Why ampacity has to be derated at all
A conductor's ampacity is not a property of the copper. It is the current at which the conductor reaches its insulation's temperature limit, and that depends on how fast heat escapes as much as on how fast heat is produced. NEC Table 310.16 fixes both sides of that balance: it assumes a 30 °C (86 °F) ambient and no more than three current-carrying conductors in the raceway or cable. Change either assumption and the table number is no longer the right answer.
Raise the ambient and the conductor starts closer to its limit, so less self-heating is available before it gets there. Add conductors to the raceway and each one heats the others, so the same current produces a higher rise. The NEC handles the first with a temperature correction factor and the second with an adjustment factor, and it requires you to apply both, multiplying them together against the table value.
The two factors come from different places. The correction factor is derived, not tabulated: NEC 310.15(B) gives the equation √((Tc − Ta)/(Tc − 30)), which is the square root because conductor temperature rise is proportional to I²R. Table 310.15(B)(1) is that equation evaluated at each ambient band and rounded to two decimals, which is why this calculator reproduces the table almost exactly while giving you the unrounded value. The adjustment factors in Table 310.15(C)(1) are empirical, derived from the heat-transfer work behind the code rather than from a closed-form equation.
None of this touches voltage drop, which is a separate and often more binding constraint on long runs — the voltage drop calculator handles that. Nor does it touch conduit fill, which limits how many conductors physically fit; see the conduit fill calculator for the cross-sectional area rules of Chapter 9.
The two factors, and the termination limit that often overrides both
Temperature correction. Take the conductor's insulation rating Tc, subtract the actual ambient, divide by Tc minus 30, and take the square root. A 90 °C conductor at 40 °C gives √(50/60) = 0.9129, so it keeps 91% of its table ampacity. The same conductor at 55 °C gives √(35/60) = 0.7638. Notice that a 90 °C insulation loses ampacity far more slowly than a 60 °C one as the ambient rises: at 45 °C the 90 °C conductor keeps 86.6% while the 60 °C conductor keeps only √(15/30) = 70.7%. That is the whole argument for 90 °C-rated wire in hot locations.
Bundling adjustment. Count the conductors that actually carry current. Four to six take 80%, seven to nine take 70%, ten to twenty take 50%, twenty-one to thirty take 45%, thirty-one to forty take 40%, and forty-one or more take 35%. Equipment grounding conductors are never counted. A neutral that carries only the unbalanced current of a 3-wire single-phase or 4-wire three-phase wye circuit is not counted either, but a neutral carrying harmonic current on a system supplying nonlinear loads is, and so is the neutral of a 3-wire circuit derived from a three-phase wye system.
The termination limit. NEC 110.14(C) says that a circuit's ampacity may not exceed the lowest temperature rating of any connected termination. Most equipment rated 100 A or less is listed for 60 °C conductors, and most equipment above that for 75 °C, though listings vary and the equipment marking governs. You are still permitted to derate from the 90 °C column — that is precisely why 90 °C insulation is worth buying — but the final answer is clamped at the 60 or 75 °C table value for that conductor size. Enter that clamp in the termination field and this calculator applies it.
Apply the continuous-load rule last. NEC 210.19(A) and 215.2(A) require the conductor to be sized for 125% of any load that runs three hours or more. That multiplier is a property of the load, not of the conductor's heat balance, so it never interacts with the correction and adjustment factors — it changes what you compare the result against.
Worked example: 3/0 THHN copper, six conductors, 40 °C attic
A 150 A feeder is run in 3/0 THHN copper. The raceway carries two three-wire circuits, so six current-carrying conductors, through an attic space that reaches 40 °C (104 °F). The equipment at both ends is listed for 75 °C terminations.
- Base ampacity. From NEC Table 310.16, 3/0 copper in the 90 °C column is 225 A. THHN is 90 °C rated in dry locations, so this column is available for derating.
- Temperature correction. Tc = 90, Ta = 40. F_temp = √((90 − 40)/(90 − 30)) = √(50/60) = √0.833333 = 0.912871. Table 310.15(B)(1) rounds this to 0.91.
- Adjustment. Six current-carrying conductors falls in the 4–6 band, so F_adj = 0.80.
- Multiply. 225 × 0.912871 × 0.80 = 205.396 × 0.80 = 164.32 A.
- Termination limit. 3/0 copper in the 75 °C column of Table 310.16 is 200 A. The derated 164.32 A is below that, so 110.14(C) does not bind and the circuit ampacity stays 164.32 A.
- Compare with the load. 164.32 A available against 150 A required leaves 14.32 A of margin, so the conductor passes.
Change one thing and the answer flips. Make the load continuous and the required ampacity becomes 150 × 1.25 = 187.5 A, which exceeds the 164.32 A available, so the feeder fails. Move up to 4/0, which is 260 A in the 90 °C column: 260 × 0.912871 = 237.35, and 237.35 × 0.80 = 189.88 A. The 4/0 termination value in the 75 °C column is 230 A, so the clamp does not bind. 189.88 A against 187.5 A required passes, with 2.38 A to spare.
Change a different thing: use 75 °C THW instead of 90 °C THHN for the original 3/0. The base becomes 200 A and the correction becomes √((75 − 40)/(75 − 30)) = √(35/45) = 0.881917, so 200 × 0.881917 × 0.80 = 141.11 A — below the 150 A load. The 90 °C insulation bought 23.2 A on the same copper.
