To derate a wire, take its ampacity from Table 310.16 of the 2020 NEC (NFPA 70) and multiply by a temperature correction factor. If more than three current-carrying conductors share a raceway or cable, also multiply by an adjustment factor. Then check the terminal rating and use the lower number. Four 3 AWG THHN copper wires in a 100°F room work out to 115 × 0.91 × 0.80 = 83.7 amps.
| Quick facts | Ampacity derating (2020 NEC) |
|---|---|
| Starting ampacity | Table 310.16: 30°C (86°F) ambient, three or fewer current-carrying conductors |
| Temperature correction | 310.15(B)(1) and Table 310.15(B)(1) |
| Rooftop sun | 310.15(B)(2): add 33°C (60°F) when less than ⅞ inch above the roof |
| More than three wires | 310.15(C)(1) and Table 310.15(C)(1) |
| Terminal limit | 110.14(C): usually 60°C for 100 amps or less, 75°C over 100 amps |
| Small wire cap | 240.4(D): 14 AWG copper 15 A, 12 AWG 20 A, 10 AWG 30 A |
Wire derating calculator (Table 310.16)
Pick the wire, insulation, terminals, room temperature and how many current-carrying conductors share the raceway or cable. The calculator runs the four steps below and shows its math.
Uses Table 310.16 (raceway, cable or earth). Count only current-carrying conductors (310.15(E) and (F)). Temperatures between table rows round up. Doesn't apply the 310.15(C)(1) exceptions, such as raceways 24 in. or shorter.
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Step 1: Start with Table 310.16
Use the column that matches the insulation. THHN, THWN-2 and XHHW-2 are in the 90°C column. THWN and THW are in the 75°C column. For example, 3 AWG copper is 85, 100 or 115 amps in the 60°C, 75°C and 90°C columns.
Step 2: Apply the temperature correction factor
If the ambient isn't 30°C (86°F), multiply by the Table 310.15(B)(1) factor from the same insulation column. Hotter means lower: at 96–104°F, 90°C wire takes 0.91 and 60°C wire takes 0.82.
Rooftops: in direct sun less than ⅞ inch above the roof, add 33°C (60°F) first. A 95°F day becomes 155°F, so 90°C wire gets 0.58. XHHW-2 is exempt.
Step 3: Adjust for more than three current-carrying conductors
Table 310.15(C)(1) cuts ampacity as the count rises: 80% for 4–6 conductors, down to 35% at 41 or more. The calculator picks the right percentage.
- Count: spares and each wire of a parallel set.
- Count the neutral: on a 3-wire circuit (two phases plus neutral) from a 4-wire, 3-phase wye, or a 4-wire wye circuit with mostly nonlinear loads.
- Don't count: a neutral carrying only unbalanced current, or grounding and bonding conductors.
- Length: raceways 24 inches or shorter get no adjustment. Cables run without spacing for more than 24 inches do.
Step 4: Check the terminal limit
Under 110.14(C), unless the equipment is listed and marked otherwise:
- 100 amps or less, or terminals marked for 14 AWG through 1 AWG: use the 60°C ampacity.
- Over 100 amps, or terminals marked for wire larger than 1 AWG: use the 75°C ampacity.
Ampacity derating worked example
Four current-carrying 3 AWG THHN copper conductors, 100°F ambient, terminals listed and marked 75°C.
- Table 310.16, 90°C column: 115 amps.
- Correction: 100°F is in the 96–104°F row, so 0.91.
- Adjustment: four conductors, so 80%.
- Multiply: 115 × 0.91 × 0.80 = 83.7 amps.
- Terminal check: the 75°C column allows 100 amps. 83.7 is lower, so the ampacity is 83.7 amps.
Small-wire check: six current-carrying 12 AWG THHN at 86°F is 30 × 0.80 = 24 amps. The 60°C terminal limit and 240.4(D) still cap it at 20 amps.
Derating questions stack two or three tables in one problem. JCR's self-study package drills them with video lessons and a 500+ question mock exam.
Common exam traps
- Counting every neutral. Check 310.15(E). Only some neutrals count.
- Stopping at the derated number. Compare it with the terminal column. For 14, 12 and 10 AWG, 240.4(D) applies after derating, and 90°C insulation doesn't raise it.
- Using Table 310.12 anyway. The 83% dwelling service table is off once correction or adjustment applies. See JCR's dwelling service load calculation.
For raceway sizing, see the conduit fill guide.
Ampacity adjustment and correction factor questions
What is the derating for 4 to 6 current-carrying conductors?
80%. Table 310.15(C)(1) drops it to 70% for 7–9 and 50% for 10–20 current-carrying conductors. Apply it on top of any temperature correction.
Does the neutral count as a current-carrying conductor?
Sometimes. A neutral carrying only unbalanced current doesn't count. One on a 3-wire circuit from a 4-wire, 3-phase wye does. So does one on a 4-wire wye circuit with mostly nonlinear load.
Does the ground wire count when derating?
No. Section 310.15(F) leaves grounding and bonding conductors out of the 310.15(C)(1) count. They do count in conduit fill, which is a separate calculation.
Can I derate from the 90°C column with THHN?
Yes. Sections 310.15(A) and 110.14(C) allow derating from the wire's own rating. The final number still can't exceed the terminal column, usually 60°C or 75°C.
What is the ampacity of 12 AWG THHN with nine current-carrying conductors?
At 86°F or below, 30 × 0.70 = 21 amps, but a 60°C terminal and 240.4(D) hold it to 20 amps. At 104°F, add the correction factor and it drops to 19.1 amps.
What is the ampacity of nine 10 AWG THHN conductors in a raceway?
At 86°F, 40 × 0.70 = 28 amps. The 60°C terminal column allows 30 amps, so 28 amps stands. Section 240.4(D) still limits the breaker to 30 amps.
Work derating problems with JCR
Run enough derating problems and the four steps become habit. JCR's $495 self-study package walks through Article 310 on video and gives you a mock exam to practice on. Pick the version for your state's exam below.
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Related guides: 200 amp service wire size (Table 310.12) · Conduit fill calculation · Voltage drop calculation · Motor breaker sizing (Table 430.52)
More worked examples: all NEC Answers.
References
- NFPA 70, National Electrical Code, 2020 edition
- Table 310.16 and 310.16 conditions of use
- 310.15(A), Ampacity tables, general
- 310.15(B)(1) and Table 310.15(B)(1), Ambient temperature correction
- 310.15(B)(2), Rooftop
- 310.15(C)(1) and Table 310.15(C)(1), Adjustment factors
- 310.15(E) and (F), Neutral and grounding conductors
- 110.14(C), Temperature limitations
- 240.4(D), Small conductors
- 310.12, Single-phase dwelling services and feeders
JCR checked each ampacity, factor and section number here against the 2020 NEC on October 21, 2026.