Preliminary numbers for estimating and for checking a design or a quote. The equipment label and the local authority having jurisdiction come first. A licensed electrician sizes and installs the circuit.

Maintained by BuildSolver · Updated October 23, 2026

Calculator · Electrical

Wire ampacity derating calculator

A 12 AWG copper THHN wire is rated 30 A at 90°C (194°F), but that is a table value. In a raceway with four current-carrying conductors it is 30 A x 0.80 = 24 A, and on 75°C (167°F) terminals it cannot go above 25 A anyway. This page takes the wire and the conditions and gives the ampacity after the NEC corrections and the terminal limit, and the largest breaker the wire allows: 20 A for this one, because 240.4(D) caps 12 AWG copper. It is the reverse of the wire size calculator, which finds the wire for a load.

Pick the size, copper or aluminum, and the insulation printed on the jacket, or NM-B for Romex-type cable. Pick the terminal rating of the breaker and the equipment.
Enter the air temperature around the wire, say whether the raceway or cable sits in sunlight less than 7/8 in. above a roof, the number of current-carrying conductors, and whether the raceway is longer than 24 in.
The result shows the corrected ampacity, the terminal limit, the usable ampacity and the largest breaker the wire allows. Add a load to check that the wire and the breaker cover it.

One copper or aluminum conductor, 14 AWG to 4/0, in Table 310.16. The count of current-carrying conductors and the terminal rating are your entries. A licensed electrician installs the circuit.

Wire size

14 AWG to 4/0. Aluminum starts at 12 AWG, because Table 310.16 has no 14 AWG aluminum row.

Conductor material

Conductor insulation rating

Read it off the wire jacket. Outdoors, on a roof and in conduit outdoors are wet locations, the way the site's AC wire page treats conduit outdoors. There, wire dual-marked THHN/THWN is 75°C (167°F), wire marked THHN alone is not permitted, and only wire marked THWN-2 or XHHW-2 is 90°C (194°F). Ascent's specification marks 14 to 10 AWG as THWN, not THWN-2, and 8 AWG and larger as THWN-2.

Terminal rating at both ends

The lower rating of the breaker lugs and the equipment lugs. On a circuit of 100 A or less, or for 14 to 1 AWG, terminals count as 60°C (140°F) unless the equipment is marked and listed for 75°C (167°F); above 100 A, or for conductors larger than 1 AWG, they count as 75°C (167°F) (110.14(C)(1)).

Ambient temperature around the conductors, °F

86°F or lower takes no correction here, and cool air earns no credit. A hot attic takes one. For a roof, enter the outdoor air temperature and answer the question below.

Raceway or cable in sunlight on a roof

A raceway or cable in sunlight less than 7/8 in. above a roof adds 60°F to the outdoor temperature (310.15(B)(2)). XHHW-2 conductors are not subject to the adder.

Current-carrying conductors in the raceway or cable

Two for a 240 V circuit with no neutral. Four or more are derated. An equipment grounding conductor is not counted. What counts for a neutral is in 310.15(E), which this page does not work.

Raceway or cable length

The conductor-count table does not apply to a nipple 24 in. or shorter.

Load current, A (optional)

The current of the load itself, to check a circuit. Do not add 125 percent: the question below does that where the NEC asks for it. Leave blank to see the ampacity only.

Continuous load

A continuous load is counted at 125 percent against the terminal column and the breaker. The conductor itself, after the corrections, only has to carry the load.

Example result for the default inputs

12 AWG copper, 90°C (194°F) insulation, 75°C (167°F) terminals, 86°F, 4 current-carrying conductors.

Usable ampacity

24 A

Largest breaker

20 A

Set by

Corrections

Calculation details
Table 310.16 value	12 AWG copper: 30 A in the 90°C (194°F) column.
Ambient correction, Table 310.15(B)(1)(1)	1.00 (30°C (86°F) or lower, no correction)
Conductor count, Table 310.15(C)(1)	0.80 (4 to 6 current-carrying conductors)
Ampacity after the corrections	30 A x 1.00 x 0.80 = 24 A
Terminal and cable limit	25 A in the 75°C (167°F) column. Terminals marked and listed for 75°C (167°F) conductors (110.14(C)).
Usable ampacity	24 A, the lower of 24 A and 25 A. The corrections set it: they bring the ampacity below the terminal column.
Largest breaker, with 240.4(B)	20 A. NEC 240.4(D) caps a breaker on 12 AWG copper at 20 A, even though 24 A of ampacity would allow 25 A under 240.4(B).
Largest breaker, if 240.4(B) does not apply	20 A, the largest standard size at or below 24 A. 240.4(B) is not available for the conductors of a multioutlet branch circuit that supplies receptacles for cord-and-plug connected portable loads, so use this figure there.

