|NEC Article 300 - Wiring Methods
|By Mike Holt for EC&M Magazine
Article 300 benefits extend beyond
The primary benefit of following the adequate requirements of
Article 300 is the safeguarding of people and property. But, these requirements
also reduce conductor failure rates and increase operational efficiency. For
power quality problem prevention, Article 300 ranks second only to Article
Article 300 requirements do not apply to signaling and
communications systems, except where specifically mentioned. In addition, the
requirements of this article do not apply to the internal parts of electric
equipment. See 90.7 and Figure 300-1.
Note: Graphics are not included in
this newsletter, but are available (see link below)
You must install individual conductors in a raceway, cable,
or enclosure [300.3]. Further, you must put all conductors of a circuit in the
same raceway, cable, trench, cord, or cable tray (there are exceptions to
this-see Figure 300-2). This requirement minimizes inductive heating of metallic
raceways and enclosures, plus it reduces impedance should a ground fault
When separate conductors are in a nonmetallic raceway as permitted
in 300.5(I) Ex. 2, you can minimize the inductive heating of the metal enclosure
by using aluminum locknuts and by cutting a slot between the individual holes
through which the conductors pass (Figure 300-50). Because aluminum is
nonmagnetic, it eliminates heating due to hysteresis. However, aluminum conduit,
locknuts, and enclosures do carry eddy currents [300.20(B) FPN].
conductors of different systems can occupy the same raceway, cable, or enclosure
if all conductors have an insulation voltage rating not less than the maximum
circuit voltage (Figure 300-5). You must separate control, signal, and
communications wiring from power and lighting circuits-so the higher voltage
conductors do not accidentally energize them. Exceptions to this requirement
allow power conductors to terminate to listed signaling equipment, if the power
conductors maintain a minimum of 1/4 in. separation from the low voltage and
limited-energy conductors.Where cables and raceways are underground, apply the
requirements of 300.5 and Table 300.5. To protect underground cables and
conductors from damage, follow the requirements of 300.15.
raceways must be continuous between all points of the system. This means you
can't place splices in the raceway, except as permitted by 376.56, 378.56,
384.56, 386.56, or 388.56.
In multiwire branch circuits, the removal of a
wiring device (e.g., a receptacle) must not interrupt the continuity of the
grounded (neutral) conductor. Therefore, you must splice the grounded (neutral)
conductors together and provide a pigtail for device terminations (Figure
300-31). The opening of the ungrounded or grounded (neutral) conductor of a
2-wire circuit during the replacement of a device does not cause a safety
hazard, so pigtailing of these conductors is not required.
A closer look
Use raceways, fittings, and supports that are suitable for
the environment in which you'll install them (Figure 300-19). Where corrosion
protection is necessary (underground and wet locations) and the conduit is
threaded in the field, coat the threads with an approved electrically
conductive, corrosion-resistant compound.
If condensation from
temperature changes might be a problem, fill the raceway with a material
approved by the AHJ, to prevent the circulation of warm air to a colder section
of the raceway or sleeve. An explosionproof seal is not required for this
purpose. See Figure 300-20.
You must provide raceways with expansion
fittings where necessary to compensate for thermal expansion and contraction.
Table 352.44(A) provides the expansion characteristics for PVC rigid nonmetallic
conduit. To determine the expansion characteristics for metal raceways (EMT IMC
and RMC), multiply the values from Table 352.44(A) by a multiplier of
If you use an expansion fitting with a metal raceway, you must use
a bonding jumper to maintain the equipment grounding path [250.98 and 300.10].
Join all metal raceways, cable, boxes, fittings, cabinets, and enclosures to
form a continuous low-impedance ground-fault current path. The ground-fault
current path must have adequate capacity to carry any fault likely to be imposed
on it. See 110.10, 250.4(A)(3), 250.22, and Figure 300-23.
designed for the exclusive use of electrical conductors and cables, and cannot
contain nonelectrical components (e.g., lines for steam, water, or gas). See
Figure 300-22. Raceways and cable sheaths must be mechanically continuous
between terminations [300.12]. However, short lengths of metal raceways used for
cable protection or support are not required to be electrically continuous,
mechanically continuous, or bonded.
Securely fasten in place raceways,
cable assemblies, boxes, cabinets, and fittings. You cannot use ceiling support
wires or the ceiling grid to support raceways or cables [300.11].
support electrical wiring within the cavity of a floor-ceiling or roof-ceiling
assembly to independent support wires that are secured at each end
If this cavity is fire-rated, the support wires must be
distinguishable from the suspended ceiling support wires by color, tagging, or
other effective means. If this cavity is non-fire-rated, the support wires do
not have to be distinguishable from the suspended ceiling support wires
You can support outlet boxes [314.23(D)] and luminaires to
the suspended ceiling if you securely fasten them to the ceiling-framing member
You cannot use raceways to support other raceways, cables, or
equipment (Figure 300-26). However, you can support Class 2 and 3 cables to the
raceway that supplies power to the equipment controlled by the circuit. Because
Class 2 and 3 cables cannot be installed in the same raceway with the power
conductors [725.55(A)], the next best thing is to attach them to the raceway.
See 725.58 and Figure 300-27.
You cannot use type AC, NM, or MC cable to
support other cables, raceways, or nonelectrical equipment. You can at times use
raceways to support threaded boxes, conduit bodies [314.23(E) and (F)], and
If the vertical rise of a raceway exceeds the
values of Table 300.19(A), you must support the conductors at the top-or as
close to the top as practical (Figure 300-48). The weight of long vertical runs
of conductors can cause the conductors to drop out of the raceway, if you don't
secure them properly. There have been cases where conductors in a vertical
raceway were released from the pulling basket (at the top), and the conductors
fell down and out of the raceway injuring the workers!
You must install
electrical circuits and equipment in a way that doesn't substantially increase
the possible spread of fire or products of combustion. This means sealing
openings in fire-rated walls, floors, and ceilings for electrical equipment. Use
fire-stop material listed for the specific types of wiring methods and
construction structures [300.21].
In general, you must keep wiring and
air handling separate (Figure 300-52). You can wire in environmental air space
under certain conditions listed in 300.21(C) A space not used for environmental
air-handling purposes has no restrictions on wiring methods, so you can use
nonplenum cables there (Figure 300-54).
While Article 300 provides a
basis for safeguarding people and equipment, it also provides a basis for good
system performance. For example, a small engine plant experienced severe power
quality problems. Investigation showed the wireways were not electrically
continuous. Bonding the wireways eliminated the power quality problems. An
extrusion plant was experiencing high scrap rates on a new system installed with
signal and power wiring in the same wireways. Separating these dropped the scrap
rates to less than one percent.