Conductor Fill and Derating in a Crowded Conduit
A 1¼ in. EMT run of 40 ft carries twelve 10 AWG THHN copper current-carrying conductors and four 12 AWG THHN equipment grounding conductors between a panel and a junction box. The conduit passes through a boiler room where the ambient temperature is 45°C for 20 ft. The electrician used Table 310.16 ampacity for 10 AWG THHN (35 A at 90°C) and sized the 20 A breaker accordingly, claiming the 90°C column applies. The plan reviewer notes the termination rating at the panel is 75°C and that the conduit fill exceeds 40%.
1. Determine the correct ampacity of the 10 AWG THHN conductors after applying fill and temperature correction.
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Twelve current-carrying conductors in a single conduit require a 50% adjustment factor (Table 310.15(C)(1)). Four EGCs are not current-carrying and are excluded from the count. At 45°C ambient, the 90°C correction factor is 0.82 (Table 310.15(B)(1)). Start from the 90°C ampacity of 10 AWG THHN (40 A per Table 310.16 - note: THHN 10 AWG is 40 A at 90°C, not 35 A). Apply both: 40 × 0.50 × 0.82 = 16.4 A. Because the terminations are 75°C, the conductor's ampacity is also limited by the 75°C column (35 A) before adjustment, but the adjusted value of 16.4 A governs. A 20 A breaker is therefore not permitted on these conductors; the load must be reduced, the conduit fill reduced, or the conductor upsized. The electrician's use of the 90°C column without adjustment is incorrect.
2. Does the 40 ft length and the 20 ft hot section affect the analysis?
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Ambient correction applies to the portion of the run in the higher-temperature environment. Under 310.15(B), where conductors are exposed to different ambient temperatures, the ampacity is based on the higher temperature for the length involved, and the correction factor is applied to the entire circuit unless the higher-temperature section is short and the ampacity is calculated for that section. A 20 ft section in a 45°C boiler room is not a trivial nipple; the correction applies. The 40 ft total length does not trigger voltage-drop considerations under the Code (that is an informational note in 210.19(A) Informational Note 4), but the reviewer should verify voltage drop separately for the actual load.
3. What options does the electrician have to keep the 20 A circuits?
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Options: (1) reduce the number of current-carrying conductors in the conduit (split into two conduits) to lower the adjustment factor; (2) upsize to 8 AWG THHN (55 A at 90°C × 0.50 × 0.82 = 22.6 A, still marginal - 6 AWG gives 75 × 0.5 × 0.82 = 30.8 A, acceptable); (3) route the conduit around the boiler room to avoid the 45°C ambient; (4) use a wiring method with higher ampacity or reduce load. Simply changing the breaker is not an option because the conductors are protected at their adjusted ampacity. The cleanest fix is splitting the circuits and rerouting to avoid the hot section.