral conductor must not be bonded to the
equipment grounding (bonding) conductor on the load side of service
equipment, except as permitted by 250.142(A).
Exception No. 1: The grounded neutral conductor can serve as the
effective ground-fault current path for existing ranges, dryers, and
ovens [250.140 Ex].
Exception No. 2: The grounded neutral conductor can be bonded to the
meter enclosure on the load side of the service disconnecting means
if:
- No service ground-fault protection is installed,
- Meter enclosures are located immediately adjacent to the service
disconnecting means, and
- The grounded neutral conductor is sized no smaller than specified
in
Table 250.122.
Editor's note: This information was extracted from Mike
Holt's
textbook, Understanding
the National Electrical Code
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Code Challenge
What's Wrong Here?
By Joe Tedesco
Think you know how this installation violates the
NEC?
Visit EC&M's
Web site to see the answer.
Hint: Power in a pinch
Code Q&A
By Mike Holt
Q. Can I install conductors from different
panels
in the same raceway?
Visit EC&M's Web
site to see the answer.
Code Quiz
By Steven Owen
Q. What is the minimum size and metal thickness
required for a junction box constructed of sheet steel, with one 4-inch
RMC conduit (per wall) connected on opposite walls of the box (i.e.,
directly across from one another)? The conductors within the raceways
are 600kcmil type XHHW. The outer diameter (O.D.) of each of these
insulated conductors is 1.2 inches.
- 24 inch by 24 inch. There is no requirement for metal thickness.
- 32 inch by 24 inch. Metal thickness = 1.35 millimeters (0.053
inches).
- 32 inch by 32 inch. Metal thickness = 1.35 millimeters (0.053
inches).
- 43.2 inches by 43.2 inches. Metal thickness = 1.35 millimeters
(0.053 inches).
Visit EC&M's
Web site for the answer and explanation.
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