Number of 6 AWG and Smaller Conductors in Boxes and Conduit Bodies
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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
Number of 6 AWG and Smaller Conductors in Boxes and Conduit
By Mike Holt
Boxes containing 6 AWG and smaller conductors must be
sized to provide sufficient free space for all conductors, devices, and
fittings. In no case can the volume of that box, as calculated in
314.16(A), be less than the volume requirement as calculated in
314.6(B). Conduit bodies must be sized in accordance with 314.16(C).
Note: The requirements for sizing boxes and conduit bodies
containing conductors 4 AWG and larger are contained in 314.28.
The volume of a box includes the total volume of its assembled
including plaster rings, extension rings, and domed covers that are
either marked with their volume in cubic inches or are made from boxes
listed in Table 314.16(A).
The calculated conductor volume determined by (1) through (5) and
Table 314.16(B) are added together to determine the total volume of the
conductors, devices, and fittings. Raceway and cable fittings,
locknuts and bushings, are not counted for box fill calculations.
Each conductor that runs through a box and does not have 6 inches of
free conductor for splices or terminations in accordance with 300.14,
and each conductor that terminates in a box is counted as a single
conductor volume in accordance with Table 310.16(B). Conductors that
originate and terminate within the box, such as pigtails, aren't
at all. An Exception notes: Equipment grounding (bonding) conductors,
and not more than four 16 AWG and smaller fixture wires, can be omitted
from box fill calculations if they enter the box from a domed luminaire
or similar canopy, such as a ceiling paddle fan canopy. Caution:
Conductor loops occupy space and a box can be excessively filled if we
do not take into consideration the increased conductor volume. This can
create a serious fire hazard, especially when an electronic device is
installed in an outlet box without adequate room for heat dissipation.
One or more internal cable clamps count as a single conductor volume
in accordance with Table 310.16(B), based on the largest conductor that
enters the box. Cable connectors that have their clamping mechanism
outside the box aren't counted.
Each luminaire stud or luminaire hickey counts as a single conductor
volume in accordance with Table 310.16(B), based on the largest
conductor that enters the box.
Each device yoke (regardless of the ampere rating of the device)
counts as two conductor volumes in accordance with Table 310.16(B),
based on the largest conductor that terminates on the device.
All equipment grounding (bonding) conductors in a box count as a
single conductor volume in accordance with Table 310.16(B), based on
largest equipment grounding (bonding) conductor that enters the box.
Equipment grounding (bonding) conductors for isolated ground circuits
count as a single conductor volume in accordance with Table 310.16(B).
Splices are only permitted in conduit bodies that are legibly
by the manufacturer, with their volume. The maximum number of
permitted in a conduit body is limited in accordance with 314.16(B).
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
Web site to see the answer.
Hint: Indecent exposure.
By Mike Holt
Q. When a conduit leaves a Class I, Division 1
classified paint spray room and goes under the concrete floor into a
cabinet in the motor control room that is non-classified, is one
fitting in the paint mix room sufficient, or is an additional sealing
fitting required in the non-classified area as well?
Visit EC&M's Web
site to see the answer.
By Steven Owen
Q. When installing cable- or raceway-type wiring
methods, installed in exposed or concealed locations under
metal-corrugated sheet roof decking, the cable- or raceway-type wiring
method shall be installed and supported so that the nearest outside
surface of the cable or raceway is not less than 1 1/2 inches from the
nearest surface of the roof decking. This rule applies to all cable- or
raceway-type wiring methods except which one of the following?
- No exceptions. It applies to all cable- or raceway-type wiring
- MI cable. It does not apply to MI cable with a 2-hour fire rating.
- Type TC -- ER cable as noted in 336.10(7).
- Rigid metal conduit (RMC) and intermediate metal conduit
Web site for the answer and explanation.
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than 60 years. Seven Code change conferences have been scheduled in the
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Education provider Program (RCEPP), professional engineers attending
of our 2008 Code change conferences will receive Professional
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The conferences are also approved by every state that has a continuing
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