Capacity and Rating for Optional Standby Systems
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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.
The Top 2008 Code Change item from the last issue
(700.9 Wiring for Emergency Power Systems) has caused some
confusion. To rectify this situation, we offer the following text to
help clarify when and where these requirements apply.
These wiring requirements apply to single or multiple (two or more)
generators connected in parallel configurations.
An Exception to 700.9(B)(5)(b) states, "Overcurrent protection shall
be permitted at the source or for the equipment, provided the
overcurrent protection is selectively coordinated with the downstream
In addition to a "generator source", the new subsection (5) applies
to other power sources such as storage batteries, uninterruptible power
supplies, a separate service (i.e., redundant utility feeder), fuel
systems, and unit equipment.
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Top 2008 Code Changes
Capacity and Rating for Optional Standby Systems
By Mike Holt
The sizing for optional standby power systems is now
based on the type of transfer switch used: manual versus automatic.
Code section was extensively revised and new subsections were added in
response to the growth of generator installations — and the concern
about the sizing of an optional standby source that uses automatic
(A) Available Short Circuit Current. Optional standby system
equipment shall be rated for the maximum available short circuit
current at its terminals.
(B) System Capacity. The calculated load on the standby
shall be in accordance with Art. 220 or by a method approved by the
Authority Having Jurisdiction (AHJ).
(1) Manual Transfer Equipment. The optional standby power
source shall have adequate capacity for all equipment intended to
operate at one time. The user of the optional standby system selects
loads to be connected to the system.
When a manual transfer switch is used, the user of the optional
standby system selects the loads to be connected to the system, which
determines the system size.
(2) Automatic Transfer Equipment.
(a) Full Load. The optional standby power source shall have
capacity to supply the full load transferred.
If an automatic transfer switch is used in an optional system, the
power source — typically a generator — must be capable of supplying
the full load transferred. The load is determined by using Art. 220 as
basis on system sizing or an alternate method approved by the AHJ.
For existing facilities, the optional standby source can be sized to
the maximum demand data available for one year or the average power
demand of a 15-min. period over a minimum of 30 days [220.87].
However, the conditions are not the same for optional standby power
supply when a manual transfer switch is used. In this case, the user of
the optional standby system selects the loads to be connected to the
system, which determines the system size.
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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: A shining example of poor construction
By Mike Holt
Q. Can the ground wire from a ground rod
terminate in the meter socket enclosure, or must it terminate in the
Visit EC&M's Web
site to see the answer.
By Steven Owen
When grounding network interface units containing
protectors, network interface units with metallic enclosures, primary
protectors, and the metallic members of the network-powered broadband
communications cables that are intended to be grounded, which of the
following requirements apply to the grounding conductor?
- The grounding conductor shall be insulated. The grounding conductor
shall be copper or other corrosion-resistant conductive material,
stranded or solid.
- The grounding conductor shall not be smaller than 14 AWG copper,
have a current-carrying capacity approximately equal to that of the
grounded metallic members and protected conductors of the
network-powered broadband communications cable. In addition, the
grounding conductor shall not be required to exceed 6 AWG.
- The grounding conductor shall be as short a practicable, be run in
straight line, and be protected where exposed to physical damage.
- All of the above requirements apply.
Web site for the answer and explanation.
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find up-to-date UL Listed products at the UL Code Correlation
Code News Update
Comments Sought on Proposed
As published in the March issue of NFPA News,
following Tentative Interim Amendment (TIA) has been proposed to the
NFPA 70-2008 and proposed 2011 Edition, National Electrical
TIA Log No. 941
Submitter: Robert Torbin, Cutting Edge Solutions LLC
Revise 250.104 to read as follows:
250.104 Bonding of Piping Systems and Exposed Structural
(B) Other Metal Piping. Where installed in or attached to a
or structure, a metal piping system(s), including gas piping, that is
likely to become energized shall be bonded to the service equipment
enclosure, the grounded conductor at the service, the grounding
electrode conductor where of sufficient size, or the one or more
grounding electrodes used.
The bonding jumper(s) shall be sized in
accordance with 250.122, using the rating of the circuit that is likely
to energize the piping system(s). The equipment grounding conductor for
the circuit that is likely to energize the piping shall be permitted to
serve as the bonding means. The points of attachment of the bonding
jumper(s) shall be accessible.
(1) Other than Corrugated Stainless Steel Tubing (CSST).
The bonding jumper(s) shall be sized in accordance with 250.122, using
the rating of the circuit that is likely to energize the piping
system(s). The equipment grounding conductor for the circuit that is
likely to energize the piping shall be permitted to serve as the
(2) CSST. Corrugated stainless steel tubing gas piping
shall be bonded by connection to a metallic piping segment or fitting,
either outside or inside the building, between the individual gas meter
and the first CSST fitting. The bonding jumper shall be sized in
accordance with Table 250.66 based on the size of the service-entrance
conductor or feeder supplying each occupancy and as permitted in
250.66(A), (B), and (C), but not smaller than 6 AWG copper (or
To review the submitter's reason (substantiation and technical
and emergency nature text for this change, click
here and review the text on pages 1, 2, and 3.
This TIA has been published for public review and comment. Comments
should be filed with the Secretary, Standards Council by April 17,
You should identify the TIA number when submitting your comments. The
Standards Council will review the technical committees’ ballot
results, the public comments, and any other information that has been
submitted when it considers the issuance of the TIA at its meeting on
August 4-6, 2009.
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- Clarifying NFPA 70E and IEEE 1584 Requirements
- Impact of System and Operating Conditions on Arc Flash Energy
April 21, 2009
- High Resistance Grounding and Arc Flash Accident
May 7, 2009
- Mitigation of Arc Flash Hazards Using Fuses
May 12, 2009
- Arc Flash Testing Updates
June 16, 2009
- Arc Flash Hazard Mitigation Case Studies
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