Conductor Derating in Conduit (NEC 310.15)
NEC 310.15 · Derating · Updated 2026-08-07
Put more than three current-carrying conductors in one conduit and the NEC cuts the ampacity of every wire inside. The adjustment factors in NEC 310.15(C)(1) are simple: 4 to 6 conductors keep 80 percent of their ampacity, 7 to 9 keep 70 percent, and 10 to 20 keep just 50 percent. Bundled wires cannot shed heat the way a lone conductor can, so the code shrinks what each one is allowed to carry.
This guide covers the full adjustment table, which conductors actually count, why THHN derates from the 90C column of NEC 310.16, and the small-conductor caps in 240.4(D) that quietly override your math. The worked examples show every step of the arithmetic, including the painful jump from nine to ten conductors. As always, the adopted code edition and your local AHJ have the final word.
The 60-second answer
Derating, formally called ampacity adjustment, applies whenever more than three current-carrying conductors share a conduit or other raceway. You multiply each conductor's table ampacity by the adjustment factor, and the result becomes the new maximum current that wire may carry. The factors come from NEC 310.15(C)(1), and they apply to the ampacity values of NEC 310.16 - see the wire size chart for the full table - not to the breaker size directly.
| Current-carrying conductors | Percent of table ampacity |
|---|---|
| 4 to 6 | 80% |
| 7 to 9 | 70% |
| 10 to 20 | 50% |
| 21 to 30 | 45% |
| 31 to 40 | 40% |
| 41 and up | 35% |
Notice the cliff between 9 and 10 conductors: the factor drops from 70 to 50 percent in one step. That single extra wire can turn a compliant 20 A circuit into a 15 A circuit, which is why an accurate count matters more than anything else in this calculation.
Which conductors count as current-carrying
Only conductors that normally carry load current enter the count, and the count is taken per raceway, for the stretch the wires share.
- Ungrounded (hot) conductors always count. Every phase or hot leg of every circuit in the conduit is current-carrying.
- Equipment grounding conductors never count. They carry current only during a fault, so they are excluded from the derating count, although they still take up space in the conduit fill calculation and must be sized per the ground wire size chart.
- Neutrals depend on the circuit. A neutral carrying only the unbalanced current of a multiwire circuit is generally not counted, while the neutral of a 2-wire circuit carries full load current and always counts. Heavy harmonic loads can push a neutral into the count, so confirm the case in the code book.
- Derating counts conductors, not circuits. Three 2-wire circuits in one pipe put six current-carrying conductors in the count, and five 2-wire circuits put ten.
Why THHN derates from the 90C column
THHN is a 90C-rated conductor, so the adjustment factor is applied to the 90C ampacity in NEC 310.16. That is the good news: the 90C column gives you headroom before derating starts to hurt. The result still has to clear two more hurdles, though. First, breaker and device terminations are commonly rated 75C, so the final ampacity can never exceed the 75C column value for that size. Second, the small-conductor rule in 240.4(D) caps overcurrent protection at 15 A for 14 AWG, 20 A for 12 AWG, and 30 A for 10 AWG copper, no matter what the derated math says.
| Size | 90C base | 4-6 wires (80%) | 7-9 wires (70%) | 10-20 wires (50%) | Max breaker per 240.4(D) |
|---|---|---|---|---|---|
| 14 AWG | 25 A | 20 A | 17.5 A | 12.5 A | 15 A |
| 12 AWG | 30 A | 24 A | 21 A | 15 A | 20 A |
| 10 AWG | 40 A | 32 A | 28 A | 20 A | 30 A |
| 8 AWG | 55 A | 44 A | 38.5 A | 27.5 A | no small-conductor cap |
| 6 AWG | 75 A | 60 A | 52.5 A | 37.5 A | no small-conductor cap |
Read the table in both directions. Nine 12 AWG THHN conductors still support a 20 A breaker because 21 A clears 20 A; ten do not, because 15 A falls short. And thanks to the 240.4(D) caps, the mild 80 percent band usually costs small conductors nothing: 12 AWG derated to 24 A was headed for a 20 A breaker either way.
How to derate, step by step
- Count the current-carrying conductors: Tally every hot conductor in the raceway, plus any neutral that counts. Ignore equipment grounds. If the total is three or fewer, no adjustment applies and you are done.
- Look up the 90C ampacity: For THHN copper, use the 90C column of NEC 310.16: 25 A for 14 AWG, 30 A for 12, 40 A for 10, 55 A for 8, and 75 A for 6.
- Multiply by the adjustment factor: Apply the 310.15(C)(1) percentage for your count: 80 percent for 4 to 6 conductors, 70 percent for 7 to 9, 50 percent for 10 to 20.
