NEC 310.15(C)(1) Ampacity Adjustment Factors
NEC 310.15(C)(1) requires ampacity derating when more than 3 current-carrying conductors share a raceway over 24 inches.
NEC 310.15(C)(1) requires ampacity adjustment when more than three current-carrying conductors (CCCs) share a single raceway, cable, or earth trench longer than 24 inches. The rule is built into NFPA 70 (2023) because heat dissipation from bundled conductors is dramatically reduced - the table values in 310.16 assume only three CCCs at 30C ambient. Derating protects the insulation from cooking when more conductors crowd a pipe.
The Adjustment Factors Table
NEC Table 310.15(C)(1) - 2023 values:
| Number of CCCs | Adjustment Factor |
|---|---|
| 4 - 6 | 80% |
| 7 - 9 | 70% |
| 10 - 20 | 50% |
| 21 - 30 | 45% |
| 31 - 40 | 40% |
| 41 and above | 35% |
These factors are applied to the 90C-column ampacity from Table 310.16 (the conductor's intrinsic thermal limit), not the 75C terminal-limited value.
Why the Rule Exists
A single conductor in still air sheds I^2 R heat to the surrounding insulation, jacket, and conduit wall. With more conductors packed together, the inner conductors have nowhere to dump heat except outward through the conduit. Bench testing during the 1950s and 60s by IEEE and ICEA showed that for raceways longer than two feet, the heat builds toward a steady-state temperature that scales with the number of loaded conductors. The 310.15(C)(1) factors are a conservative envelope around those tests. Short nipples (24 inches or less) do not develop enough length for the heat to accumulate, which is why Chapter 9 Note 4 exempts them.
Counting Current-Carrying Conductors
NEC 310.15(E) governs how to count CCCs:
- Phase (hot) conductors: Always count
- Neutral on 2-wire circuit: Counts (it carries the same current as the hot)
- Neutral on 3-wire single-phase or balanced 3-phase 4-wire wye: Does not count, since it only carries unbalance
- Neutral on 3-phase 4-wire wye with non-linear loads (more than 50%): Does count per 310.15(E)(3) - triplen harmonics flow on the neutral
- Equipment grounding conductor (EGC): Does not count - it carries fault current only
- Grounded conductor of a 3-phase 3-wire delta system: Counts
Worked Example
A 1-inch EMT 40 feet long contains:
- Two 3-wire single-phase MWBC (multi-wire branch circuits) - 4 hots + 0 effective neutrals = 4 CCC
- One 2-wire 20A circuit - 1 hot + 1 neutral = 2 CCC
- Total = 6 CCC plus EGCs (not counted)
All conductors are 12 AWG THHN copper. Base 90C ampacity from Table 310.16 is 30A. The 4-6 CCC factor is 80%. Adjusted ampacity = 30 x 0.80 = 24A. Per NEC 110.14(C)(1), the terminal limit is the 75C column at 25A. The lower of (24A derated, 25A terminal) is 24A. NEC 240.4(D) caps overcurrent protection for 12 AWG at 20A regardless, so the 20A breaker stands. The derating did not force a wire upsize this time, but if a fourth MWBC were added (now 8 CCC, 70% factor), adjusted ampacity would drop to 21A - still above the 20A breaker, just barely.
Common Pitfalls
- Counting EGCs as CCCs. EGCs are exempt - they only carry fault current.
- Forgetting the 24-inch nipple exemption. A 22-inch sleeve from a panel to an LB elbow does not require 310.15(C)(1) derating, even with 30 conductors. See our nipple rule guide.
- Applying the factor to the 60C or 75C column. Always start from the 90C ampacity, then compare to the terminal rating after derating.
- Missing the harmonic-neutral case. Office buildings, data centers, and LED-lighting installations frequently push neutral harmonic content above 50%, triggering NEC 310.15(E)(3).
Cross-References
- NEC 310.16 - base ampacity table; see our Table 310.16 guide
- NEC 310.15(B)(1) - ambient temperature correction (applied as a separate multiplier)
- NEC 310.15(E) - neutral-counting rules
- NEC 110.14(C)(1) - terminal temperature rule
- NEC Chapter 9 Note 4 - nipple exemption (no derating for raceways 24" or shorter)
- NEC 240.4(D) - small conductor cap
How WireFillChart Implements It
Our conduit fill calculator flags raceways exceeding 3 CCCs and shows the applicable adjustment factor inline, drawing on Table 310.15(C)(1). The fill calculation itself is independent of derating, but understanding both rules together is essential to a complete raceway design. See our current-carrying conductor reference for definitions.