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NEC 310.15(B)(1)

Ampacity Derating Calculator

The ambient temperature correction factor at the temperature you actually have. The code prints fifteen 5°C bands plus an open-ended “10 or less” row, and it also prints the equation behind them, so we compute the equation and give you the factor at the ambient you actually have.

Quick answer: at 103°F on 90°C insulation the correction factor is 0.918, computed from NEC Equation 310.15(B)(1). Table 310.15(B)(1)(1) bands that temperature and prints 0.91, because a band is evaluated at its hottest end. NEC 310.15(B)(1) permits either route. Multiply by the conductor count adjustment from 310.15(C)(1) as well, then hold the result to the terminal rating under 110.14(C).

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Correction Factors at 5°F Steps

Computed from Equation 310.15(B)(1) for every 5°F across roughly the span the code’s table spans. The 2023 edition publishes sixteen bands and nothing between them, so this grid is finer than the one it prints. Use the calculator above for a temperature that falls between these rows.

Correction factors computed every 5°F from 50°F to 185°F using NEC Equation 310.15(B)(1) (NFPA 70-2023). Three decimals, because the factor is continuous and the rounding is ours to choose. “None” means the ambient has reached the insulation’s own rating and there is no ampacity left to correct. A dagger marks a value the code’s own table does not print: NEC Table 310.15(B)(1)(1) stops publishing a factor for 60°C and 75°C insulation well before those conductors reach their rating. 310.15(B)(1) permits calculating there, but you would be computing where the code declined to tabulate. These factors are larger than the banded values in the code’s table, which is evaluated at the hot end of each band, and a larger factor permits a smaller conductor. Both routes are permitted. Use these only if the ambient is the one you actually have, and hold the result to the termination rating (110.14(C)) and 240.4(D).
Ambient °F	Ambient °C	60°C wire	75°C wire	90°C wire
50	10.0	1.291	1.202	1.155
55	12.8	1.255	1.176	1.134
60	15.6	1.217	1.149	1.114
65	18.3	1.179	1.122	1.093
70	21.1	1.139	1.094	1.072
75	23.9	1.097	1.066	1.050
80	26.7	1.054	1.036	1.027
85	29.4	1.009	1.006	1.005
90	32.2	0.962	0.975	0.981
95	35.0	0.913	0.943	0.957
100	37.8	0.861	0.909	0.933
105	40.6	0.805	0.875	0.908
110	43.3	0.745	0.839	0.882
115	46.1	0.680	0.801	0.855
120	48.9	0.609	0.762	0.828
125	51.7	0.527	0.720	0.799
130	54.4	0.430	0.676	0.770
135	57.2	0.304 †	0.629	0.739
140	60.0	none	0.577	0.707
145	62.8	none	0.521	0.674
150	65.6	none	0.458	0.638
155	68.3	none	0.385	0.601
160	71.1	none	0.294 †	0.561
165	73.9	none	0.157 †	0.518
170	76.7	none	none	0.471
175	79.4	none	none	0.419
180	82.2	none	none	0.360
185	85.0	none	none	0.289

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Where these derating numbers come from

The correction factor on this page is CALCULATED, not looked up, and the code is what says we may calculate it. Everything downstream of it (the usable ampacity, the largest breaker, the upsize) is computed by the engine from the NEC sections below. These figures live in one locked module rather than being retyped page by page, so every place this number appears is the same number. Every release runs this site's own checks against that locked module before it ships, and the source is cited above so you can hold the number to the document yourself.

