Ampacity calculator (NEC)

How to use the ampacity calculator

Pick the NEC edition, the conductor metal and size, and the insulation printed on the jacket. Then say how the conductor is installed: in a raceway, cable or the ground (Table 310.16), or as single conductors in free air (Table 310.17). Enter the hottest ambient temperature the run passes through, the number of current-carrying conductors sharing the raceway or cable, and whether it crosses a roof in the sun. Finally choose the temperature rating of the terminals at the ends.

You get the table value, the factors, the corrected ampacity, the usable ampacity once the terminals have their say, and the largest breaker or fuse allowed on that conductor. The default answer shows why one size has three different numbers: 12 AWG THHN is 30 A in the 90 °C column, 25 A on 75 °C terminals, and may be protected at no more than 20 A.

What ampacity means

Ampacity is the current a conductor can carry continuously, under its conditions of use, without exceeding its temperature rating. The tables assume a 30 °C (86 °F) ambient and no more than three current-carrying conductors together. Anything hotter or more crowded lowers the figure, because the conductor sheds heat into a warmer or more crowded space:

The insulation decides the column. TW and UF are 60 °C types; THW, THWN and wet-location XHHW are 75 °C; THHN, THWN-2, XHHW-2 and RHW-2 are 90 °C. Dual-rated THHN/THWN is a 90 °C conductor in dry locations and a 75 °C conductor in wet ones, and the wet rating governs outdoors and underground.

Copper, aluminum and copper-clad aluminum

Aluminum conducts about 61 % as well as copper, so a given size carries less current. Across Table 310.16 the 75 °C aluminum value runs from 75 % to 86 % of the copper value for the same size, which is why aluminum feeders are usually one or two sizes larger: a 200 A dwelling service needs 2/0 AWG copper or 4/0 AWG aluminum. Copper-clad aluminum shares the aluminum column. Aluminum and copper-clad aluminum start at 12 AWG for building wire; the 2026 edition adds 14 AWG copper-clad aluminum only. Whatever the metal, the lugs must be listed for it (marked AL/CU for aluminum), and the connections need the torque and, where specified, the compound that the listing calls for.

Terminals usually decide, not the insulation

A 90 °C conductor bolted to a 75 °C lug would cook the lug, so 110.14(C)(1) limits the conductor to the terminal’s column. For equipment on circuits of 100 A or less, or with lugs for 14 to 1 AWG, the default is 60 °C unless the equipment is listed and marked for 75 °C; above 100 A, the default is 75 °C. The 90 °C value is still useful: it is the number you correct and adjust, and the result is then capped at the terminal column.

That is why 10 AWG THHN in a hot, full raceway still works. Six current-carrying conductors at 104 °F give 40 A × 0.91 × 0.80 = 29.12 A, below the 75 °C value of 35 A, so the corrected figure governs, and 240.4(B) lets a 30 A breaker protect it.

Ambient temperature

The correction factors fall faster for lower-rated insulation, and a 60 °C conductor may not be used above 55 °C at all. The Code also allows the factor to be calculated: for a 90 °C conductor at 40 °C the equation gives 0.913, close to the tabulated 0.91. Where a temperature falls between two bands, this calculator uses the warmer band. Here is what heat does to common copper THHN sizes:

Attics, boiler rooms and rooftop runs are where these numbers bite. On a roof, a raceway or cable in direct sunlight that sits less than 0.75 in. above the surface (0.875 in. in 2017 and 2020) adds 33 °C (60 °F) to the outdoor temperature. At 95 °F and 0.8 in. above the roof, 6 AWG THHN keeps 65 A under the 2023 rule but drops to 43.5 A under 2020’s. XHHW-2 is exempt from the adder in every edition.

More than three current-carrying conductors

Put more than three current-carrying conductors in one raceway or cable, or bundle cables for more than 24 in., and each one runs hotter. The adjustment factor drops to 80 % at four to six conductors, 70 % at seven to nine and 50 % from ten to twenty. Count spares. Leave out the equipment grounding conductor, and a neutral that carries only the unbalanced current of its own circuit. Count the neutral of a three-wire circuit (two phases and a neutral) taken from a 4-wire wye system, and a wye neutral that mostly feeds nonlinear loads such as LED drivers and computers:

NM cable starts from its 90 °C conductor rating for these factors but may never exceed its 60 °C ampacity, which is why 12 AWG NM is 20 A on its own and 15 A when twelve current-carrying conductors are bundled through the same bored joists.

Small conductors: 240.4(D)

For 14, 12 and 10 AWG the breaker limit is not the ampacity. 240.4(D) caps the overcurrent device at 15 A for 14 AWG copper, 20 A for 12 AWG copper and 30 A for 10 AWG copper, and at 15 A and 25 A for 12 and 10 AWG aluminum or copper-clad aluminum, after any correction and adjustment. 18 AWG and 16 AWG copper are limited to 7 A and 10 A. The 2023 edition added 14 AWG copper-clad aluminum at 10 A (8 A continuous). Motor, air-conditioning and a few other circuits have their own rules that permit more.

What changed in 2026

Every value from 14 AWG to 2000 kcmil in Table 310.16 is the same in 2017, 2020, 2023 and 2026, and Table 310.17 did not change. The 2026 edition does three things at the small end. It adds 16 AWG copper at 10 A in the 60 °C column, for the new 10 A lighting branch circuits, and 14 AWG copper-clad aluminum at 10 A. It deletes the 18 AWG row and moves the 18 and 16 AWG 90 °C values to notes that apply only to Types MI and MTW. And the minimum conductor size becomes 16 AWG copper, 14 AWG copper-clad aluminum and 12 AWG aluminum (310.5(A), renumbered from 310.3(A)).

We have these changes from one source, NFPA’s 2026 Second Draft working text, and have not yet checked them against the printed book. The calculator computes with them for NEC 2026 and attaches a warning to every result that depends on them.

Worked examples

Common mistakes

• Quoting the 90 °C column as the breaker size. It is the starting point for derating, not the answer.
• Using a wet-location conductor at its dry rating. Conduit outdoors and underground is a wet location.
• Forgetting the factors compound. Heat and bundling multiply; 0.82 × 0.70 leaves only 57 % of the table value.
• Counting the equipment grounding conductor. It carries no current in normal operation and is never counted.
• Using Table 310.17 for conductors in a tray or bundle. Free air means spaced single conductors.
• Sizing a circuit from ampacity alone. The 125 % rule for continuous loads and voltage drop come on top; the wire size calculator runs all the checks together.

Method and verification

The values come from our research copy of Tables 310.16 and 310.17, the 30 °C correction table and the adjustment table, each checked cell by cell against at least two independent copies, with every edition’s changes from 2017 to 2026 applied. Between printed temperature bands the warmer band is used. The tests cover hand-worked cases such as the 10 AWG example above, the Annex D Example D7 conductors, the rooftop threshold change, NM cable and the 2026 additions, plus properties such as “a larger conductor never has less ampacity” and “a hotter location never has more”. For the physical data behind each size, see the wire size chart.

Updated 2026-10-14. Found a problem? Report it; every fix is logged publicly.