NEC Wire Ampacity Calculator
Copper conductor ampacity from NEC Table 310.16 with temperature correction and conductor count derating. Free. No account required.
Wire & Conditions
Auto-selects temperature column: 90°C / 194°F
No derating (1–3 conductors)
Ampacity Result
NEC Table 310.16, Copper, 90°C / 194°F column
Adjusted Ampacity
30.0A
Step-by-step calculation
NEC Table 310.16, Quick Reference (Copper, 60/75/90°C)
| Size | 60°C | 75°C | 90°C |
|---|---|---|---|
| 14 AWG | 15* | 20 | 25 |
| 12 AWG | 20* | 25 | 30 |
| 10 AWG | 30 | 35 | 40 |
| 8 AWG | 40 | 50 | 55 |
| 6 AWG | 55 | 65 | 75 |
| 4 AWG | 70 | 85 | 95 |
| 3 AWG | 85 | 100 | 115 |
| 2 AWG | 95 | 115 | 130 |
| 1 AWG | 110 | 130 | 145 |
| 1/0 AWG | 125 | 150 | 170 |
| 2/0 AWG | 145 | 175 | 195 |
| 3/0 AWG | 165 | 200 | 225 |
| 4/0 AWG | 195 | 230 | 260 |
| 250 kcmil | 215 | 255 | 290 |
| 300 kcmil | 240 | 285 | 320 |
| 350 kcmil | 260 | 310 | 350 |
| 400 kcmil | 280 | 335 | 380 |
| 500 kcmil | 320 | 380 | 430 |
| 600 kcmil | 350 | 420 | 475 |
| 700 kcmil | 385 | 460 | 520 |
| 750 kcmil | 400 | 475 | 535 |
| 800 kcmil | 410 | 490 | 555 |
| 900 kcmil | 435 | 520 | 585 |
| 1000 kcmil | 455 | 545 | 615 |
* Per NEC 240.4(D), overcurrent protection limited to 15A for 14 AWG, 20A for 12 AWG, 30A for 10 AWG.
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This calculator references NEC Table 310.16 values. Study page held for review. Always verify with your local Authority Having Jurisdiction (AHJ) and the current NEC edition adopted in your jurisdiction. Values shown are for copper conductors only.
How the wire ampacity calculation works
Ampacity is the current a conductor can carry continuously without exceeding its temperature rating. Table 310.16 gives the base value for insulated conductors rated up to 2,000 V, in raceway, cable, or direct buried, with not more than three current-carrying conductors, at an ambient of 30 degrees C.
Those three conditions in the table heading are the whole game. Break any of them and the base number has to be corrected. Above or below 30 C ambient you apply the correction factors in 310.15(B)(1). With more than three current-carrying conductors bundled or in the same raceway for more than 24 inches you apply the adjustment factors in 310.15(C)(1). When both apply you multiply them together.
Then 110.14(C) caps everything. Even if the conductor's 90 C column allows more, the terminations set the ceiling: 60 C for equipment rated 100 A or less unless it is listed otherwise, 75 C for larger equipment. Apply correction and adjustment factors using the applicable conductor temperature rating, then compare the corrected result against the applicable termination limit and take the lower value.
Step by step
- 1
Pick the base ampacity
Table 310.16, correct column for the insulation temperature rating (60, 75, or 90 C) and the right material.
- 2
Apply ambient correction
Table 310.15(B)(1) if the ambient is anything other than 30 C.
- 3
Apply bundling adjustment
Table 310.15(C)(1) if more than three current-carrying conductors run together more than 24 inches. Neutrals count only when they carry unbalanced current, 310.15(E).
- 4
Multiply the factors
Corrected ampacity = base x ambient factor x adjustment factor.
- 5
Apply the termination limit
110.14(C). Compare against the 60 C or 75 C column value for the same size and take the lower of the two.
- 6
Size the overcurrent device
240.4 and 240.4(B), with the small-conductor rules in 240.4(D) for 14, 12, and 10 AWG.
Worked example
Six current-carrying 8 AWG THHN copper conductors share a raceway in a 45 degree C ambient. Equipment is rated 75 C. What is the usable ampacity?
- Table 310.16, 90 C copper column, 8 AWG = 55 A.
- Table 310.15(B)(1), 41 to 45 C ambient at 90 C rating = 0.87.
- Table 310.15(C)(1), 4 to 6 conductors = 80 percent.
- 55 x 0.87 x 0.80 = 38.3 A.
- 110.14(C) check: 75 C column for 8 AWG copper = 50 A. The corrected 38.3 A is lower, so it governs.
Answer: 38.3 A. Per 240.4(B) the next standard overcurrent device above the ampacity is permitted here, which is 40 A, provided the conductors do not supply receptacle outlets and 40 A is not above the next standard rating.
How this is tested on the exam
This calculation combines Table 310.16, correction or adjustment factors, and 110.14(C). Use the applicable NEC provisions and official exam outline to determine whether it is relevant to your preparation.
Frequently asked questions
Which temperature column do I start in?
Start in the column matching the conductor's insulation rating, which for THHN and XHHW-2 is 90 C, and apply your ambient and bundling factors there. Then apply 110.14(C) and drop to the 60 or 75 C column value if that is lower. You do not get to use the 90 C ampacity as the final answer just because the wire is rated 90 C.
When do more than three conductors trigger derating?
310.15(C)(1) applies when more than three current-carrying conductors are bundled together or in a raceway for more than 24 inches. Under 24 inches the adjustment does not apply. Grounding conductors never count, and a neutral counts only if it carries unbalanced current, per 310.15(E).
Do ambient correction and bundling adjustment stack?
Yes. When both conditions exist you multiply both factors against the base ampacity. They are not alternatives and you do not use only the worse of the two.
Why is 12 AWG copper limited to a 20 A breaker if the table says 25 or 30?
240.4(D) sets small-conductor overcurrent limits regardless of what the ampacity table allows: 15 A for 14 AWG, 20 A for 12 AWG, 30 A for 10 AWG copper. The higher table values matter for derating headroom, not for the breaker.
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