NEC Equation 310.15(B)(1): The ambient correction itself. 310.15(B)(1) states that ampacities for ambient temperatures other than those shown in the ampacity tables shall be corrected in accordance with Table 310.15(B)(1)(1) or Table 310.15(B)(1)(2), or shall be permitted to be calculated using Equation 310.15(B)(1), which is I' = I x sqrt((Tc - Ta') / (Tc - Ta)). NFPA 70-2023, p. 70-184. This tool computes that equation at the ambient you enter, which is why it can answer temperatures the banded table does not print.
NEC Table 310.15(B)(1)(1): The banded correction factors the code prints, based on 30 C, shown beside every computed answer so the two can be compared. NFPA 70-2023, p. 70-185. The equation reproduces all 39 of its printed factors exactly at two decimals, and its 9 dashes fall exactly where the ambient reaches the conductor's own rating. The same equation also reproduces all 107 printed cells of Table 310.15(B)(1)(2), the 40 C table on the same page, which is a second check on the equation rather than on our copy of the first table.
NEC 310.15(B)(2): The rooftop option. For raceways or cables in direct sunlight where the distance above the roof to the bottom of the raceway is less than 19 mm (3/4 in), a temperature adder of 33 C (60 F) is added to the outdoor temperature. Type XHHW-2 conductors are excepted. NFPA 70-2023, p. 70-184.
NEC Table 310.15(C)(1): The conductor count adjustment for more than three current-carrying conductors bundled longer than 24 inches. Unlike the ambient correction there is no equation behind these: the committee sets the six values directly, so the tool cites and applies them rather than deriving them. NFPA 70-2023, p. 70-186.
NEC Table 310.16: The conductor ampacity the derating starts from, copper and aluminium at 60, 75 and 90 C, for conductors in a raceway or cable. NFPA 70-2023, p. 70-187. This is the feeder and branch-circuit table; the tool does not apply the separate dwelling-service allowance, which is a different table for a different case.
NEC 110.14(C): The termination limit, which is a ceiling and not a multiplier: the corrected and adjusted ampacity may not exceed the ampacity of the conductor at the temperature rating of the equipment terminations at each end. This is why a 90 C conductor derated to 68.8 A can still be held to the 75 C column, and the tool tells you which of the two is governing.
NEC 240.4(D): The small-conductor overcurrent caps, which override the ampacity outright: 15 A for 14 AWG copper, 20 A for 12 AWG, 30 A for 10 AWG, with their own lower aluminium values. Applied after derating, so the tool cannot hand back a breaker these sections forbid.
NEC 240.6(A): The standard overcurrent device ratings the breaker answer is snapped to. The tool rounds DOWN to a standard size. 240.4(B) permits the next size up on circuits not over 800 A that do not supply receptacle outlets, and the tool states that as an option rather than assuming your circuit qualifies.

The correction factor is computed, and the ampacity, breaker and upsize are computed from the NEC tables cited above, but they are only as good as the conditions you enter: the ambient is the one you tell it, and the conductor count is the one you tell it. The tool does not know your voltage drop, your load calculation, your conduit fill, or whether your terminations are actually rated what you selected. Calculating instead of using the banded table is expressly permitted by 310.15(B)(1) and produces a slightly larger factor, which permits a slightly smaller conductor; the AHJ has final say and you should be ready to show the calculation.

Conductor Bundling Adjustment (NEC 310.15(C)(1))

Unlike the ambient correction above, this one has no equation behind it. NEC 310.15(C)(1) sets these values directly, so we cite them rather than compute them. The calculator applies the one that matches your conductor count, and the count is the part people get wrong, so the rules for counting are below.

There is no adjustment for three or fewer current-carrying conductors: at that count you use the ampacity as published. Our calculators carry a 1–3 = 1.00 selection so that “no derating” is something you can pick; it is an input we add, not a factor from NEC 310.15(C)(1).

The count starts at the total number of conductors in the raceway or cable, including spare conductors, and is then adjusted: a neutral that carries only the unbalanced current from the other conductors of the same circuit is not required to be counted (NEC 310.15(E)(1)), and the equipment grounding or bonding conductor is not counted either (NEC 310.15(F)). A neutral taken from a 4-wire 3-phase wye system may be a different case: (E)(2) counts the neutral of a 3-wire circuit of two phase conductors and the neutral of such a system, and (E)(3) counts it on a 4-wire 3-phase wye circuit where the major portion of the load is nonlinear. A 4-wire wye circuit under a mostly linear load is still (E)(1). So which of the three branches your circuit is in settles whether the neutral is in this count at all. Conductors connected to equipment that cannot be energized at the same time are not counted. Leaving spares out lowers the count, which raises the adjustment factor and undersizes the conductor.

Where the Correction Factor Comes From

A conductor’s ampacity is set by how much heat it can shed before the insulation reaches its rating, and the heat it can shed goes with the square root of the temperature difference available to it. NEC 310.15(B)(1) prints that relationship as Equation 310.15(B)(1): I' = I × sqrt((Tc − Ta') / (Tc − Ta)), where Tc is the conductor temperature rating, Ta' is your ambient and Ta is the ambient the table was built on, which is 30°C for Table 310.15(B)(1)(1). The same section says ampacities may be corrected using the table or calculated using the equation.

