What ampacity is, and why one table is never the answer
Ampacity is the current a conductor can carry continuously without exceeding its temperature rating. NEC Table 310.16 gives that current for conductors in a raceway, cable or earth, at an ambient of 30 °C, with no more than three current-carrying conductors together. Almost no real installation matches all three of those conditions, which is why picking a size is a four-step process rather than a table lookup.
The physics is straightforward. Current through resistance produces heat; the insulation has a temperature it must not exceed; the conductor sheds heat to its surroundings. Anything that makes shedding harder — a hot attic, nine conductors bundled in one pipe, an insulated wall — reduces the current the conductor can carry, and that is what the correction and adjustment factors quantify.
Then a rule that has nothing to do with the conductor takes over. Under 110.14(C), the ampacity you are allowed to claim is limited by the temperature rating of the terminations at each end — the breaker lug, the disconnect, the equipment terminal. Most equipment is listed for 75 °C and a good deal of residential equipment is listed for 60 °C, so the 90 °C column of a THHN conductor is almost never the column you actually get to use for the final answer.
The four tests, in the order you apply them
1. Work out the required ampacity. Under 210.19 for branch circuits and 215.2 for feeders, the conductor must have an ampacity of at least 125% of the continuous load plus 100% of the non-continuous load. A continuous load is one expected to run for three hours or more, which covers most lighting, EV charging and electric heating. A 40 A continuous load therefore needs 50 A of conductor.
2. Correct for ambient temperature. Table 310.16 assumes 30 °C. Table 310.15(B)(1) lists the factor for other ambients, and 310.15(B)(2) gives the formula behind it: F = √((Tc − Ta) ÷ (Tc − 30)), where Tc is the conductor's temperature rating and Ta the new ambient. For a 90 °C conductor at 50 °C, that is √(40 ÷ 60) = 0.8165, which the table rounds to 0.82. Note that the conductor's own rating drives the correction, so 90 °C wire tolerates heat far better than 60 °C wire even when the terminations force you back to the 75 °C column afterwards.
3. Adjust for bundling. When more than three current-carrying conductors share a raceway or cable for more than 600 mm (24 in.), Table 310.15(C)(1) applies: 80% for four to six, 70% for seven to nine, 50% for ten to twenty, 45% for twenty-one to thirty, 40% for thirty-one to forty, and 35% beyond that. Equipment grounding conductors never count. A neutral counts only in the situations 310.15(E) describes, chiefly where it carries harmonic current from electronic loads or is the only return in a three-wire circuit from a four-wire system.
4. Apply the termination limit. Correction and adjustment are applied to the conductor's own column. The termination check is separate and uses the uncorrected Table 310.16 value in the termination temperature column. Both must be at least the required ampacity. On top of that, 240.4(D) caps the overcurrent device at 15 A for 14 AWG copper, 20 A for 12 AWG copper, 30 A for 10 AWG copper, 15 A for 12 AWG aluminium and 25 A for 10 AWG aluminium, whatever the table says.
Worked example: 100 A continuous in a 50 °C attic
You are running a 100 A continuous feeder in EMT through an attic that reaches 50 °C, using THHN copper on equipment listed for 75 °C terminations, with three current-carrying conductors in the raceway.
- Required ampacity. Continuous, so 100 × 1.25 = 125 A.
- Ambient correction. THHN is a 90 °C conductor, so F = √((90 − 50) ÷ (90 − 30)) = √(40 ÷ 60) = 0.8165.
- Adjustment. Three current-carrying conductors, so the factor is 1.00 and the combined derating is 0.8165.
- Try 1 AWG. Table 310.16, copper, 90 °C column: 145 A. Corrected: 145 × 0.8165 = 118.4 A. That is short of 125 A, so 1 AWG fails.
- Try 1/0 AWG. 90 °C column: 170 A. Corrected: 170 × 0.8165 = 138.8 A, which clears 125 A.
- Check the terminations. 1/0 copper in the 75 °C column is 150 A, which is at least the 125 A required, so 110.14(C) is satisfied. 240.4(D) does not apply above 10 AWG.
- Size the breaker. The governing ampacity is the smaller of 138.8 A and 150 A, so 138.8 A. The largest standard rating in 240.6(A) at or below that is 125 A, which also happens to equal the required ampacity, so a 125 A device is the answer.
Had you used 75 °C XHHW instead of THHN, the correction would have been √(25 ÷ 45) = 0.745 applied to the 75 °C column, giving 1/0 only 150 × 0.745 = 111.8 A and forcing you up to 2/0. That is the practical value of buying 90 °C wire even when the terminations are 75 °C.
How to read the answer
Look first at which rule governed. If the termination limit governed, buying a higher-temperature conductor will not help; you need either a larger conductor or equipment listed for a higher termination temperature. If the corrected ampacity governed, a 90 °C conductor, a cooler route, or fewer conductors in the raceway will each buy you headroom. If 240.4(D) governed, you are on a small conductor and no amount of derating arithmetic will let you put a bigger breaker on it.
Then check the breaker. The largest standard device at or below the governing ampacity is what protects the conductor. Where the conductor ampacity lands between two standard ratings, 240.4(B) lets you round up to the next standard size, provided the device does not exceed 800 A and the circuit does not supply receptacle outlets — a genuinely useful allowance that people forget. Motor circuits follow Article 430 instead and can carry short-circuit protection far above the conductor ampacity, which is why they get the motor overload and breaker sizing calculator.
Finally, check the length. Ampacity keeps the conductor from overheating; it says nothing about whether enough voltage arrives. On any run past about 100 ft, put the same circuit through the voltage drop calculator before you buy wire, because drop routinely upsizes conductors beyond what this page requires. When it does, 250.122(B) requires the equipment grounding conductor to grow in proportion.
