12 AWG copper rated 75 °C keeps its 25 A table ampacity at 30 °C with up to three current-carrying conductors; nine in one raceway cut it to 17.5 A (× 0.70).
Calculator
Derated Ampacity
25.0A
Base Ampacity
25A
Temp Correction Factor
1.00
Conduit Adjustment Factor
1.00
Ambient 26–30 °C → 1.00 for 75 °C conductors
Up to 3 current-carrying conductors → 1.00
The usable ampacity is also limited by the terminals: 20 A at 60 °C / 25 A at 75 °C.
Per NEC 2023 Table 310.16, NEC 2023 Table 310.15(B)(1)(1)
Table 310.16 ampacity × 1.00 × 1.00 for 75 °C conductors at 30 °C ambient with 3 current-carrying conductors, in amperes, before any terminal or 240.4(D) limit. A dash marks a size the table has no value for.
Size
Copper (A)
Aluminum (A)
#14 AWG
20.0
—
#12 AWG
25.0
20.0
#10 AWG
35.0
30.0
#8 AWG
50.0
40.0
#6 AWG
65.0
50.0
#4 AWG
85.0
65.0
#3 AWG
100.0
75.0
#2 AWG
115.0
90.0
#1 AWG
130.0
100.0
1/0 AWG
150.0
120.0
2/0 AWG
175.0
135.0
3/0 AWG
200.0
155.0
4/0 AWG
230.0
180.0
250 kcmil
255.0
205.0
300 kcmil
285.0
230.0
350 kcmil
310.0
250.0
400 kcmil
335.0
270.0
500 kcmil
380.0
310.0
600 kcmil
420.0
340.0
700 kcmil
460.0
375.0
750 kcmil
475.0
385.0
800 kcmil
490.0
395.0
900 kcmil
520.0
425.0
1000 kcmil
545.0
445.0
1250 kcmil
590.0
485.0
1500 kcmil
625.0
520.0
1750 kcmil
650.0
545.0
2000 kcmil
665.0
560.0
How it’s calculated
Iderated = Ibase × Ftemp × Fadj
Ibase
the Table 310.16 ampacity for the size, metal and insulation rating (Wire Lookup), or the ampacity you type (Direct Entry), in amperes
Ftemp
the ambient temperature correction factor for the insulation rating (Table 310.15(B)(1)(1))
Fadj
the adjustment factor for the number of current-carrying conductors in one raceway or cable (Table 310.15(C)(1))
Table 310.15(B)(1)(1) is on a 30 °C basis: the factor is 1.00 at 26–30 °C. Enter whole degrees Celsius; the bands are in whole degrees. Below 26 °C the factors are above 1, up to 1.29 for 60 °C conductors at 10 °C or less.
Some ratings cannot be used in the hotter bands: 60 °C conductors above 55 °C and 75 °C conductors above 70 °C. Above 85 °C no rating can be used. Where a rating cannot be used, the calculator shows no result.
Table 310.15(C)(1) by count: 1.00 for up to 3, 0.80 for 4–6, 0.70 for 7–9, 0.50 for 10–20, 0.45 for 21–30, 0.40 for 31–40 and 0.35 for 41 or more current-carrying conductors.
The derated ampacity is printed to 0.1 A, the base to 1 A and the factors to 0.01.
Assumptions
The result is not limited by the terminal rating (110.14(C)) or by the 240.4(D) small-conductor limits. The wire size calculator applies both.
No rooftop adder (310.15(B)(2)): include it in the ambient yourself.
The 310.15(C)(1) exemptions are not applied: the factor follows the count you enter.
A typed base ampacity is taken to be on a 30 °C basis, like Table 310.16.
You decide which conductors count (310.15(E) for neutrals, 310.15(F) for grounding and bonding conductors).
One ambient along the whole run, and one insulation rating for every conductor.
Worked examples
Nine current-carrying 12 AWG copper THHN conductors in one raceway
THHN is rated 90 °C in dry and damp locations, so the correction and the adjustment may start from the 90 °C column (110.14(C)). The raceway is at 30 °C and feeds a 20 A circuit whose terminals are not marked for 75 °C.