For estimation purposes only. Consult a licensed engineer for final designs.

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How the ampacity and the breaker are worked

Start from the Table 310.16 value in the conductor's insulation column, multiply it by the ambient-temperature factor and by the conductor-count factor, and hold the result to the table value in the column the terminals allow. Whichever of the two is lower is the usable ampacity. A breaker on it may be no larger than the next standard size above that ampacity, and no larger than the 240.4(D) limit of a small conductor.

Table value: I table = Table 310.16, insulation column12 AWG copper is 20, 25 and 30 A in the 60°C (140°F), 75°C (167°F) and 90°C (194°F) columns. THHN and THWN-2 take the 90°C (194°F) column. NM-B cable also starts from the 90°C (194°F) value for the corrections (334.80), then is held to 60°C (140°F).
Ambient factor: f amb = Table 310.15(B)(1)(1), at outdoor °F (+60°F on a roof)Table 310.15(B)(1)(1) is read in the insulation column. 86°F (30°C) or less is 1.00. 104°F (40°C) is 0.91 and 122°F (50°C) is 0.82 for 90°C (194°F) wire. A raceway or cable in sunlight less than 7/8 in. above a roof adds 60°F (310.15(B)(2)): 6 AWG THWN-2 at 90°F becomes 150°F and the factor 0.58.
Conductor-count factor: f adj = Table 310.15(C)(1)Three or fewer conductors take 1.00, 4 to 6 take 0.80, 7 to 9 take 0.70, 10 to 20 take 0.50, 21 to 30 take 0.45, 31 to 40 take 0.40 and 41 or more take 0.35. The factor applies to a raceway longer than 24 in., not to a nipple of 24 in. or less.
Corrected ampacity: I corrected = I table x f amb x f adjIn Holt's example a 12 THWN-2 wire is 30 A at 90°C (194°F) with four conductors, so 30 A x 0.80 = 24 A. With eight conductors it is 30 A x 0.70 = 21 A.
Usable ampacity: I usable = the lower of I corrected and the terminal-column valueThe terminal column is the lower of the insulation and terminal ratings, and 60°C (140°F) for NM-B. 6 AWG THHN in three conductors at 86°F corrects to 75 A, but 75°C (167°F) terminals allow 65 A, so 65 A is usable. On 60°C (140°F) terminals it is 55 A.
Largest breaker: next standard size above I usable (240.4(B)), no more than the 240.4(D) limit8 AWG THHN in four conductors is 44 A. That falls between the 40 A and 45 A standard sizes, and 240.4(B) lets the next one, 45 A, protect it. Where 240.4(B) does not apply, the largest standard size at or below is 40 A. 240.4(D) caps 14 AWG copper at 15 A, 12 AWG at 20 A and 10 AWG at 30 A, so 12 AWG at 24 A takes 20 A.

Eight cases through the same formula

Rows 1, 3, 7 and 8 are worked examples from Mike Holt's EC&M articles: 30 A x 0.80 = 24 A for four conductors, 21 A for eight, 44 A for 8 AWG, and 43.5 A for 6 AWG THWN-2 on a roof at 90°F (150°F with the adder). The other 12 AWG rows show how conductor count and heat move the same wire. All rows are copper THWN-2 or THHN (90°C (194°F)) on 75°C (167°F) terminals.

Table value, factors, corrected ampacity, usable ampacity and largest breaker for eight copper wires
Case	Table 90°C (194°F), A	Ambient factor	Count factor	Corrected, A	Usable, A	Largest breaker, A
12 AWG, 4 conductors (Holt 2021)	30	1.00	0.80	24.00	24.00	20
12 AWG, 3 conductors	30	1.00	1.00	30.00	25.00	20
12 AWG, 8 conductors (Holt 2021)	30	1.00	0.70	21.00	21.00	20
12 AWG, 10 conductors	30	1.00	0.50	15.00	15.00	15
12 AWG, 3 conductors at 104°F	30	0.91	1.00	27.30	25.00	20
12 AWG, 3 conductors at 122°F	30	0.82	1.00	24.60	24.60	20
8 AWG, 4 conductors (Holt 2017)	55	1.00	0.80	44.00	44.00	45
6 AWG THWN-2 on a roof at 90°F (Holt 2021)	75	0.58	1.00	43.50	43.50	45

