60 Modern Commercial Wiring
Copyright Goodheart-Willcox Co., Inc.
Securing and Supporting
Wiring Systems
All wiring must be secured to and supported by
structural members. Normally, wiring systems are not
used to support other wiring systems or nonelectric
equipment. However, Section 300.11(B) of the Code
specifies some conditions under which one raceway can
be supported by another raceway.
Mechanical and electrical continuity is required. Metal
conduit, cable sheaths, cable armor, and nonmetallic
raceway must be continuous between boxes, enclosures,
fittings, and cabinets. All connections between the compo-
nents must be secure prior to pulling conductors. The
equipment grounding conductor should be independent
of the device connection, so if the device is removed the
continuity is not interrupted.
Conductor support in vertical raceway
In buildings where raceways are run vertically, the
conductors must be supported at assigned intervals. One
cable support at the top of the raceway run and at intervals
indicated in the table in Figure 5-5 are required. This table
is a condensed version of Table 300.19(A) from the Code.
Support of vertical conductors can be accomplished
in several ways, including the following:
• Placing insulated wedges between the cable and
the inside wall of the raceway or using insulated
clamps within an enclosure.
• Installing junction boxes with supports installed
and attached to the cable, Figure 5-6.
• Installing junction boxes and offsetting the cable
at least 90° and using tie wires within the box.
NEC NOTE 300.19(C)(3)
When the cable is supported by bending it at least
90° in a junction box and securing with tie wires,
support intervals cannot exceed 20% of the normal
maximum support distance. The bend in the conductor
must extend horizontally at least twice the diameter of
the cable.
Wiring within Air-Handling Spaces
No wiring is permitted within air-handling ducts
used for the purpose of transporting dust, flammable
vapors, or cooking equipment ventilation. In plenums
used for environmental air only, wiring methods
employing type MI or MC cable are permitted. Flexible
Figure 5-4. Conduit must be installed to protect cables and
conductors emerging from underground.
Conduit extends
8′ above ground
or to point of entry
into building
Conduit extends down
to minimum cover
requirement (18″ max.)
Bushing
Figure 5-5. Condensed version of Table 300.19(A) of the Code.
Vertical Spacing for Conductor Supports
Conductor Size
Maximum Distance between Supports (in feet)
Copper Conductors Aluminum Conductors
18 AWG through 8 AWG 100 100
6 AWG through 1/0 AWG 100 200
2/0 AWG through 4/0 AWG 80 180
Over 4/0 AWG through 350 kcmil 60 135
Over 350 kcmil through 500 kcmil 50 120