Table ampacity of 12 AWG copper in the 90 °C column (Table 310.16)Ibase= 30 A
Temperature correction: 30 °C is in the 26–30 °C bandFtemp= 1.00
Adjustment: nine conductors are in the 7–9 bandFadj= 0.70
Derated ampacityIderated = 30 × 1.00 × 0.70= 21.0 A
The terminals: on a circuit of 100 A or less, equipment not marked otherwise is taken at 60 °C (110.14(C)(1)(a)), and the result may not exceed that column, 20 A for 12 AWG copper. The 240.4(D) limit for 12 AWG copper is 20 A as well.min(21.0, 20)= 20 A usable
Starting from the 75 °C column instead understates the conductor:25 × 1.00 × 0.70= 17.5 A, short of 20 A, which would move the circuit to a larger size for no reason
Answer: 21.0 A derated; on this 20 A circuit, 20 A usable (the 60 °C terminal column and 240.4(D)).
The raceway now runs through a space at 40 °C (104 °F); everything else stays the same.
Temperature correction: 40 °C is in the 36–40 °C bandFtemp= 0.91
Derated ampacityIderated = 30 × 0.91 × 0.70= 19.1 A
That is less than the 20 A circuit, so 12 AWG is too small. The wire size calculator, with the same conditions and the terminals at 60 °C, moves to 10 AWG:40 × 0.91 × 0.70, within the 30 A of the 60 °C column= 25.5 A usable
Answer: 12 AWG carries only 19.1 A here; the 20 A circuit needs 10 AWG copper (25.5 A usable).
Section numbers are from NEC 2023 (NFPA 70). This page explains them in its own words and quotes no code text.
NEC 2023 Table 310.16: ampacities up to 2000 V for 60, 75 and 90 °C insulation, for up to three current-carrying conductors in a raceway, a cable or buried directly, at 30 °C (86 °F) ambient; the aluminum columns also cover copper-clad aluminum
NEC 2023 Table 310.15(B)(1)(1): correction factors for an ambient other than 30 °C, for the tables on a 30 °C basis (310.16 and 310.17)
NEC 2023 Table 310.15(C)(1): how much the ampacity drops when one raceway or cable holds four or more current-carrying conductors
NEC 2023 110.14(C): a conductor with a higher temperature rating may start from its higher ampacity for correction and adjustment, but the result may not exceed the column of the lowest-rated termination
NEC 2023 310.15(B)(2): raceways and cables in sunlight on or above a roof, less than 3/4 in. above it, take a 33 °C (60 °F) adder; XHHW-2 is exempt
NEC 2023 310.15(C)(1)(b)–(d): where the adjustment factors do not apply: raceways 24 in. or shorter, some short underground sleeves and the cable bundling rules
NEC 2023 310.15(E): which neutral conductors count as current-carrying
NEC 2023 310.15(F): grounding and bonding conductors do not count
NEC 2023 240.4(D): overcurrent limits for small conductors: copper 14, 12 and 10 AWG: 15, 20 and 30 A; aluminum and copper-clad aluminum 12 and 10 AWG: 15 and 25 A
NEC 2023 334.80: NM cable uses the 60 °C ampacity (the 90 °C value may be the starting point for derating) and has its own bundling triggers
NEC 2023 Table 310.15(B)(1)(2): correction factors on a 40 °C basis, for the tables that use it (310.18 to 310.21), not for Table 310.16
Starting from the 75 °C column, or keeping a 90 °C result
A 90 °C conductor may start from its 90 °C ampacity (110.14(C)), which gives the larger derated value. Tools that start from 75 °C show less; tools that report the derated 90 °C value as the answer skip the terminal limit. In the worked example the 90 °C start gives 21.0 A, the 75 °C start 17.5 A, and the terminals hold the circuit to 20 A.
Check: which column the tool starts from, and whether it applies the terminal limit afterwards.
