When do I apply the more-than-three-conductor adjustment factor?

NEC Table 310.15(C)(1) applies once a raceway or cable holds more than three current-carrying conductors. Four to six conductors derate to 80 percent, seven to nine to 70 percent, ten to twenty to 50 percent, and it steps down from there. Three or fewer conductors get no adjustment.

Do the ambient correction and conductor-count factors multiply together?

Yes. NEC 310.15 applies both factors to the base ampacity as multipliers: corrected ampacity equals the Table 310.16 base ampacity times the ambient correction factor times the conductor-count adjustment factor. For example, 55 A base times 0.82 ambient times 0.70 count equals 31.57 A.

Which temperature column do I use for the correction factor?

Use the conductor insulation rating column. The NEC lets you correct and adjust starting from the 90C column for a 90C-rated conductor such as THHN, even though the final connection may be limited to a 60C or 75C termination. That termination limit (NEC 110.14(C)) is applied to the result separately and is handled by the Wire Ampacity Calculator.

Why is my corrected ampacity higher than the table value in cold weather?

Below the 30C basis the ambient correction factor is greater than 1.0, so the corrected ampacity rises above the Table 310.16 value. This tool reports that raw corrected number. In practice NEC 110.14(C) still caps the conductor at its base ampacity for the termination rating, so a cold-ambient boost usually does not raise the usable ampacity unless conductor-count derating is also present.

Ampacity Correction & Adjustment Calculator

Read the NEC Table 310.15(B)(1) ambient temperature correction factor and the Table 310.15(C)(1) more-than-three-conductor adjustment factor for a conductor, and get the corrected and adjusted ampacity in one step. This is the focused factor tool; for the full termination-limited result use the Wire Ampacity Calculator.

How to use this calculator

Pick the conductor material, size, and insulation rating. The insulation rating sets both the base ampacity column and the correction-factor column.
Enter the ambient temperature in degrees C or F. Values are placed in the NEC 5-degree range that covers them, rounding up in the conservative direction.
Enter the number of current-carrying conductors bundled in the same raceway or cable (count per NEC 310.15(E); three or fewer means no count adjustment).
Read the two factors and the corrected and adjusted ampacity. The notes show the arithmetic and flag any cold-ambient factor above 1.0.

NEC reference

Factors come from NEC 2020 Table 310.15(B)(1) (ambient temperature correction, based on a 30C / 86F ambient) and Table 310.15(C)(1) (adjustment for more than three current-carrying conductors), applied to the base ampacity in Table 310.16. The factor values are unchanged across the 2017, 2020, and 2023 editions; only the table numbers differ (in 2017 they were Tables 310.15(B)(2)(a), 310.15(B)(3)(a), and 310.15(B)(16)). The NEC 110.14(C) termination-temperature limit is applied to the corrected result separately and is out of scope for this tool.

Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.

The two factors, and how they combine

Table 310.16 lists ampacity at a 30C ambient with no more than three current-carrying conductors. Real installations rarely match both conditions, so NEC 310.15 corrects for a hotter or colder ambient and adjusts for extra bundled conductors. Both are simple multipliers applied to the base ampacity:

corrected ampacity = base (Table 310.16) x ambient factor (310.15(B)(1)) x count factor (310.15(C)(1))

The insulation rating picks the column for both the base ampacity and the ambient factor, so a 90C conductor derates from the 90C column. That is why running THHN (90C) instead of THWN (75C) leaves more ampacity after derating, even when the terminations limit the final connection to 75C.

NEC Table 310.15(B)(1) ambient correction factors

Factors are based on a 30C (86F) ambient. A blank cell means the conductor rating is too low to be used at that ambient.

Ambient (C)	60C	75C	90C
10 or less	1.29	1.20	1.15
11-15	1.22	1.15	1.12
16-20	1.15	1.11	1.08
21-25	1.08	1.05	1.04
26-30	1.00	1.00	1.00
31-35	0.91	0.94	0.96
36-40	0.82	0.88	0.91
41-45	0.71	0.82	0.87
46-50	0.58	0.75	0.82
51-55	0.41	0.67	0.76
56-60	-	0.58	0.71
61-65	-	0.47	0.65
66-70	-	0.33	0.58
71-75	-	-	0.50
76-80	-	-	0.41
81-85	-	-	0.29

Values from NEC 2020 Table 310.15(B)(1) (identical in the 2017 and 2023 editions).

NEC Table 310.15(C)(1) conductor-count adjustment

Current-carrying conductors	Adjustment factor
1-3	1.00 (none)
4-6	0.80
7-9	0.70
10-20	0.50
21-30	0.45
31-40	0.40
41 and above	0.35

Values from NEC 2020 Table 310.15(C)(1) (was Table 310.15(B)(3)(a) in 2017).

Worked example

Take 8 AWG copper THHN (90C) in a raceway with seven current-carrying conductors in a 46C (about 115F) ambient. The base ampacity from Table 310.16 at the 90C column is 55 A. The 46-50C ambient factor is 0.82, and the seven-conductor factor is 0.70. So the corrected and adjusted ampacity is:

55 A x 0.82 x 0.70 = 31.57 A

That 31.57 A is the derated ampacity before the termination-temperature check. If the terminations are rated 75C, apply NEC 110.14(C) with the Wire Ampacity Calculator, which caps the result at the 75C base value and reports the usable ampacity and the governing limit.

How this differs from the Wire Ampacity Calculator

This page answers one question: what are the ambient and conductor-count factors, and what is the derated ampacity they produce. It stops there on purpose, so the two multipliers are front and center. The Wire Ampacity Calculator takes the same two factors and then applies the NEC 110.14(C) termination cap and the 240.4(D) small-conductor overcurrent note to give the final usable ampacity for a real circuit. Use this tool to check a factor; use that one to size a run end to end.

Frequently asked questions

What is the temperature correction factor for ampacity?

The correction factor from NEC Table 310.15(B)(1) scales a conductor ampacity when the ambient is not 30C (86F). A 90C conductor at 40C ambient uses 0.91, turning a 30 A base into 27.3 A. Below 30C the factor climbs above 1.0.

Do neutrals count as current-carrying conductors?

It depends. Under NEC 310.15(E) the grounded neutral of a balanced circuit usually does not count, but a neutral carrying the major harmonic current of a nonlinear load does. Equipment grounding conductors never count. Enter the count you arrive at; the tool applies Table 310.15(C)(1) to that number.

Does the 30C basis change with the NEC edition?

No. The 30C (86F) basis and every factor value are the same in the 2017, 2020, and 2023 editions. Only the table designations changed when Article 310 was reorganized for the 2020 code.

Is there a separate rooftop adder?

The 2017 code carried a rooftop temperature adder (Table 310.15(B)(3)(c)) for raceways in sunlight, but it was deleted in the 2020 NEC. Current practice is to use the actual ambient. This tool applies only the Table 310.15(B)(1) ambient correction and the Table 310.15(C)(1) conductor-count adjustment.

Related calculators

Wire Ampacity Calculator: the full result with the 110.14(C) termination cap and the 240.4(D) OCPD note.
Wire Gauge Calculator: reverse lookup, minimum AWG for a required ampacity after derating.
Wire Ampacity Chart: printable Table 310.16 base ampacities for copper and aluminum.
Wire Size Calculator: size a conductor for a load and distance with voltage drop and OCPD.

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Correction & adjustment factors