Home run split above the ceiling
What's wrong here?
- The 1/2 in. EMT is filled past the Chapter 9 limit
- The 4-11/16 in. box is past its box-fill limit
- Blue is not permitted for an ungrounded conductor
- The 3/4 in. EMTs are filled past the Chapter 9 limit
Show the answer and the code reference
The answer
Fourteen 12 AWG THHN conductors are pulled through a 1/2 in. EMT, which is limited to 9 of them by the 40 percent fill rule.
Correct choice: 1. The 1/2 in. EMT is filled past the Chapter 9 limit
358.22 limits the number of conductors in EMT to the fill percentage in Chapter 9, Table 1, which is 40 percent for three or more conductors. 1/2 in. EMT has 0.304 sq in of internal area (Table 4), so 40 percent is 0.122 sq in; 12 AWG THHN is 0.0133 sq in each (Table 5), which allows 9 conductors (also shown in Informative Annex C, Table C.1). Fourteen conductors is about 61 percent fill: the pull strips insulation, the conductors run hot, and with this many current-carrying conductors the ampacity adjustment of 310.15(C)(1) would also apply.
How to fix it
Replace the 1/2 in. home run with at least a 3/4 in. EMT (16 maximum for 12 AWG THHN) or split the circuits into two raceways, and apply the 310.15(C)(1) adjustment factors for the number of current-carrying conductors in each raceway.
Code references
NEC 2023 358.22 — Number of Conductors
The number of conductors in EMT shall not exceed that permitted by the percentage fill specified in Table 1, Chapter 9.
Drills citing 358.22NEC 2023 Chapter 9, Table 1 — Percent of Cross Section of Conduit and Tubing for Conductors and Cables
Maximum fill is 53 percent for one conductor, 31 percent for two conductors, and 40 percent for over two conductors.
Drills citing Chapter 9, Table 1NEC 2023 Chapter 9, Tables 4 and 5 — Dimensions and Percent Area of Conduit and Tubing; Dimensions of Insulated Conductors and Fixture Wires
Trade size 1/2 EMT has a total internal area of 0.304 sq in (0.122 sq in at 40 percent); a 12 AWG THHN/THWN conductor has an approximate area of 0.0133 sq in.
Drills citing Chapter 9, Tables 4 and 5NEC 2023 310.15(C)(1) — More Than Three Current-Carrying Conductors
Where the number of current-carrying conductors in a raceway or cable exceeds three, the ampacity of each conductor shall be reduced per Table 310.15(C)(1). The adjustment does not apply to conductors in raceways 600 mm (24 in.) or less in length.
Drills citing 310.15(C)(1)
2026 NEC:same rule and number. 358.22 still points to the Chapter 9, Table 1 fill limits, and the 40 percent limit for more than two conductors is unchanged in 2026. Secondary reference 310.15(C)(1) was not separately checked. Confidence: medium
Minnesota
Minnesota adopted the 2023 NEC as the Minnesota Electrical Code effective July 1, 2023 (Minn. Stat. 326B.35; Minn. Rules 1315.0200) and adopts the NEC without section-by-section amendments. No Minnesota change to 358.22 or the Chapter 9 fill tables is known. Newer permits may fall under a later NEC edition; see the Minnesota page. The Board of Electricity adopted the 2026 NEC, also without amendment, for permits filed on or after Aug. 17, 2026 (exams moved to the 2026 NEC on Sept. 8, 2026); see the 2026 NEC note on this drill.
How Minnesota adopts the NECWhy the other choices are wrong
- The 4-11/16 in. box is past its box-fill limit. A 4-11/16 x 2-1/8 in. box is 42 cu in; 13 conductors at 2.25 cu in each plus one allowance for the EGC is about 31.5 cu in, so box fill under 314.16 is fine.
- Blue is not permitted for an ungrounded conductor. The NEC reserves white or gray for grounded conductors (200.6) and green or bare for EGCs (250.119). Blue is a common phase color on 208Y/120 V systems; where a building has more than one voltage system, 210.5(C)(1) requires the identification scheme to be posted or documented.
- The 3/4 in. EMTs are filled past the Chapter 9 limit. 3/4 in. EMT is permitted 16 conductors of 12 AWG THHN (Informative Annex C, Table C.1); seven per side is well within fill.