Which conductors count
Some tools count every wire in the raceway. Grounding and bonding conductors never count (310.15(F)). A neutral that carries only the unbalanced current of the other conductors does not count, while some neutrals of wye systems do (310.15(E)). Spare conductors are sometimes counted so that the raceway still works once they are connected.
Check: what the count in the other tool includes.
Cold spaces
Below 26 °C the correction factors are above 1.00: 1.04 for 90 °C conductors at 21–25 °C, up to 1.15 at 10 °C or less. Tools that stop at 1.00 show less.
Check: whether the tool raises the ampacity in cold spaces.
Fahrenheit near a band edge
The bands are whole degrees Celsius. 96 °F is 35.6 °C: rounded down it reads the 31–35 °C band (0.96 for 90 °C conductors), rounded up the 36–40 °C band (0.91). This calculator takes whole °C; round up.
Check: how the other tool converts and rounds the temperature.
The rooftop adder
Raceways and cables in sunlight on or above a roof, less than 3/4 in. above it, add 33 °C (60 °F) to the ambient (310.15(B)(2)); XHHW-2 is exempt. Some tools add it for you; this calculator does not, so enter the ambient with the adder included.
Check: whether the other tool’s ambient already includes the adder.
Adjustment exemptions
Raceways 24 in. or shorter, some short underground sleeves and some cable bundles need no adjustment (310.15(C)(1)(b)–(d)), and NM cable has its own bundling triggers (334.80). A tool that applies an exemption shows 1.00 where this calculator applies the count.
Check: whether the run qualifies for an exemption.
A base that is not on a 30 °C basis
NEC 2023 Table 310.15(B)(1)(2) has factors on a 40 °C basis for the tables that use it (310.18 to 310.21). Applying the 30 °C factors to such an ampacity, or the 40 °C factors to a Table 310.16 value, gives the wrong result.
Check: which table the base ampacity comes from.
Rounding
This calculator prints the derated value to one decimal and rounds an exact half to the even digit: 21 current-carrying 12 AWG copper conductors rated 75 °C give 25 × 1.00 × 0.45 = 11.25, printed 11.2 A. Tools that round halves up print 11.3 A, and some round to whole amperes.
Check: how many decimals the other tool shows and how it rounds.
Questions
What is the ampacity of nine 12 AWG THHN conductors in one raceway?
30 × 1.00 × 0.70 = 21.0 A at 30 °C, and 30 × 0.91 × 0.70 = 19.1 A at 40 °C. Both are before the terminal limit (20 A in the 60 °C column for a circuit of 100 A or less) and the 240.4(D) limit of 20 A for 12 AWG copper.
What is the temperature correction factor at 40 °C?
0.82 for 60 °C conductors, 0.88 for 75 °C and 0.91 for 90 °C. 40 °C falls in the 36–40 °C band of a table on a 30 °C basis.
Does a cold space raise the ampacity?
Yes. Below 26 °C the factors are above 1.00: a 90 °C conductor gets 1.04 at 21–25 °C and 1.15 at 10 °C or less. The terminal and 240.4(D) limits still apply.
Can I use the derated ampacity by itself?
No. It is the conductor’s ampacity under these conditions, before two more limits: the result may not exceed the column of the lowest-rated termination (110.14(C)), and small conductors keep their 240.4(D) limits (12 AWG copper: 20 A). The wire size calculator applies both.
Check the drop on a long run once the size is settled.
About this calculator
Written and maintained by Yusuf Can Samiloglu: MEP/HVAC design background, mechanical-engineering degree, not a licensed electrician.
How it’s checked: the calculation is the method of the Electric Master app, ported to the web and tested case by case against the app’s source code. Every table value was checked against the NEC 2023 values in at least two independent published sources.
The App Store version of Electric Master (2.2.0) predates the NEC corrections this page follows; until its next update, its result can differ from this page for some inputs. Version 2.2.0 takes an ambient between 10 and 80 °C and has no factors below 21 °C, so in a cold space it shows less ampacity than this page.