NEC Table 310.16 ampacities at 30 °C, three conductors or fewer
| Size | Cu 60 °C | Cu 75 °C | Cu 90 °C | Al 60 °C | Al 75 °C | Al 90 °C |
|---|---|---|---|---|---|---|
| 14 AWG | 15 | 20 | 25 | — | — | — |
| 12 AWG | 20 | 25 | 30 | 15 | 20 | 25 |
| 10 AWG | 30 | 35 | 40 | 25 | 30 | 35 |
| 8 AWG | 40 | 50 | 55 | 30 | 40 | 45 |
| 6 AWG | 55 | 65 | 75 | 40 | 50 | 55 |
| 4 AWG | 70 | 85 | 95 | 55 | 65 | 75 |
| 3 AWG | 85 | 100 | 115 | 65 | 75 | 85 |
| 2 AWG | 95 | 115 | 130 | 75 | 90 | 100 |
| 1 AWG | 110 | 130 | 145 | 85 | 100 | 115 |
| 1/0 AWG | 125 | 150 | 170 | 100 | 120 | 135 |
| 2/0 AWG | 145 | 175 | 195 | 115 | 135 | 150 |
| 3/0 AWG | 165 | 200 | 225 | 130 | 155 | 175 |
| 4/0 AWG | 195 | 230 | 260 | 150 | 180 | 205 |
| 250 kcmil | 215 | 255 | 290 | 170 | 205 | 230 |
| 350 kcmil | 260 | 310 | 350 | 210 | 250 | 280 |
| 500 kcmil | 320 | 380 | 430 | 260 | 310 | 350 |
Aluminium is not listed at 14 AWG. Dwelling-unit services and feeders have a separate allowance in 310.12 that is not applied here.
The 90 °C column is a derating start point, not a rating you can use
NEC 110.14(C) is explicit: the conductor ampacity used for the connected equipment must not exceed the value in the column matching the lowest temperature rating of any terminal, device or conductor in the circuit. You may start from the 90 °C column when applying correction and adjustment factors, because those govern the conductor's own heating, but the final answer must still stand up in the 60 °C or 75 °C column. Absent a marking to the contrary, 110.14(C)(1) treats circuits rated 100 A or less, and terminations for 14 through 1 AWG conductors, as 60 °C.
Mistakes that produce an undersized conductor
- Using the 90 °C column as the final ampacity. It is a start point for derating only. The termination column decides what you may claim.
- Forgetting the 125% continuous factor. It applies to the conductor and the overcurrent device, and it catches EV chargers and lighting circuits constantly.
- Counting the ground as a current-carrying conductor. It never counts for 310.15(C)(1). Counting it makes the derating harsher than the code requires.
- Skipping ambient correction on a rooftop or attic run. A conduit exposed to sunlight on a roof can run far above the shade air temperature, and 310.15(B)(3) addresses that case directly.
- Ignoring 240.4(D). No amount of favourable derating lets you put a 30 A breaker on 12 AWG copper outside the specific allowances of 240.4(E) and 240.4(G).
What this calculator assumes
It assumes conductors in a raceway, cable or earth, which is the scope of Table 310.16, at up to 2,000 V. It does not use Table 310.17 for single conductors in free air, or Table 310.20 for messenger-supported wiring, both of which give higher ampacities.
It does not apply the dwelling-unit allowance of 310.12, which lets a 120/240 V single-phase dwelling service or main feeder be sized at 83% of the service rating; that shortcut has its own conditions and belongs in the dwelling load calculation. It does not handle motor circuits, where Article 430 replaces most of this arithmetic, and it does not handle welders, transformers or capacitor circuits, which have their own articles.
Finally, it treats the ambient as one number. Where a run passes through spaces at different temperatures, the code intent is to correct for the worst portion the conductor is exposed to. If part of the run is on a rooftop in sunlight, check 310.15(B)(3) before assuming the shade temperature applies, and if a portion of the run is a nipple no longer than 600 mm (24 in.), Chapter 9 Note 4 exempts that portion from the bundling adjustment — a detail the conduit fill calculator covers alongside the fill limits.
Where conductor sizing sits among the other checks
Sizing a circuit properly means running four separate calculations and taking the most demanding answer from each. Ampacity, on this page, keeps the conductor from cooking. Voltage drop keeps enough voltage at the load, and wins on long runs. Conduit fill decides whether the conductors you have chosen physically fit the raceway you have specified. And the derating calculation is worth running on its own when you are trying to work out how many circuits you can share one pipe before the adjustment factor forces everything up a size.
Two downstream consequences follow automatically. The equipment grounding conductor is sized from the overcurrent device by Table 250.122, and it must be enlarged in proportion whenever the ungrounded conductors are enlarged for any reason — that is the equipment grounding conductor calculator. And the overcurrent device itself has its own sizing rules for continuous loads, which the continuous load breaker sizing calculator works through in more detail than this page does.
Key terms
- Ampacity
- The maximum current a conductor can carry continuously under the conditions of use without exceeding its temperature rating. It is a property of the installation, not of the wire alone.
- Continuous load
- A load whose maximum current is expected to continue for three hours or more. Conductors and overcurrent devices are sized at 125% of it.
- Correction factor
- The multiplier applied for an ambient other than 30 °C, from Table 310.15(B)(1) or the 310.15(B)(2) formula.
- Adjustment factor
- The multiplier applied when more than three current-carrying conductors share a raceway or cable, from Table 310.15(C)(1).