Usable is the lower of the corrected ampacity and the 75°C (167°F) column of the terminals. 12 AWG copper has 25 A there, so in three conductors the terminals set 25 A, and the 240.4(D) limit of 20 A sets the breaker. The 8 AWG row, 44 A, is not a standard size, so 240.4(B) lets 45 A protect it; where 240.4(B) does not apply, the largest is 40 A. Cool air earns no credit on this page: 12 AWG THHN at 50°F is 30 A, where Table 310.15(B)(1)(1) would give 34.5 A.

Standards and data sources

NEC 2023, Table 310.16, Table 310.15(B)(1)(1), Table 310.15(C)(1), Table 240.6(A), 110.14(C), 240.4(B), 240.4(D) and 334.80 - the ampacity columns, the ambient and conductor-count factors, the terminal limit, the standard ratings, the next-higher-rating rule, the small-conductor limits and the NM-B limit. The 2023 text was not read in NFPA's viewer; section and table numbers are as printed in 2023, where the 2020 Table 310.15(B)(1) is Table 310.15(B)(1)(1).
HELUKABEL, Allowable Ampacity Tables, NFPA 70: NEC 2023 - the 2023 Table 310.16, Table 310.15(B)(1)(1) and Table 310.15(C)(1) values this page uses, as a manufacturer reproduces them.
Mike Holt in EC&M, two 2021 articles on conductor ampacity adjustment (July 21, 2021) and on ambient temperature correction (September 23, 2021), 2020 NEC - the conductor-count rule for a raceway longer than 24 in. and the worked examples this page starts from: 12 THWN-2, 30 A x 80% = 24 A with four conductors and 30 A x 70% = 21 A with eight. The ambient article gives 6 THWN-2 at 50°C (122°F) as 75 A x 0.82 = 61.5 A, and 6 THWN-2 in a raceway 1/2 in. above a roof at 90°F as 150°F, factor 0.58, 43.5 A. His 12 THHN at 50°F example, 30 A x 1.15 = 34.5 A, is the cool-air credit this page does not take. The ambient article is at ecmweb.com under Stumped by the Code, NEC requirements for ambient temperature correction to conductor ampacity.
Mike Holt in EC&M, Code Q&A on sizing branch-circuit conductors (February 2, 2017) - the 90°C (194°F) ampacity used for the corrections with the result held to the terminal column, which gives 8 AWG, 55 A x 0.80 = 44 A, and 6 AWG THHN, 75 A x 0.80 = 60 A. Also 210.19(A)(1): 125 percent of a continuous load against the terminal column, and the load itself after the corrections.
Ascent (IEWC) Product Specification No. 10521 rev 01, THHN THWN-2 600V (May 16, 2025) - the wet and dry ratings of dual-marked wire: THHN is 90°C (194°F) in a dry location, THWN-2 is 90°C wet or dry, and THWN is 75°C (167°F) in a wet location. THHN is marked in all sizes, THWN-2 in 8 AWG and larger and THWN in 14, 12 and 10 AWG.
IAEI Magazine, Domitrovich, 100% vs 80%: Choosing the right OCPD solution (2016, 2014 NEC) - 210.20(A): the rating of the overcurrent device is not less than the noncontinuous load plus 125 percent of the continuous load, which this page uses for the breaker check.

