Journeyman Electrician Exam (NEC)Wiring Methods and MaterialsMedium
A commercial building requires a new 277-volt lighting circuit. The branch circuit will consist of three 10 AWG THHN conductors and one 10 AWG bare copper grounding conductor, run in a 3/4-inch Rigid Metal Conduit (RMC) for 75 feet. What is the minimum size square junction box required for a straight pull through these four conductors without splices?
- A4-inch x 4-inch
- B8-inch x 8-inch
- C6-inch x 6-inch
- D10-inch x 10-inch
Show answer & explanationAnswer & explanation
Correct answer: C. 6-inch x 6-inch
For straight pulls, the minimum length of the box must not be less than eight times the trade diameter of the largest raceway. In this case, 8 times 3/4-inch = 6 inches. Therefore, a 6-inch x 6-inch box is the minimum size.
Why the other options are wrong
- A. Incorrect. 4 inches is less than 8 times the raceway diameter, violating NEC 314.28(A)(1).
- B. Incorrect. While larger boxes are acceptable, 8 inches is not the minimum required.
- D. Incorrect. While larger boxes are acceptable, 10 inches is not the minimum required.
Junction Box Straight Pull
For straight pulls in junction boxes without splices, the box length must be at least eight times the trade diameter of the largest raceway.
- NEC 314.28(A) governs box dimensions for conductors 4 AWG or larger.
- This rule applies to the box dimension 'in the direction of the pull'.
- Ensures adequate space for pulling conductors without damage.
Memory trick: Eight times the pipe, or conductors will gripe!