Code Quiz

Sep 8, 2004 9:26 AM, By Steven Owen

According to 314.28 of the 2002 NEC, what are the minimum dimensions required for a metal junction box that has the following raceways (conduits) connected to it? The junction box has two 4-inch EMT raceways connected directly across from one another on opposite walls. Each raceway contains three 250 kcmil XHHW conductors. A 2-inch EMT raceway, which contains three 4/0 AWG XHHW conductors, is connected to the same wall as one of the 4-inch EMT raceways. In effect, one wall has a 4-inch and a 2-inch raceway. The opposite wall has only a 4-inch raceway. The wall at a right angle to the 4-inch and 2-inch raceways has a single 2-inch EMT raceway connected to it. The 4/0 AWG conductors are installed between the 2-inch raceways and are passing through the junction box. There's no raceway on the wall opposite the single 2-inch EMT raceway.

A) 32 inches x 12 inches
B) 32 inches x 32 inches
C) 26 inches x 12 inches
D) 34 inches x 12 inches

Answer: A)

Explanation: When sizing the junction box, based on a straight pull, 314.28(A)(1) requires a (minimum) multiplier of 8 times the trade diameter of the largest raceway (enclosing the conductors for the straight pull only). (8 x 4-in. raceway = 32 in. minimum) For angle or U-pulls, 314.28(A)(2) requires a minimum of 6 times the trade diameter of the largest raceway (containing conductors that are included in the angle or U-pull), plus the sum of the diameters of the other raceways on the same wall in the same group. Don’t forget to apply the 6 times multiplier only to the raceway(s) that contains the conductors that are included in the angle pull or U-pull.

In this example, the 4-inch raceways are aligned for a straight pull. The 2-inch raceways are aligned in an angle pull layout. When both straight pulls and angle pulls are involved on the same wall, both must be calculated. Use the greater dimension obtained by comparing both calculations.

Example:
Calculation #1 for wall 1: (side wall – horizontal)
Straight pull: 8 x 4 in. = 32 in.
Angle pull for same wall: 6 x 2 in. = 12 in. + 4 in. (for the 4-inch raceway) = 16 in.
The straight pull gives the larger dimension of 32 inches, which shall be used.

Calculation #2 for wall 2:
Angle pull: 6 x 2 in. = 12 in.
This is the only raceway on the bottom or top wall - (vertical).

Minimum dimensions: 32 inches (wide) x 12 inches (height)