Q. What is the minimum size and metal thickness required for a junction box constructed of sheet steel, with one 4-inch RMC conduit (per wall) connected on opposite walls of the box (i.e., directly across from one another)? The conductors within the raceways are 600kcmil type XHHW. The outer diameter (O.D.) of each of these insulated conductors is 1.2 inches.

  1. 24 inch by 24 inch. There is no requirement for metal thickness.
  2. 32 inch by 24 inch. Metal thickness = 1.35 millimeters (0.053 inches).
  3. 32 inch by 32 inch. Metal thickness = 1.35 millimeters (0.053 inches).
  4. 43.2 inches by 43.2 inches. Metal thickness = 1.35 millimeters (0.053 inches).

Answer: C

As per 314.28(A)(1), the length of the box shall not be less than eight times the metric designator of the largest raceway. Therefore, the calculation is as follows: (4-inch trade size diameter) x (8) = 32 inches.

As per 314.40(C), metal boxes constructed of sheet steel more than 1,650 cubic centimeters (100 cubic inches) in size shall not have an uncoated metal thickness of less than 1.35 millimeters (0.053 inches).

Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at necexpert@aol.com.