Limitations of this calculator

This page gives preliminary numbers for estimating and for checking a design or a quote. A licensed electrician sizes and installs the circuit under the permit and the inspection of the local authority having jurisdiction (AHJ). Nothing here is an approval of an installation.
The NEC text was not read in NFPA's own viewer, which needs a login. The rules come from section numbers as Mike Holt's EC&M articles (2020 NEC, and 2017 for one example) and the HELUKABEL tables (2023) print them. Table numbers are as printed in 2023. The 2026 NEC was not read. Check the edition and the local amendments your AHJ uses.
The page works 14 AWG to 4/0 in Table 310.16: conductors in a raceway, a cable or the earth, with the conductor-count factor applied above three. It does not work free air (Table 310.17), cable tray, sizes above 4/0 or aluminum 14 AWG, which the table does not list.
The count of current-carrying conductors is your entry, from 1 to 60. The rules for what counts, such as neutrals on nonlinear loads and conductors that are not used at the same time (310.15(E) and (F)), are not worked here. Count them before you enter the number.
Cool air earns no credit. Table 310.15(B)(1)(1) gives a factor above 1.00 below 78°F; Mike Holt's example is 12 THHN at 50°F, 30 A x 1.15 = 34.5 A. This page holds the factor at 1.00, so it is cautious in cold places and cannot be used to claim the higher figure.
The terminal rating is your entry. Equipment is marked for 60°C (140°F) or 75°C (167°F) conductors, and the label and the installation instructions come first (110.14(C)). Aluminum needs terminals identified for it.
The rooftop answer follows Holt's reading of 310.15(B)(2): a raceway or cable in direct sunlight less than 7/8 in. above a roof adds 60°F to the outdoor temperature, and XHHW-2 is not subject to it. The 2023 text was not opened. The page ignores other heat, such as a hot attic fed from a duct, other than the temperature you enter.
The largest breaker is the most the wire allows, not the breaker to buy. The load check follows 210.19(A)(1) and 210.20(A) as Holt (2017 NEC) and IAEI Magazine (2014 NEC) print them, and the 2023 text was not opened. Equipment rules can be stricter: motors and air conditioners (Articles 430 and 440), the equipment label, tap and transformer rules (240.4(E) to (G)) are not applied. 240.4(B) is summarized as not available for the conductors of a multioutlet branch circuit that supplies receptacles for cord-and-plug connected portable loads; the 2023 text of that exclusion was not opened.
NM-B cable is not permitted in wet or damp locations by NEC 334.12 (read in the 2017 text), so not on a roof. Voltage drop is not worked; use the voltage drop calculator for the length of the run.

Common mistakes when derating wire

Derating from the 75°C (167°F) column instead of 90°C (194°F). For 90°C (194°F) wire the corrections start from the 90°C (194°F) value, and the 75°C (167°F) or 60°C (140°F) column only caps the result (110.14(C)). 8 AWG THHN in four conductors is 55 A x 0.80 = 44 A. Starting from the 75°C (167°F) column gives 40 A and throws away 4 A. 12 AWG THWN-2 is 24 A, not 20 A.
Forgetting the terminal limit. 6 AWG THHN is 75 A at 90°C (194°F), and with three conductors at 86°F nothing reduces it. The lugs on a 75°C (167°F) device stop it at 65 A, and on 60°C (140°F) terminals at 55 A. The corrected figure counts only when it is lower than the terminal value.
Counting the equipment grounding conductor as current-carrying. Three current-carrying conductors and a ground are four wires, but only three count, and three take no factor. Entering 4 gives the 4 to 6 factor of 0.80: 8 AWG THHN drops from 50 A to 44 A. An equipment grounding conductor carries no current in normal operation. Which neutrals count depends on 310.15(E), which this page does not work, so count before you enter.
Forgetting the rooftop adder. 6 AWG THWN-2 in a raceway on a roof at 90°F is 150°F with the adder, factor 0.58, 43.5 A. At 90°F with no roof it is 0.96 and 65 A, the 75°C (167°F) terminal limit. If the conductor is XHHW-2 there is no adder and the answer is 65 A.
Applying the conductor-count factor to a short nipple. The conductor-count table is for a raceway longer than 24 in. Ten 12 AWG conductors in a nipple of 24 in. or less keep their full 30 A before the terminal limit; the same ten in a longer raceway are 15 A.
Reading NM-B cable from the 90°C (194°F) column. The conductors in NM-B are 90°C (194°F) wire and the corrections start from that value, but 334.80 holds the result to the 60°C (140°F) column. 12 AWG NM-B is 30 A in the 90°C (194°F) column and 20 A usable. 14 AWG is 15 A, 10 AWG 30 A, 8 AWG 40 A and 6 AWG 55 A.

Common questions about wire ampacity derating

What is the ampacity of 12 AWG THHN wire?

Table 310.16 gives 30 A for 12 AWG copper at 90°C (194°F). On 75°C (167°F) terminals that is held to 25 A, and on 60°C (140°F) terminals to 20 A. Four to six conductors in a raceway take 0.80 off the 30 A, which is 24 A. The breaker on 12 AWG copper is no more than 20 A (240.4(D)).

How many amps can 10 gauge wire carry?

10 AWG copper THHN is 40 A at 90°C (194°F) and 35 A on 75°C (167°F) terminals, and the breaker is limited to 30 A (240.4(D)). 10 AWG aluminum is 35 A at 90°C (194°F), 30 A on 75°C (167°F) terminals, with a 25 A breaker limit. Four to six conductors take 0.80 off first.

