Conductor Derating in Conduit (NEC 310.15)
NEC 310.15 · Derating · Updated 2026-07-10
Conductors bundled in a raceway can't shed heat like a single wire, so the NEC makes you reduce their ampacity as the count climbs. This is the rule that turns "16 wires fit in 3/4 EMT" into "you only want 9 in there."
Two factors stack: the adjustment factor for conductor count and the correction factor for ambient temperature. Both come off the conductor's 90°C rating (for THHN), then the result is capped by the 75°C termination rule.
Adjustment factors for more than 3 current-carrying conductors
| Current-carrying conductors | Adjustment factor |
|---|---|
| 4 - 6 | 80% |
| 7 - 9 | 70% |
| 10 - 20 | 50% |
| 21 - 30 | 45% |
| 31 - 40 | 40% |
| 41+ | 35% |
Which conductors count
- Hots: always count.
- Neutrals: count only when they actually carry current continuously - the shared neutral of a balanced 3-wire MWBC doesn't count, but a neutral serving nonlinear loads (electronic ballasts, VFDs, most LED drivers at scale) does (310.15(E)).
- Equipment grounds: never count (310.15(F)).
- Spares: unenergized spare conductors don't count - but they still count toward conduit fill.
Worked example: nine #12 THHN in one EMT run
- Start at the 90°C column for #12 THHN: 30 A.
- Nine current-carrying conductors → 70% factor: 30 × 0.70 = 21 A.
- Termination cap: #12 is limited to a 20 A breaker by 240.4(D) anyway → each circuit still delivers its full 20 A. Pass.
- Add a tenth conductor and the factor drops to 50%: 30 × 0.50 = 15 A - now your 20 A circuits fail. That's why nine is the magic number.
Ambient temperature correction
| Ambient | Factor |
|---|---|
| 26-30°C (up to 86°F) | 1.00 |
| 31-35°C (87-95°F) | 0.96 |
| 36-40°C (96-104°F) | 0.91 |
| 41-45°C (105-113°F) | 0.87 |
| 46-50°C (114-122°F) | 0.82 |
Attics, boiler rooms, and sun-exposed rooftops all trigger correction, and the factors multiply with the conductor-count adjustment. Two exceptions worth knowing: adjustment factors don't apply inside conduit nipples of 24 inches or less, and raceways buried underground get their own treatment.
Derating without the table-flipping. Count, ambient, insulation in - defensible ampacity out, cited to 310.15 and 310.16. Free on the App Store. Download Conduit Fill & Bending Calc on the App Store.
Frequently asked questions
How many current-carrying #12 THHN can I run in one conduit for 20 A circuits?
Nine. At 7-9 conductors the 70% factor leaves 21 A (from the 90°C base of 30 A), which still supports a 20 A breaker. The tenth conductor drops you to 50% and 15 A, failing a 20 A circuit.
Do neutral wires count for derating?
Only when they carry current in normal operation. The neutral of a balanced multiwire branch circuit doesn't count; a neutral on a 2-wire circuit does (it carries the same current as the hot), and neutrals feeding nonlinear loads count per 310.15(E).
Do derating factors apply in short conduit nipples?
No. Where the raceway is a nipple 24 inches or less, the adjustment factors of 310.15(C)(1) don't apply - and the nipple may also be filled to 60% instead of 40%.
Why do I start derating from the 90°C column?
Because THHN/THWN-2 is a 90°C-rated conductor, the code lets you apply adjustment and correction factors to the 90°C ampacity. The final result just can't exceed what the 75°C terminations allow (110.14(C)) - that's the cap, not the starting point.