That is why the two answers track each other so closely: the table is a rounded tabulation of the equation, evaluated at the top of each 5°C band. It is also why the table stops printing a factor for 60°C insulation above 55°C ambient. Once the ambient reaches the conductor’s own rating there is no temperature difference left to carry heat away, the term (Tc − Ta') goes to zero, and the conductor has no ampacity to correct. The dash in the book and the refusal in the calculator are the same statement.

Worked Example

#6 THHN, 108°F attic, 6 current-carrying conductors

Base: #6 THHN at 90°C = 75A (NEC Table 310.16)

Temperature: sqrt((90 − 42.2) / (90 − 30)) = 0.8924

Conductor count: 6 conductors = 0.80 (NEC 310.15(C)(1))

75 × 0.8924 × 0.80 = 53.5A

Result: 53.5A of usable ampacity, well under the 65A that #6 copper carries at the 75°C terminal column, so the derated figure governs. This is why a #6 that looks good for 65A ends up on a 50A load once it is in a hot, crowded raceway.

Frequently Asked Questions

What is the temperature correction factor at 103 degrees F?

At 103 degrees F on 90 C insulation the computed factor is 0.918. NEC Table 310.15(B)(1)(1) puts that temperature in a band and prints 0.91 for it, because each band is evaluated at its hottest end. Both are permitted: 310.15(B)(1) says ampacities may be corrected using the table or calculated using Equation 310.15(B)(1).

Can I calculate a correction factor instead of using the NEC table?

Yes. NEC 310.15(B)(1) states that ampacities for ambient temperatures other than those shown in the ampacity tables shall be corrected in accordance with Table 310.15(B)(1)(1) or Table 310.15(B)(1)(2), or shall be permitted to be calculated using Equation 310.15(B)(1). The equation is I' = I x sqrt((Tc - Ta') / (Tc - Ta)), where Tc is the conductor temperature rating, Ta' is your ambient and Ta is the ambient the table is based on, which is 30 C for Table 310.15(B)(1)(1).

Why does the calculated factor differ from the table?

The table is banded in 5 C steps and each band is evaluated at its hottest end, so a conductor at the cool end of a band is corrected as if it were at the hot end. The equation uses your actual ambient. At the top of a band, wherever the code prints a factor at all, the two agree to two decimals. Below it the calculated factor is larger, which permits a smaller conductor, so use it only when the ambient you entered is the real one.

How do I derate ampacity for more than 3 conductors?

Per NEC 310.15(C)(1), for 4 to 6 current-carrying conductors multiply ampacity by 0.80, for 7 to 9 by 0.70, for 10 to 20 by 0.50, for 21 to 30 by 0.45, for 31 to 40 by 0.40, and for 41 or more by 0.35. The adjustment applies when conductors are bundled longer than 24 inches. This one is a table of values the code sets; there is no equation behind it to compute.

What happens on a rooftop in direct sunlight?

NEC 310.15(B)(2) adds 33 C (60 F) to the outdoor temperature for raceways or cables exposed to direct sunlight where the distance from the roof to the bottom of the raceway is less than 19 mm (3/4 in). Type XHHW-2 conductors are excepted. A 95 F afternoon becomes a 155 F ambient for correction purposes, which is why rooftop runs get spaced off the deck.

Does the neutral count as a current-carrying conductor?

It depends on which of the three printed conditions in NEC 310.15(E) describes the circuit. Under (E)(1) a neutral that carries only the unbalanced current from the other conductors of the same circuit is not required to be counted, which is the case for a 120/240V three-wire circuit as the text is printed. Under (E)(2) the neutral of a 3-wire circuit taken from a 4-wire, 3-phase, wye-connected system is counted. Under (E)(3) the neutral of a 4-wire, 3-phase wye circuit is counted where the major portion of the load consists of nonlinear loads. The NEC prints no percentage for that phrase. The equipment grounding conductor is never counted, under 310.15(F). A 2-wire circuit is a fourth case the three conditions do not name: it has no other conductor to be unbalanced against, so its neutral carries the full load current and (E)(1) does not exclude it. Which branch a circuit falls in is worked out by the neutral current-carrying conductor tool.

Which ampacity column do you start derating from?

Start from the 90 C column of NEC Table 310.16 for THHN, THWN-2, and XHHW-2, apply the correction and adjustment factors, then confirm the derated result does not exceed the 60 C or 75 C terminal rating of the equipment per NEC 110.14(C).

Sizing a conductor, not just a factor?

The wire size calculator takes the load, pulls the base ampacity, applies temperature and conductor count, and enforces the terminal rating in one pass.

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