- Check termination limits: With common 75C-rated terminations, the final ampacity cannot exceed the 75C column value for that conductor size. Take the lower of the derated number and the 75C number.
- Apply the small-conductor cap: For 14, 12, and 10 AWG copper, overcurrent protection cannot exceed 15 A, 20 A, and 30 A respectively under 240.4(D), regardless of what the math produced.
- Sanity-check the rest of the run: Derating is only one gate. Confirm the raceway passes conduit fill, and check long runs against the 3 percent voltage drop recommendation.
Worked examples with verified math
Example 1: nine 12 AWG THHN in 3/4 inch EMT. Fill first: 3/4 inch EMT accepts up to 16 conductors of 12 AWG THHN per Annex C, so nine fit easily. Ampacity next: the 90C base is 30 A, nine conductors takes the 70 percent factor, and 30 x 0.70 = 21 A. Termination check: 21 A is below the 25 A value in the 75C column, so 21 A stands. Cap check: 240.4(D) limits 12 AWG to a 20 A breaker anyway. Result: all nine conductors can serve 20 A circuits.
Example 2: one more wire ruins the party. Add a tenth current-carrying 12 AWG conductor to the same conduit and the factor drops to 50 percent: 30 x 0.50 = 15 A. Every circuit in the pipe must now be protected at 15 A. Splitting the run across two conduits, or landing that circuit elsewhere, restores the 70 percent factor.
Example 3: six 8 AWG THHN. A 3/4 inch EMT holds exactly six 8 AWG THHN per Annex C. Six current-carrying conductors takes the 80 percent factor: the 90C base is 55 A, and 55 x 0.80 = 44 A. The 75C termination value for 8 AWG is 50 A, and 44 A is the lower number, so each conductor is good for 44 A. There is no small-conductor cap at this size.
Example 4: twelve 10 AWG THHN. Twelve current-carrying conductors falls in the 10 to 20 band, so the factor is 50 percent: 40 x 0.50 = 20 A. Even though 240.4(D) would allow a 30 A breaker on 10 AWG, the derated ampacity of 20 A now governs, and 30 A protection is off the table. To keep 30 A circuits, cut the count to six or fewer conductors, where 40 x 0.80 = 32 A clears 30 A, or move up a wire size.
Derating and conduit fill are separate checks
Fill and derating answer different questions, and both must pass. Fill asks whether the wires physically fit, at the NEC Chapter 9 limit of 40 percent of internal area for 3 or more conductors. Derating asks whether those wires can safely carry their load once packed together. A 1 inch EMT legally holds 26 conductors of 12 AWG THHN by fill, but 26 current-carrying conductors lands in the 45 percent band: 30 x 0.45 = 13.5 A per wire. The conduit is perfectly legal and nearly useless for 20 A circuits. In practice, derating - not fill - is what limits how many circuits belong in one pipe. Run both checks against the conduit fill chart before you pull.
Common mistakes
- Counting equipment grounds. They are not current-carrying and never enter the derating count, though they do count toward fill.
- Derating from the wrong column. THHN derates from the 90C column. Starting at the 60C or 75C value throws away headroom the code gives you and can force needless upsizing.
- Forgetting the 240.4(D) cap. Derated 12 AWG at 24 A does not permit a 25 A breaker; the cap holds it at 20 A.
- Missing the 10-conductor cliff. Ten current-carrying conductors takes 50 percent, not 70 percent. Count twice before you pull.
- Treating fill as the only limit. A raceway can pass fill and still fail derating badly, as the 26-conductor example above shows.
- Ignoring ambient heat. NEC 310.15 also applies a temperature correction when conduit runs through hot spaces such as attics and rooftops, and it multiplies together with the bundling factor. The correction values live in the code book.
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Frequently asked questions
How many wires can be in a conduit before derating?
Derating starts with the fourth current-carrying conductor in the raceway. At 4 to 6 conductors each wire keeps 80 percent of its table ampacity, at 7 to 9 it keeps 70 percent, and at 10 to 20 it keeps 50 percent.
Do equipment grounding conductors count toward derating?
No. Equipment grounds carry current only during a fault, so they are excluded from the adjustment-factor count. They still count toward conduit fill and box fill.
Do I derate THHN from the 90C column?
Yes. THHN is a 90C conductor, so you apply the adjustment factors to the 90C ampacity in NEC 310.16. The result then has to clear the 75C termination limit and the small-conductor caps in 240.4(D).
Can I still use a 20 A breaker on 12 AWG with nine wires in the conduit?
Usually yes. Nine current-carrying conductors takes 70 percent of the 30 A 90C ampacity, which is 21 A, so 20 A protection still works. At ten conductors the factor falls to 50 percent and the ampacity drops to 15 A.