Grounding and Bonding of Separately Derived AC Systems
for 2008 NEC
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The designations "National Electrical Code” and “NEC” refer to the
National Electrical Code®, which is a registered
trademark of the
National Fire Protection Association.
Top 50 NEC Rules
Grounding and Bonding of Separately Derived AC Systems
By Mike Holt
A separately derived system is a premises wiring system
with no direct electrical connection to conductors originating from
another system [Art. 100 definition and 250.20(D)]. All transformers,
except autotransformers, are separately derived because the primary
circuit conductors do not have any direct electrical connection to the
secondary circuit conductors. Generators that supply a transfer switch
that opens the grounded neutral conductor would be considered
derived [250.20(D) FPN 1].
250.30(A) Grounded Systems. Separately derived systems must
system bonded and grounded in accordance with the following:
A neutral-to-case bond must not be on the load side of the system
bonding jumper, except as permitted by 250.142(B).
(1) System Bonding Jumper. Bonding the metal parts of the
separately derived system to the secondary grounded neutral terminal by
the installation of a system bonding jumper ensures that dangerous
voltage from a secondary ground fault can be quickly removed by opening
the secondary circuit's overcurrent protection device [250.2(A)(3)].
The system bonding jumper must be sized in accordance with Table
250.66, based on the area of the largest ungrounded secondary conductor
[250.28(D)]. The system bonding jumper can be installed at the
separately derived system, the first system disconnecting means, or any
point in between the separately derived system and the first
disconnecting means -- but not at both locations.
In addition, the system bonding jumper must be installed at the same
location where the grounding electrode conductor terminates to the
grounded neutral terminal of the separately derived system, which can
at the separately derived system, the first system disconnecting means,
or any point in between -- but not at more than one location
Exception 2: A system bonding jumper can be installed at both the
separately derived system and the secondary system disconnecting means
where doing so doesn't establish a parallel path for neutral
For all practical purposes, this isn't possible except in a wood
frame building that doesn't have any conductive metal.
to read the rest of this article.
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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: New math: Addition = Violation
By Mike Holt
Q. If an outdoor generator has a disconnect, do
need an additional disconnect where the conductors enter the building?
Web site to see the answer.
By Steven Owen
Q. What is the proper size branch-circuit,
short-circuit, ground-fault protective device required for a hermetic
compressor with a nameplate rated load of 40A? The branch-circuit
selection current on the nameplate is rated at 38A. Note: Do not
exceed the values obtained from the lower multiplier.
Web site for the answer and explanation.
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Code News Update
Report on Proposals for 2008
The Code is the definition of a "living document."
Constantly changing, it's the subject of thousands of proposals for
revision throughout each cycle. Earlier this year, the Code Making
Panels acted on 3,688 proposals associated with the 2008 NEC. Now the
deadline for comments is fast approaching.
If you're interested in submitting comments on any of the proposed
changes, you better get moving. All comments must be submitted to the
NFPA by 5 p.m. EST on October 20, 2006. You can submit your
comments online or download
a comment form.
Cool Electronic Cabinets
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an ordinary supply of compressed air to cool electrical controls.
Thermostat control minimizes air usage. Maintains the NEMA 4, 4X
(stainless steel) and 12 rating of the enclosure. Web site offers
detailed information, downloadable drawings and PDF literature.
Shows and Events
Visit the 2006 EC&M E-TradeShow, a Year-Long
Virtual Business Event
Don't miss these scheduled seminars on October 18 in the EC&M
Before attending these events, make sure you visit the 2006 EC&M
e-TradeShow, a year long virtual business event. In addition to
attending live activities at conference sessions scheduled throughout
the year, you can meet with exhibitors in virtual exhibit halls. You
also access past presentations that are archived in the e-TradeShow.
Employing the latest interactive 3D technology, sponsors use online
trade-show booths to generate leads on a continuous basis throughout
year, while interacting live with customers and prospects during
scheduled events. Free access and all the information you need are
available at the event's Web
- "The Basics of Insulation Resistance Testing," by John Olobri, Dir.
Sales and Marketing, AEMC Instruments (9:00 am EST and PST, Conference
- "Good Project Management, Enhancing the Bottom Line," by John
Senior Director, Editorial and EC&M Development (11:00 am EST and PST,
Conference Room A)
- "Presenting AEMC's 3-Phase PQ Analyzer," by Ed Cunie, Eastern
Regional Manager, AEMC Instruments (10:00 am EST and PST, Conference
- "Electrical Market News Update," by Jim Lucy, Chief Editor,
Electrical Wholesaling, and Dale Funk, Chief Editor, Electrical
Marketing (10:00 am EST and PST, Conference Room B)
NFPA World Safety Conference &
It's not too early to start making plans for the NFPA
World Safety Conference & Exposition, to be held at the Boston
Convention & Exhibition Center June 3-7, 2007, in Boston. A popular
segment of this show is the necforum™, a track emphasizing the
most important issues relating to the National Electrical Code (NEC).
This coverage includes seminars on new electrical design issues,
successful maintenance programs, best practices in electrical
contracting, effective inspection techniques, and practical electrical
safety programs. Make sure you save the date for this upcoming event.
For more information, visit the show's Web
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