Ampacity Derating Calculator
Apply NEC 310.15(B) ambient temperature correction and 310.15(C)(1) bundling adjustment to a Table 310.16 base ampacity. Free, no account required.
Conductor & Conditions
Neutral counts only when it carries current (310.15(E)). Grounds do not count.
40.0 A derated
Adjusted and corrected ampacity for the conditions of use.
NEC 310.15 Reference
- 310.15(B): Multiply by the ambient temperature correction factor for the conductor's temperature column.
- 310.15(C)(1): When 4 or more current-carrying conductors share a raceway or cable, apply the adjustment factor (80% / 70% / 50% / 45% / 40% / 35%).
- Start at the 90°C column for derating when the conductor is rated 90°C, even if the final answer must be limited at the termination.
Don't forget the termination limit
Per NEC 110.14(C), the final circuit ampacity is also capped by the temperature rating of the terminations (usually 60°C up to 100 A, 75°C above). Derate from the 90°C column, then compare to the 75°C termination value and use the lower.
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This calculator uses NEC 2023 Table 310.16 with the ambient and adjustment factors of 310.15. Always verify with your local AHJ and the NEC edition adopted in your jurisdiction.
How the ampacity derating calculation works
Derating is the two-factor correction applied to a Table 310.16 base ampacity. The code separates them deliberately: 310.15(B)(1) handles ambient temperature, because a hotter environment leaves the conductor less headroom before it hits its insulation rating, and 310.15(C)(1) handles bundling, because conductors packed together cannot shed heat into open air.
Both factors are multipliers against the base ampacity of the conductor's own temperature column, and when both conditions exist they multiply together rather than being applied in the alternative. A 90 C conductor at 46 C ambient with seven conductors in the raceway takes both hits.
Counting current-carrying conductors is where candidates lose points. Equipment grounding conductors and bonding jumpers never count. A grounded neutral counts only when it actually carries unbalanced current, which 310.15(E) spells out: in a three-wire circuit from a single-phase three-wire system, or a four-wire three-phase wye where the load is majority nonlinear, the neutral does count.
Step by step
- 1
Get the base ampacity
Table 310.16, at the conductor's own insulation temperature column.
- 2
Find the ambient factor
Table 310.15(B)(1), indexed by ambient range and by the conductor's temperature rating.
- 3
Count current-carrying conductors
Apply 310.15(E). Exclude EGCs. Include neutrals only where they carry unbalanced or harmonic current.
- 4
Find the adjustment factor
Table 310.15(C)(1): 4 to 6 is 80 percent, 7 to 9 is 70 percent, 10 to 20 is 50 percent, 21 to 30 is 45 percent, 31 to 40 is 40 percent, 41 and above is 35 percent.
- 5
Multiply through
Derated ampacity = base x ambient factor x adjustment factor.
- 6
Check terminations
110.14(C) still caps the result at the 60 or 75 C column value.
Worked example
Nine current-carrying 10 AWG THHN copper conductors in one raceway, ambient 38 degrees C. What is the derated ampacity?
- Table 310.16, 90 C copper, 10 AWG = 40 A.
- Table 310.15(B)(1), 36 to 40 C at 90 C rating = 0.91.
- Table 310.15(C)(1), 7 to 9 conductors = 70 percent.
- 40 x 0.91 x 0.70 = 25.5 A.
Answer: 25.5 A. Note that 240.4(D) still limits 10 AWG copper to a 30 A overcurrent device, and here the derated ampacity of 25.5 A is the binding constraint, so a 25 A device is the appropriate protection.
How this is tested on the exam
Derating problems are designed to see whether you can count conductors correctly and whether you know the two factors stack. Expect at least one question where an equipment grounding conductor is included in the count to bait you into the wrong adjustment factor, and one where the ambient is given in Fahrenheit so you have to convert before entering the table.
Frequently asked questions
Does the equipment grounding conductor count toward the conductor count?
No. Grounding and bonding conductors are not current-carrying under normal operation and are excluded from the count in 310.15(C)(1).
Does the neutral count?
Only sometimes. Per 310.15(E), a neutral carrying only the unbalanced current from other conductors is not counted. It does count in a three-wire circuit from a single-phase three-wire system, and in a four-wire three-phase wye circuit where more than half the load is nonlinear.
Does derating apply to conduit nipples?
No. 310.15(C)(1) applies where conductors are bundled or in a raceway longer than 24 inches. A nipple of 24 inches or less is exempt from the adjustment factors, though ambient correction still applies.
Do I derate from the 90 C column even on 75 C terminations?
Yes. Apply the factors to the conductor's own insulation rating, then compare the result against the termination limit from 110.14(C) and use the lower value. Deriving from 90 C and finishing at 75 C is the standard sequence.