How much do I derate for 4, 6 or 9 conductors?

Table 310.15(C)(1) gives 0.80 for 4 to 6 current-carrying conductors and 0.70 for 7 to 9. For 12 AWG THHN that is 24 A for 4 conductors, 24 A for 6 and 21 A for 9. Ten to 20 conductors take 0.50, 15 A. The factor applies to a raceway longer than 24 in.

What is the 90°C (194°F) column for if my terminals are 75°C (167°F)?

It is the starting point for corrections. The NEC lets you derate from the 90°C (194°F) value, then hold the result to the terminal column (110.14(C)). For 8 AWG THHN in four conductors, 55 A x 0.80 = 44 A, under the 50 A of the 75°C (167°F) column, so 44 A is usable. Starting from the 75°C (167°F) column would give 40 A. With three or fewer conductors at 86°F the 90°C (194°F) value is higher than the terminals allow, so the terminal column sets it: 8 AWG is 50 A.

What is the ampacity of NM-B (Romex) cable?

NM-B is held to the 60°C (140°F) column (334.80). 14 AWG is 15 A, 12 AWG 20 A, 10 AWG 30 A, 8 AWG 40 A and 6 AWG 55 A. The conductors are 90°C (194°F) wire, so the corrections for heat or many conductors start from the 90°C (194°F) value, and the result may not exceed the 60°C (140°F) value. NM-B is not allowed in wet or damp locations.

How does a hot attic or a rooftop change the ampacity?

12 AWG THHN in three conductors is 30 A before the terminal limit at 86°F, 27.3 A at 104°F (factor 0.91) and 24.6 A at 122°F (0.82). A raceway or cable in sunlight less than 7/8 in. above a roof adds 60°F to the outdoor temperature, so 6 AWG THWN-2 at 90°F is evaluated at 150°F, factor 0.58, 43.5 A. XHHW-2 is not subject to the adder.

Is THHN wire 90°C (194°F) in conduit on a roof?

Only if it also carries a 90°C (194°F) wet marking. A roof and conduit outdoors are wet locations. Wire marked THHN alone is not permitted there, wire dual-marked THHN/THWN is 75°C (167°F) there, and wire marked THWN-2 or XHHW-2 is 90°C (194°F). Ascent's specification marks 14, 12 and 10 AWG as THWN and 8 AWG and larger as THWN-2; other makers differ, so read the jacket. For 6 AWG on a roof at 90°F in three conductors, wire marked THWN-2 gives 43.5 A and a 45 A breaker, and wire marked THWN only gives 21.45 A and a 25 A breaker.

How is a continuous load checked against the wire?

After the corrections, the conductor has to carry the load itself. The terminal column, before corrections, and the breaker are checked at 125 percent of a continuous load (210.19(A)(1), 210.20(A)). For 10 AWG THHN in seven conductors, a 24 A continuous load is 28 A after the 0.70 factor, which covers it. The terminal column is 35 A against 30 A, and the largest breaker is 30 A, which is just enough.

Does cold weather increase the ampacity?

Table 310.15(B)(1)(1) gives a factor above 1.00 below 78°F. Mike Holt's example is 12 THHN at 50°F: 30 A x 1.15 = 34.5 A, round to 35 A. This page does not take that credit. It holds the factor at 1.00, so 12 AWG THHN at 50°F is 30 A before the terminal limit. That is the cautious reading.

What size breaker can protect wire with a derated ampacity?

At most the next standard size above the derated ampacity (240.4(B)), up to 800 A, and not for a multioutlet branch circuit that supplies cord-and-plug receptacles. 8 AWG THHN in four conductors is 44 A, so 45 A is the most with 240.4(B) and 40 A without it. On 14, 12 and 10 AWG copper, 240.4(D) also caps the breaker at 15, 20 and 30 A, so 12 AWG at 24 A takes 20 A.

Does a short nipple need a derating factor?

No. The conductor-count table applies to a raceway longer than 24 in. Ten 12 AWG conductors in a nipple of 24 in. or less keep the full 30 A of the 90°C (194°F) column before the terminal limit; in a longer raceway they are 15 A.

Does this replace an electrician?

No. This page is for estimating and for checking numbers on a design or a quote. A licensed electrician sizes and installs the circuit under a permit, and the local authority having jurisdiction inspects it.