Induced Currents in Metal Parts
Comments Sought on
Tentative Interim Amendments
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Top 50 NEC Rules
Induced Currents in Metal Parts
By Mike Holt
To minimize induction heating of ferrous metallic
raceways and enclosures -- and to maintain an effective ground-fault
current path -- all conductors of a circuit must be installed in the
same raceway, cable, trench, cord, or cable tray. See 250.102(E),
300.3(B), 300.5(I), and 392.8(D).
When alternating current (AC) flows through a conductor, a pulsating
or varying magnetic field is created around the conductor. This
field is constantly expanding and contracting with the amplitude of the
AC current. In the United States, the frequency is 60 cycles per
Since AC reverses polarity 120 times per second, the magnetic field
surrounds the conductor also reverses its direction 120 times per
second. This expanding and collapsing magnetic field induces eddy
currents in the ferrous metal parts that surround the conductors,
causing the metal parts to heat up from hysteresis.
Hysteresis heating affects only the ferrous metals with magnetic
properties, such as steel and iron, but not aluminum. Simply put, the
molecules of steel and iron align to the polarity of the magnetic field
and when the magnetic field reverses, the molecules reverse their
polarity as well. This back and forth alignment of the molecules heats
up the metal -- the greater the current flow, the greater the heat
rise in the ferrous metal parts.
When conductors of the same circuit are grouped together, the
magnetic fields of the different conductors tend to cancel each other
out, resulting in a reduced magnetic field around the conductors. The
lower magnetic field reduces induced currents in the ferrous metal
raceways or enclosures, which reduces hysteresis heating of the
surrounding metal enclosure.
When single conductors are installed in nonmetallic raceways as
permitted in 300.5(I) Ex. 2, the inductive heating of the metal
enclosure can be minimized by the use of aluminum locknuts and by
cutting a slot between the individual holes through which the
Aluminum conduit, locknuts, and enclosures carry eddy currents, but
because aluminum is nonferrous, it doesn't heat up.
Editor's note: This information was extracted from Mike
the National Electrical Code
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What's Wrong Here?
By Joe Tedesco
Think you know how this installation violates the
NEC? Visit EC&M's
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Hint: A case of the bends.
By Mike Holt
Q. What is the minimum service drop clearance
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Visit EC&M's Web
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By Steven Owen
Q. A construction site has 10 office trailers.
Each unit is identical. The HVAC unit for each trailer has a
load current of 27A and a branch circuit selection current of 30A at
208VAC, 3-phase. What is the proper size for the ground-fault,
short-circuit protective device for each HVAC unit?
Web site for the answer and explanation.
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Code News Update
Comments Sought on Tentative Interim
According to August/September 2007 issue of NFPA
News, two Tentative Interim Amendments (TIAs) to the 2008 version
the National Electrical Code have been proposed to the NFPA. The
TIAs are now available for public review and comment. Any comments on
these TIAs should be filed with the Secretary Standards Council by
September 17, 2007. The NEC technical committee will consider all
comments prior to taking final action on these items. The Standards
Council will then review the technical committees' ballot results, the
public comments, and any other information that has been submitted to
determine whether to issue the TIA at its meeting October 3-4, 2007.
The two TIAs in question are as follows:
TIA Log No. 881 -- "Add the following Exception to Articles
374.17, 366.23(A)(B), 372.11, 372.17, 374.5, 374.17, 376.22, 378.22,
384.22, 386.22, 388.22, 392.9, 392.10, and 392.11: Exception No. 1:
Additional conductors shall not be added unless with a tested and
or recognized method or device that maintains spacing for air to
surround the conductors, and it is to be followed by its installation
instructions with its limitations. Article 310.15(B)(a) spacing shall
TIA Log No. 891 -- "Revise the following section to read as
follows: (374.17) The ampacity adjustment factors in Table
shall not apply to cellular metal floor raceways."
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