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450.3
Overcurrent Protection
What's
Wrong
Here?
Code Q&A
Code Quiz
Comments Sought on
Proposed TIA
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The designations "National Electrical Code? and ?NEC? refer to the
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Top 50 NEC Rules
450.3
Overcurrent Protection
By Mike Holt
Overcurrent protection of the primary winding of a
transformer not exceeding 600V must comply with (B). Fine print note
No.
2 states that 4-wire, 3-phase 120/208V or 277/480V systems that supply
nonlinear line-to-neutral loads can overheat because of triplen
harmonics (3rd, 9th, 15th, 21st, etc.) [450.9 FPN 2].
The primary winding of a transformer rated less than 600V must be
protected against overcurrent in accordance with the percentages listed
in Table 450.3(B) and all applicable notes.
Let’s review an example to better explain these requirements.
Question: What is the primary protection device rating and
conductor size required for a 45kVA, 3-phase, 480V-120/208V transformer
that is fully loaded? Terminals are rated 75°C.
Answer: Step 1. Calculate the primary current.
I = VA ÷ (E x 1.732) = 45,000VA ÷ (480V x 1.732) = 54A
Step 2. Identify the primary protection device rating per
240.6(A).
54A x 1.25 = 68A, next size up is 70A, Note 1
Step 3. The primary conductor must be sized to carry 54A
continuously
(54A x 1.25 = 68A) [215.2(A)(1)] and be protected by a 70A protection
device [240.4(A)]. A 4 AWG conductor rated 85A at 75°C meets all of
the
requirements [110.14(C)(1) and 310.16].
Step 4. Calculate the secondary current.
I = VA ÷ (E x 1.732) = 45,000VA ÷ (208V x 1.732) = 125A
Secondary conductors having a maximum length of 25 feet that
terminate in an overcurrent protection device that doesn’t exceed the
ampacity of the conductors, must be sized at 125% of the continuous
load
[215.2(A)(1) and 240.21(C)(6)].
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: The 72-hour shunt
Code Q&A
By Mike Holt
Q. Does the NEC require a connection of a
parking
lot lighting pole to a ground rod?
Visit EC&M's Web
site to see the answer.
Code Quiz
By Steven Owen
Q. When installing wiring methods in ducts or
plenums used for environmental air, all but which of the following
wiring methods are permitted?
- Flexible metal conduit no greater than 4 feet in length
- EMT
- IMC
- MC cable with overall nonmetallic covering
Visit EC&M's
Web site for the answer and explanation.
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Code News Update
Comments Sought on Proposed
TIA
As published in the March issue of NFPA News,
the
following Tentative Interim Amendment (TIA) has been proposed to the
NFPA.
NFPA 70-2008, National Electrical Code
TIA Log No. 904
Reference: 645.17
Submitter: William Burke, American Power Conversion
1. Delete 645.17 as follows: “645.17 Power Distribution
Units. Power distribution units that are used for information
technology equipment shall be permitted to have multiple panelboards
within a single cabinet, provided that each panelboard has no more than
42 overcurrent devices and the power distribution unit is utilization
equipment listed for information technology application.”
Substantiation: A longstanding requirement for the NEC has
been the limitation that no more than 42 overcurrent devices of a
lighting and appliance branch-circuit panelboard be installed in any
one
cabinet or cutout box (reference Section 408.35 of the 2004 NEC).
Equipment intended for use in an information technology equipment
(ITE) room (Article 645 installation) has traditionally been
constructed
using multiple 42-circuit panelboards within the same enclosure. To
account for this reality, a proposal was processed for the 2005 Code
that resulted in Section 645.17. This new section clarified that
distribution units used for ITE are permitted to have multiple
panelboards within a single cabinet, provided that each panelboard has
no more than 42 overcurrent devices, and the unit is listed for
information technology applications.
This requirement was conceived with the traditional 42-circuit
limitation in place and was intended as a relaxation of the limitation
of no more than 42 circuits within a single enclosure.
With the revisions to Section 408.36 in the 2008 NEC, the 42-circuit
limitation has been removed. A companion proposal was not submitted to
Code-making Panel 12, and the revision processed by Code-making Panel 9
was not correlated between technical committees. The end result is the
unintended consequence that Section 645.17 is more restrictive than the
general rule in Section 408.36, without any technical reasons for the
differences.
Emergency Nature: APC, as the original submitter of the
proposal that results in Section 645.17 in the 2005 NEC (reference ROP
– May 2004, Proposal 9-110), submit that the intent of the proposal
for the 2005 NEC process was to include a provision for additional
overcurrent devices in ITE power distribution units. New requirements
in
Section 408.36, which were not coordinated with existing requirements
in
645.17, resulted in the unintended consequence of more restrictive
requirements for ITE power distribution units. Approval of this request
for a Tentative Interim Amendment will correct this error.
This TIA has been published for public review and comment. Comments
should be filed with the Secretary, Standards Council by May 2, 2008.
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 July 22-24,
2008 meeting.
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