How to read the result and what to change when it fails
Look at which constraint is binding before you reach for larger wire. If the termination limit is the number the calculator lands on, extra derating headroom is worthless and the only fixes are a larger conductor or equipment listed for a higher termination temperature. If the adjustment factor is doing the damage, splitting the circuits into two raceways is usually cheaper than upsizing every conductor — dropping from seven conductors to two raceways of three or four moves you from 70% to 100% or 80%.
If the correction factor is the problem, look at the ambient you entered. The code asks for the ambient the raceway actually sees, and only the hottest section matters, because ampacity is set by the worst point along the run. A conduit that spends five feet crossing a boiler room is rated for the boiler room. Conversely, do not use an optimistic attic figure: measured summer attic temperatures well above 50 °C are common in hot climates, and a conductor sized for 30 °C in that space is not compliant.
Read the spare ampacity against the load you will actually have, not the load on the drawing. A feeder with 2 A of margin is compliant and fragile: one added appliance, one recount of current-carrying conductors during inspection, or one revision to the continuous classification puts it over. Where the margin is thin, the calculator's ambient sweep table shows exactly how much ambient you can absorb before the conductor fails.
Finally, remember what this calculation does not decide. Overcurrent protection is a separate exercise — 240.4(D) caps small conductors regardless of ampacity, and 240.4(B) allows rounding up to the next standard device size in defined circumstances. The continuous load breaker sizing calculator covers the device side, and wire size and ampacity gets you the base table value this page starts from.
NEC Table 310.16 ampacities for copper conductors
| Size | 60 °C (TW, UF) | 75 °C (THW, THWN) | 90 °C (THHN, XHHW-2) |
|---|---|---|---|
| 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 |
| 350 kcmil | 260 | 310 | 350 |
| 500 kcmil | 320 | 380 | 430 |
Use the column that matches the conductor's insulation for derating, and the 60 or 75 °C column for the termination check. NEC 240.4(D) separately limits overcurrent protection on 14, 12 and 10 AWG copper to 15, 20 and 30 A regardless of these ampacities. Always confirm against the code book edition your jurisdiction has adopted.
Adjustment factors for more than three current-carrying conductors
| Number of current-carrying conductors | Percent of table value | Factor |
|---|---|---|
| 1–3 | 100% | 1.00 |
| 4–6 | 80% | 0.80 |
| 7–9 | 70% | 0.70 |
| 10–20 | 50% | 0.50 |
| 21–30 | 45% | 0.45 |
| 31–40 | 40% | 0.40 |
| 41 and above | 35% | 0.35 |
The jump from three to four conductors costs 20% in one step, which is why keeping a raceway to three current-carrying conductors is worth designing for.
Where derating calculations go wrong
- Counting the equipment grounding conductor. It carries no current in normal operation and is never included in the conductor count. Counting it can push a six-conductor raceway into the 70% band and cost you a wire size for nothing.
- Getting the neutral wrong in both directions. A neutral carrying only unbalanced current in a 3-wire single-phase or 4-wire wye circuit is not counted. A neutral on a system with substantial nonlinear load, where triplen harmonics add rather than cancel, is counted. So is the neutral of a 3-wire circuit taken from a three-phase wye source.
- Using the average ambient instead of the maximum. Ampacity is limited by the hottest point on the run. A conduit that passes briefly through a mechanical room is derated for that room.
- Forgetting that the 90 °C column is only for derating. You may start there, but 110.14(C) still clamps the finished number to the equipment's termination rating. A 12 AWG THHN conductor is a 30 A conductor for derating purposes and a 25 A conductor at a 75 °C termination.
- Applying the 125% continuous multiplier to the derating factors. It applies to the load, changing what the conductor must supply. Multiplying the ampacity by 0.8 to account for continuity double-counts the same rule.
- Ignoring the small-conductor rule. NEC 240.4(D) limits overcurrent protection on 14, 12 and 10 AWG copper to 15, 20 and 30 A whatever the derated ampacity says, with limited exceptions.
- Assuming a raceway is one thermal environment. Nipples shorter than 24 inches are exempt from the adjustment factors under 310.15(C)(1), and cables in free air follow different tables altogether.
How this fits into a complete conductor sizing
Sizing a conductor properly means satisfying four constraints, and derating is only one. First, ampacity after correction and adjustment must meet the load, which is what this page computes. Second, the termination temperature limit of 110.14(C) must be met. Third, overcurrent protection must coordinate with the conductor under 240.4, including the small-conductor limits of 240.4(D) and the next-size-up allowance of 240.4(B). Fourth, voltage drop on long runs frequently demands a larger conductor than any of the first three — the NEC treats this as a recommendation in informational notes rather than a requirement, but the equipment it feeds does not care about that distinction.
Two situations deserve their own treatment. Motor circuits follow Article 430, where the conductor is sized at 125% of the table full-load current rather than the nameplate current, and the branch-circuit protection is sized for starting inrush rather than for the conductor — the motor overload and breaker sizing calculator covers that path. Photovoltaic circuits follow Article 690, with their own 125% multipliers and, on rooftops, ambient temperatures that make the correction factor the dominant term.
Where a raceway carries many circuits, the arithmetic pushes hard towards a design change rather than a wire size. Twenty current-carrying conductors take a 50% adjustment, so every conductor in the pipe is worth half its table ampacity. Two pipes of ten conductors each are also at 50%; four pipes of five are at 80%. The break points in Table 310.15(C)(1) are steep enough that raceway layout is a real cost decision, not a detail.
Finally, keep the paperwork. Inspectors ask which ambient you assumed, how you counted the conductors, and which termination rating you used, because those three assumptions are the only places the number can move. Recording them alongside the calculation is what makes it reviewable a year later.