Contrary to popular electrical opinion, designated rules limit the number of conductors allowed within outlet, device, and junction boxes. Unfortunately, all too often, electricians remove junction box covers only to find the box crammed full of conductors.

Some creative methods have been devised to overpack the conductors into a junction box so that the cover can still be installed. Such methods include, but are not limited to: carefully wrapping the conductors around the inside of the box so that no air space remains; mashing the conductors down with the thumb, the heel of the hand; or even by applying force with the handle of a hammer. An optional creative method is to back out the junction box screws, insert a blank plate, then tighten the screws, thereby compressing the conductors. Of course, not one of these methods is correct. Article 370 contains detailed requirements for box fill and junction box sizing.

Introduction to Article 370

Article 370 of the National Electrical Code (NEC) covers a variety of provisions pertaining to boxes (outlet, device, pull; junction), conduit bodies, and fittings. It covers the installation and use of all boxes (and conduit bodies) used as outlet, junction, or pull boxes, depending on their use. It also covers manholes (including other electric enclosures intended for personnel entry). [370-1]

Article 370 consists of five parts: A. Scope and General, B. Installation, C. Construction Specifications, D. Manholes and Other Electric Enclosures Intended for Personnel Entry (new to the 1999 edition), and E. Pull and Junction Boxes for Use on Systems Over 600 Volts, Nominal. This month's In Focus discusses Installation (Part B).

370-15 Damp, Wet, or Hazardous (Classified) Locations

Boxes, conduit bodies, and fittings in damp or wet locations must be placed or equipped so as to prevent the entrance of, or internal accumulation of, moisture. [370-15(a)] Damp locations, as defined in Article 100, are locations subject to moderate degrees of moisture. Partially protected external locations under canopies, marquees, roofed (open) porches, and similar sites are considered damp locations as are some interior locations such as certain basements, barns, and cold-storage warehouses. Dry locations are not normally subject to moisture, except on a temporary basis, such as a building under construction.

Boxes, conduit bodies, and fittings installed in wet locations must be listed for that specific use. [370-15(a)] Installations in any of the following categories are wet locations: underground, within concrete slabs, in masonry (directly contacting the earth), areas subject to saturation (water and other liquids), and locations unprotected from weather. [Article 100]

Hazardous (classified) location installations must comply with Articles 500 through 517. [370-15(b)]

370-16 Number of Conductors in Outlet, Device, and Junction Boxes, and Conduit Bodies

Boxes and conduit bodies must be of sufficient size to provide free space for all enclosed conductors. (The phrase "free space" should not be confused with "free conductor" located in Section 300-14.)

Two sections refer to these calculations: 370-16(a) Box Volume Calculations, and 370-16(b) Box Fill Calculations. In no case shall the volume of the box, as calculated in (a), be less than the fill calculation as determined in (b). Conduit body requirements are located in Section 370-16(c). Conduit bodies, also referred to as condulets, occur in a variety of types, including: LB, LL, LR, C, etc. Unlike junction boxes and conduit bodies, terminal housings supplied with motors are not required to comply with Article 370.

Use Section 370-16 to determine the size box required for No. 6, and smaller, conductors. These boxes are calculated by using the number of conductors and their respective sizes.

370-16(a) Box Volume Calculations

Table 370-16(a) lists the most commonly used metal boxes along with the maximum number of conductors permitted in each. This table also provides the minimum cubic inch capacity, as well as the maximum number of conductors (for sizes No. 18 through No. 6) permitted for these boxes. The maximum number of conductors is based on the same size conductors in a box containing no fittings or devices, such as fixture studs, cable clamps, hickeys, switches, or receptacles. Fewer conductors are allowed in boxes that contain fittings or devices.

A 4-inch square box (also known as a 1900 box, a 4-S box, or a 4-square box) with a depth of 1.5 inches, has a volume of 21 cubic inches, as listed in Table 370-16(a). Note the volume is not calculated by using the box's outside dimension. Instead, the calculation is based on the box's actual (inside) volume. A 3-inch by 2-inch device box, 2.5 inches deep, has a volume of 12.5 cubic inches.

The volume of a wiring enclosure (box) is the total volume of the assembled sections. Additional cubic-inch capacity can be gained by using plaster (mud) rings, domed covers, extension rings, or similar devices. Besides having the cubic-inch capacity marked on (or in) the box, ring, cover, etc., the box could be made from boxes the dimensions of which are listed in Table 370-16(a). A plaster ring, clearly marked with a cubic-inch capacity, increases a box's volume by that amount. A 4-inch square extension ring, having a depth of 1.5 inches, has a capacity of 21 cubic inches because its dimensions are the same as a box listed in Table 370-16(a).

What is the maximum number of No. 12 conductors permitted in a 4-inch square box with a depth of 1.5 inches? (There will be no fittings or devices in the box.) First, look at the left side of Table 370-16(a) to find the specific box. Next, follow the row across to the column containing No. 12 conductors. The maximum number of No. 12 conductors permitted in this box is nine.

This represents a total of nine conductors, not nine conductors from each conduit entering the box.

Cubic inch capacity must be marked on all boxes with a volume of 100 cubic inches or less, except for boxes listed in Table 370-16(a). A manufacturer may choose to mark the box with the cubic-inch capacity, even though it is not mandatory when the box is listed in Table 370-16(a). Nonmetallic boxes must have the cubic-inch capacity durability and legibility marked either in, or on, the box.

Although not required, some nonmetallic boxes are marked with the maximum number of conductors permitted within the box. Usually, several size conductors are listed, such as No. 14, No. 12, and No. 10. The maximum number of conductors listed, whether in Table 370-16(a) or in a nonmetallic box, applies only to a box containing no fittings or devices. A junction box with a blank cover would be an appropriate example.

Wirenuts are not counted in box fill calculations. Some confusion exists regarding this issue. It may be thought, for instance, that a plastic box having nine No. 14 conductors listed can contain not only the nine No. 14 conductors, but also a switch or receptacle. This is incorrect. Remember a reduction of conductors must occur in boxes containing fittings or devices such as fixture studs, cable clamps, hickeys, switches, or receptacles.

Next month's column will discuss in detail the procedures for calculating box fill. Specific items will include: conductor fill, clamp fill, device or equipment fill, and equipment grounding conductor fill.

MILLER, owner of Lighthouse Educational Services, teaches custom-tailored classed and conducts seminars covering various aspects of the electrical industry. He is the author of Illustrated Guide to the National Electrical Code. For more information, visit his Web site at www.charlesRmiller.com. He can be reached by telephone at (615) 333-3336, or e-mail at charles@charlesRmiller.com.