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Wiring Methods — Part 2 of 3

April 1, 2010
NEC Article 300 provides wiring method requirements including information on sizing and securing raceways

We begin Part 2 of this three-part series by continuing our discussion of the requirements for securing and supporting raceway and other parts of electrical systems. The NEC requires raceways, cable assemblies, boxes, cabinets, and fittings to be securely fastened in place [300.11(A)] and provides a few specifics. Although Art. 300 does not go into great detail regarding securing and strapping requirements, the individual articles for the various wiring methods contained in Chapter 3 do include specifics you must adhere to, so be sure to refer to the article for each specific wiring method.

The support wires used to install suspended ceilings are not allowed to support raceways or cables. However:

  • Independent support wires that are secured at both ends (and that provide secure support) are allowed to be added for the support of wiring methods [300.11(A)].
  • Outlet boxes [314.23(D)] and luminaires may be attached to a suspended-ceiling grid that's securely fastened to ceiling-framing members [410.36(B)].

Most suspended ceiling systems aren't part of a fire-resistance rated assembly. If they are, wiring methods may be supported by independent wires attached to the ceiling assembly. However, these independent support wires must be distinguishable from the suspended-ceiling support wires by color, tagging, or other effective means in fire-rated ceiling assemblies [300.11(A)(1)]. The same installation is allowed in ceilings that are not fire-rated assemblies — the only difference is that marking the support wires is not required [300.11(A)(2)].

Raceways and cable sheaths must be mechanically continuous between boxes, cabinets, and fittings [300.12]. Two exceptions exist:

  1. Short sections of raceway used to provide support or protection of cable are not required to be mechanically continuous [300.12 Ex 1], and they are not required to be connected to an equipment grounding conductor [250.86 Ex 2].
  2. Raceways that enter the bottom of open-bottom equipment, such as switchboards, are not required to be mechanically continuous to that equipment [300.12 Ex 2], as shown in Fig. 1.


Raceways must be large enough to permit the installation and removal of conductors without damaging them and to allow the dissipation of the heat generated by current flowing through the conductors [300.17]. If a raceway is 24 in. or less in length, it can be filled to 60% of its total cross-sectional area [Chapter 9, Table 1, Note 4].

When all conductors in a raceway are the same size and of the same insulation type, Annex C can be used to determine the number of conductors that are allowed in the raceway. If the conductors in a raceway differ in insulation type or conductor size, the conductor fill must be limited to the percentages found in Table 1 of Chapter 9. Sizing a raceway is a two-step process:

  1. Determine the total area of conductors (Chapter 9, Table 5 for insulated conductors and Chapter 9, Table 8 for bare conductors).
  2. Select the raceway from Chapter 9, Table 4 using the percent fill listed in Chapter 9, Table 1.

Conductors in raceways

To protect conductors from abrasion during installation, raceways must be mechanically complete between the pulling points before the conductors are installed [300.18(A)]. Short sections of raceway used for the protection of cables from physical damage are exempt from this rule [300.18(A) Ex]. See 300.10 and 300.12 for more information on the mechanical continuity requirements for raceways.

Because of the effect of gravity on vertical raceways enclosing conductors, supports must be installed to relieve the stress on vertical conductors in runs where the vertical rise exceeds the values of Table 400.19(A). The conductors must be supported at the top, or as close as practical to the top [300.19], and intermediate support must be provided per Table 300.19(A).

To maintain an effective ground-fault current path and minimize induction heating of ferrous raceways (and enclosures), all conductors of a circuit must be installed in the same raceway, cable, trench, cord, or cable tray [300.20(A)]. See 250.102(E), 300.3(B), 300.5(I), and 392.8(D).

Single conductors are allowed to be run in nonmetallic raceways to ferrous enclosures [300.5(I) Ex 2]. Inductive heating of the enclosure can be minimized when doing this by the use of aluminum locknuts and cutting a slot between the individual knock-out holes in the wall of the cabinet or junction box through which the conductors pass (Fig. 2).

Fire containment

Install circuits and equipment in a way that doesn't substantially increase the potential spread of fire [300.21]. Fire-stop openings in fire-rated walls, floors, and ceilings using methods approved by the authority having jurisdiction (AHJ).

Use fire-stopping materials that are listed for the specific types of wiring methods and the construction of the assembly they penetrate. Directories of electrical construction materials published by qualified testing laboratories contain listing and installation restrictions for maintaining the fire-resistive rating of assemblies.

Outlet boxes must have a horizontal separation from other outlet boxes of not less than 24 in. when installed in a fire-rated assembly, unless an outlet box is listed for closer spacing or protected by fire-resistant "putty pads" per manufacturer's instructions.

Air handling spaces

Don't run wires through any ducts that transport dust, loose stock, or vapors [300.22(A)].

