310.15(C)
Conduit Fill Adjustment Factors
Imagine sitting in a packed subway car on a hot day — the more people crammed in, the hotter it gets for everyone. Wires work the same way. With three or fewer current-carrying conductors, heat dissipates well enough that the base Table 310.16 values hold up. Once you exceed three, the trapped heat starts to become a problem, and the code requires you to dial back the allowed ampacity. For 4 to 6 conductors, you reduce to 80 percent of the table value. For 7 to 9, it drops to 70 percent. The more wires you add, the steeper the reduction goes. This adjustment applies independently of temperature correction — if you are in a hot attic AND have a packed conduit, you apply both factors, and the resulting ampacity can drop significantly.
When You Need This
- Pulling a multi-circuit home run through a single conduit from a panel to a junction point
- Running multiple lighting or receptacle circuits through a shared conduit in a commercial ceiling
- Installing control wiring alongside power conductors in an industrial raceway
- Any time an inspector asks you to justify wire sizing in a conduit that obviously has more than three circuits
- Studying for the journeyman exam — conduit fill derating calculations appear frequently
Key Points
Common Mistakes
Confusing physical conduit fill (Chapter 9, Table 1 — a space limitation) with ampacity adjustment (310.15(C) — a heat limitation)
Counting equipment grounding conductors as current-carrying — they are excluded from the count
Forgetting to count the neutral on a 3-phase, 4-wire wye circuit where the neutral carries harmonic currents — it counts as a current-carrying conductor in that case
Applying the adjustment factor to the wrong starting ampacity — always start from Table 310.16, not from an already-derated value, unless also applying temperature correction
Assuming that meeting the physical fill requirement means you can skip the ampacity adjustment — both rules apply independently
Exam Tip
The exam will try to trick you by asking about neutrals. On a straight 120/240V single-phase circuit, the neutral only carries unbalanced current and is NOT counted. But on a 3-phase, 4-wire system with nonlinear (harmonic) loads, the neutral IS counted because it carries the triplen harmonics.
Frequently Asked Questions
Yes. The adjustment factors apply to cables as well as raceways. If you bundle multiple MC cables together, the current-carrying conductors inside all of them count toward the total.
Yes, and this is actually the main practical benefit of 90-degree-rated wire like THHN. You start with the higher 90-degree-C ampacity, apply the adjustment factor, and as long as the resulting value does not exceed the ampacity from the column matching your terminal temperature rating, you are code compliant.
Only count the current-carrying conductors for the adjustment factor. Equipment grounding conductors and neutrals that carry only unbalanced current are excluded from the count.
Inline Tools
Conduit Fill Calculator
Check if your conductors fit inside the raceway
Related Code Sections
This is an educational summary, not the official code text. The NEC® is a registered trademark and copyright © National Fire Protection Association (NFPA). The CEC is copyright © CSA Group. For official code text, visit nfpa.org or csagroup.org. SparkShift is not affiliated with NFPA or CSA Group.