370-1 Scope

Article 370 covers the installation and use of all boxes (and conduit bodies) functioning as outlet, junction, or pull boxes, depending on their application. It also covers manholes, inclusive of other electric enclosures intended for personnel entry. [370-1]

This article addresses installation requirements for fittings used to join raceways, and to connect raceways and cables to boxes and conduit bodies. Connectors, couplings, and conduit hubs are examples of fittings discussed.

Article 370 is subdivided into five distinct parts: “A. Scope and General; B. Installation; C. Construction Specifications; D. Manholes and Other Electric Enclosures Intended for Personnel Entry”; and “E. Pull and Junction Boxes for Use on Systems Over 600 Volts, Nominal.”

For the last three months, “In Focus” covered the first few sections of Article 370, with emphasis on box fill calculations, found in Part B, Section 370-16. Of course, box fill calculation requirements represent only a portion of Part B, Installation. This, and future issues will explain and illustrate other Part B requirements. Specifically, this month’s “In Focus” discusses Sections 370-17 through 370-19.

370-17 Conductors Entering Boxes, Conduit Bodies, or Fittings

Section 370-17 states that conductors entering boxes, conduit bodies, or fittings must be protected from abrasion. Compliance with this requirement prevents sharp edges from rubbing, scraping, or cutting into a conductor’s insulation. Without the proper protection, by fittings or other means, conductors entering knock-out openings of metal boxes (or enclosures) are susceptible to damage. Scrapped or cut conductor insulation creates a safety hazard by exposing live conductors.

Section 370-17(a) stipulates that openings through which conductors pass must be adequately closed. Compliance with this regulation is usually accomplished by simply tightening the fitting that secures the conductor to the box.

Improper selection or use of a fitting may constitute a violation of this requirement. For example, installing a 14-2 nonmetallic-sheathed cable with ground into a fitting listed for No. 8 through No. 4 nonmetallic-sheathed cables violates several requirements. Section 110-3(b) states that listed or labeled equipment must be installed and used in accordance with any instructions included in the listing or labeling. This section specifies that openings must be adequately closed and the cable must be secured to the box.

The first sentence of 370-17(b) pertains to open wiring as well as concealed knob-and-tube wiring. While requirements for open wiring on insulators are found in Article 320, concealed knob-and-tube wiring regulations are located in Article 324. Conductors in these wiring systems must enter metal boxes or conduit bodies either through insulated bushings or, in dry locations, through flexible tubing.

Wherever tubing is used, it must be firmly secured to the box, extending to the last insulating support.

The next sentence of Section 370-17(b) refers to all raceways and cables installed using metal boxes or conduit bodies. The raceway or cable must be secured to the box or conduit body. Again, using the proper connector or fitting and making sure the locknut and screws are tight will ensure compliance with the requirements as given.

370 -17(c) Nonmetallic Boxes

Nonmetallic boxes must be suitable for the lowest temperature-rated conductor entering the box. Be sure to apply any instructions found in the listing or labeling for applicable equipment. As previously stated, listed or labeled equipment must be installed and used in accordance with any instructions included in the listing or labeling. [110-3(b)]

Like the previous section, reference is made to open wiring on insulators and knob-and-tube wiring. In these systems, conductors must enter the box through individual holes.

Electricians who work on old houses or buildings containing knob-and-tube wiring must understand this requirement because violations exist all too often. For example, exposed conductors are frequently cut and spliced without using outlet or junction boxes. Where another wiring method is spliced onto open wiring or concealed knob-and-tube wiring, the installation must meet all applicable articles.

Envision a situation where exposed wiring in the basement of a one-family dwelling is damaged. An electrician replaces most of the exposed wiring with nonmetallic-sheathed cable, splicing the old wiring to the new inside nonmetallic boxes. The electrician must make sure that each individual knob-and-tube conductor enters the box through a separate (individual) hole. If flexible tubing is used to encase the knob-and-tube conductors, the tubing must extend from the last insulating support to at least 1/4 inch inside the box.

In addition, Section 370-17(c) outlines requirements for nonmetallic-sheathed cable. Nonmetallic-sheathed cable installations require that the cable assembly (sheath included) extend into the box a minimum of 1/4 inch through the cable knockout opening. This requirement refers only to nonmetallic boxes; installing the cable’s sheath 1/4 inch into metal boxes is allowed, but not required.

All permitted wiring methods must be secured to nonmetallic boxes, unless the Section 370-17(c) exception is met. If three conditions are satisfied, nonmetallic-sheathed cable and/or underground feeder and branch-circuit cable may enter a nonmetallic box without being secured to the box.

These conditions include: 1) the box must be a single-gang box no larger than a nominal size 21/4 by 4 inches, 2) the cable must be fastened within 8 inches of the box measured along the sheath, and 3) the sheath must extend through a cable knockout at least 1/4 inch.

Of course, the third condition is required regardless of the exception. Note, this exception applies only to single-gang nonmetallic boxes. Section 336-18 stipulates that nonmetallic-sheathed cable must be secured within 12 inches of each box.

But, where a cable is not secured to the box, it must be secured within 8 inches of the box. This exception also permits multiple cables to enter a single cable knockout opening.

Section 370-17(d) stipulates that the installation of No. 4 or larger conductors must comply with Section 300-4(f), which in turn states that the conductors must be protected by a substantial fitting that provides a smooth, rounded insulating surface. Conduit bushings and threaded hubs are two examples of acceptable fittings.

370-18 Unused Openings

Unused cable or raceway openings in boxes and conduit bodies must be closed so that the protection provided at least equals that of the wall of the box or conduit body.

Even though duct tape is similar in color to metal boxes, it does not provide protection equivalent to the wall of the box. Therefore, closing the hole of a knockout opening with duct tape is a violation.

Metal plugs or plates cannot be used with nonmetallic boxes or conduit bodies unless they are recessed at least 1/4 inch from the outer surface of the box.

370-19 Boxes Enclosing Flush Devices

Boxes used to enclose flush devices must, by design, completely enclose the device on the back and sides. Also, the device must be securely attached and supported, by a means other than the screws used to support the box.

Next month’s “In Focus,” beginning with Section 370-20, will continue discussion of “Article 370, Part B, Installation.” Section 370-20 covers requirements for boxes in walls or ceilings constructed of combustible, as well as, noncombustible material. Although these rules are easily understood, they are often ignored.

MILLER, owner of Lighthouse Educational Service and author of The Illustrated Guide to The National Electrical Code,can be reached by phone at (615) 333-3336, or via e-mail at charles@charlesRmiller.com.