Ducts or plenums for environmental air

In some cases, devices or equipment must directly act upon or sense the air in a duct or plenum that primarily serves to transport environmental air [300.22(B)]. This is the only time that any wiring methods are allowed inside fabricated ducts or plenums that transport environmental air. In such cases, only the following wiring methods are allowed:

  • Type MI cable.
  • Type MC cable that has a smooth or corrugated impervious metal sheath without an overall nonmetallic covering.
  • Electrical metallic tubing (EMT).
  • Flexible metallic tubing (FMC).
  • Intermediate metal conduit (IMC).
  • Rigid metal conduit (RMC) without an overall nonmetallic covering.

Flexible metal conduit can be used in lengths up to 4 ft to connect physically adjustable equipment and devices if all openings are closed.

If lighting is necessary for maintenance and repair within the plenum, install enclosed, gasketed-type luminaires.

Other space used for environmental air. The space above a suspended ceiling, if used for environmental air, is an example of spaces to which this requirement applies. It doesn't apply to habitable rooms or areas of buildings, the prime purpose of which isn't air handling. The permitted wiring methods are different from the ones we just looked at, and are listed in 300.22(C)(1). Also, any cables used in a space used for environment air must be plenum rated cables. Be sure you understand this distinction; actual fabricated plenums are not the same thing as spaces used for environmental air. We are allowed to run certain wiring methods in spaces allowed for environment air [300.22(C)(1)], but we are never allowed to run any wiring methods inside fabricated plenums, unless they are there to service very specific needs inside the plenum [300.22(B)].

In other spaces used for environmental air, only electrical equipment constructed with a metal enclosure, or a nonmetallic enclosure listed for the application are allowed [300.22(C)(2)]. Dry-type transformers rated 50kVA and less with metal enclosures can be installed above suspended ceilings used for environmental air [450.13(B)].

You can install surface metal raceways, metal wireways with metal covers, or solid bottom metal cable tray with solid metal covers in other spaces used for environmental air, if you install them so that they are accessible [300.22(C)(1)].

That brings us to another point about removable panels, such as suspended ceiling panels. If you install anything behind or above panels, ensure the panels can be removed for access to the equipment and will not be blocked by cables, raceways, and equipment [300.23]. An accumulation of cables can hinder this, so route your runs with this requirement in mind. This is a requirement for all suspended ceilings — not just those that are used for environmental air.

Splices and pigtails

Conductors in raceways must be continuous between all points of the system [300.13]. This means you can't make splices in raceways, except as permitted by 376.56, 378.56, 384.56, 386.56, or 388.56. See 300.15.

In a multiwire branch circuit, don't wire a device such that its removal breaks the continuity of the neutral conductor. The only way to meet this requirement is to splice the supply side and load side neutrals together and then pigtail to the neutral terminal of the device [300.14(B)].

The opening of the neutral of a 2-wire circuit during the replacement of a device doesn't cause a safety hazard, so you don't have to pigtail these conductors [110.14(B)].

Boxes and fittings

In each box for splices or terminations, leave at least 6 in. of free conductor, measured from the entry point [300.14].

Boxes that have openings less than 8 in. in any dimension must have at least 6 in. of free conductor and at least 3 in. of free conductor outside the box opening (Fig. 3).

A box is required at each splice or termination point [300.15], except for:

  • Cabinet or cutout boxes [312.8]
  • Conduit bodies [314.16(C)]
  • Luminaires [410.64]
  • Surface raceways [386.56 and 388.56]
  • Wireways [376.56]
  • CATV [90.3]
  • Class 2 and 3 control and signaling [725.3]
  • Communications [90.3]
  • Optical fiber [770.3]

A box is not required where:

  • Splices are made in a cabinet or cutout box containing switches or overcurrent devices, if the splices or taps don't fill the wiring space at any cross section to more than 75%, and the wiring at any cross section doesn't exceed 40% [300.15(I)]. See 312.8 and 404.3(B).
  • Splices are made underground, if you use a splicing device listed for direct burial [300.15(G)]. See 110.14(B) and 300.5(E).
  • A luminaire is used as a raceway as permitted in 410.64 and 410.65 [300.15(J)].
  • Conductors are installed in a handhole enclosure, and splices are made per 314.30.

Some general rules on fittings:

  • Use fittings only with the specific wiring methods for which they’re listed and designed [300.15].
  • If you use a raceway for the support or protection of cables, reduce the potential for abrasion by placing a fitting where the cables enter the raceway [300.15(C)].
  • You can use a fitting instead of a box or conduit body where conductors aren't spliced or terminated within the fitting, if it’s accessible after installation [300.15(F)], as shown in Fig. 4.

While Art. 300 tells you where you need a box, it doesn't provide the requirements for installing boxes. However, Art. 314 does. So in Part 3 of this series, we'll see how to correctly apply those requirements.

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (, one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.

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