A contractor is preparing a material takeoff for a new branch circuit. The circuit will consist of three 12 AWG THHN conductors and one 12 AWG bare copper grounding conductor installed in a 3/4-inch EMT conduit. What is the maximum number of additional 12 AWG THHN conductors that can be added to this conduit without exceeding the 40% fill limit, assuming a standard cross-sectional area for 12 AWG THHN?
- A1
- B0
- C3
- D2
Show answer & explanationAnswer & explanation
Correct answer: A. 1
According to CEC Chapter 9, Table 4, the approximate area for one 12 AWG THHN conductor is 0.0133 sq in. A 12 AWG bare copper grounding conductor has the same area (0.0133 sq in). The total area of the four existing conductors is 4 * 0.0133 sq in = 0.0532 sq in. For 3/4-inch EMT, the total allowable fill area at 40% (for 3 or more conductors) is 0.217 sq in * 0.40 = 0.0868 sq in. The remaining available area is 0.0868 sq in - 0.0532 sq in = 0.0336 sq in. Since one additional 12 AWG THHN conductor requires 0.0133 sq in, 0.0336 sq in / 0.0133 sq in/conductor = 2.52 conductors. Therefore, a maximum of 2 additional 12 AWG THHN conductors can be added.
Why the other options are wrong
- B. Incorrect. There is enough space for more than zero additional conductors.
- C. Incorrect. Adding three additional conductors would exceed the 40% fill limit.
- D. Correct. With 4 existing 12 AWG conductors (3 current-carrying, 1 grounding) occupying 0.0532 sq in, and a 3/4-inch EMT having an allowable 40% fill of 0.0868 sq in, there is 0.0336 sq in remaining. This space can accommodate two additional 12 AWG THHN conductors (2 * 0.0133 sq in = 0.0266 sq in), as adding a third would exceed the limit.
Conduit Fill Calculation
The process of determining the maximum number of conductors allowed in a conduit based on their cross-sectional area and the conduit's internal area, adhering to NEC/CEC fill percentages.
- Ensures proper heat dissipation and ease of pulling conductors.
- CEC Chapter 9, Table 1 specifies fill percentages (e.g., 40% for 3 or more conductors).
- CEC Chapter 9, Table 4 and 5 list conductor and conduit areas.
Memory trick: Area times Percent equals Max Wires, don't exceed the limit.