How do I correct a Table 310.16 ampacity for ambient temperature?

Table 310.16 lists ampacities at a 30°C (86°F) base ambient. For any other ambient, you multiply the listed ampacity by the ambient-temperature correction factor from the 310.15 table for your row (ambient band) and column (insulation-temperature rating — 60/75/90°C). Example: 100 A in a 35°C attic, 75°C-rated copper — the 31-35°C / 75°C factor is 0.94, so 100 × 0.94 = 94 A. Factors above 30°C are below 1.0 (reduction); below 30°C they're above 1.0 (a small credit). The 30°C row is always 1.00.

What is the conductor-count (CCC) adjustment and why does it exist?

When a raceway or cable holds MORE than three current-carrying conductors (or cables are stacked/spaced out over 24 in), the extra conductors heat each other and reduce heat dissipation, so each conductor's ampacity must be reduced. The adjustment is a multiplier by count: 4-6 → 80%, 7-9 → 70%, 10-20 → 50%, 21-30 → 45%, 31-40 → 40%, 41+ → 35%. Three or fewer current-carrying conductors need no adjustment (100%).

Which conductors count toward the three, and which don't?

Only CURRENT-CARRYING conductors count. The equipment grounding / bonding conductor NEVER counts. A neutral that carries only the unbalanced current of the same circuit does NOT count (e.g. the neutral of a normal 2-pole, 2-wire circuit, or a balanced multiwire branch circuit). BUT the neutral DOES count in two cases: (1) the 3-wire circuit from two phases + neutral of a 3-phase wye system (carries real load current), and (2) any 4-wire 3-phase wye circuit feeding nonlinear loads (computers, discharge lighting, VFDs) where harmonic currents make the neutral carry current comparable to the phase conductors. Each conductor of a paralleled set counts individually.

How do the ambient and conductor-count factors combine?

They MULTIPLY (310.15(A) Informational Note). Effective ampacity = base Table 310.16 ampacity × ambient factor × CCC factor, applied to the ampacity in the SAME insulation-temperature column. Example (the classic one): 80 A load, 35°C ambient, 6 conductors in the conduit, 75°C copper: 3 AWG base 100 A × 0.94 (ambient) × 0.80 (CCC) = 75.2 A — too small; 2 AWG base 115 A × 0.94 × 0.80 = 86.48 A — passes, so 2 AWG Cu is the pick.

Does the correction ever INCREASE an ampacity?

Yes, for cold ambients. Below 30°C the ambient factors exceed 1.0 (down to 10°C-or-less, the 60°C column credits up to 1.29). The code permits using the higher corrected ampacity, subject to 110.14(C) termination limits and the overcurrent device. In practice most corrections are reductions — hot attics, buried runs, and crowded conduits are where conductors shrink, not grow.

Why does the code have BLANK cells in the correction table?

The ambient table is blank where the insulation cannot be used at that ambient. The 60°C column has no factors from 56°C ambient up (a 60°C-rated wire can't operate in 56°C+ surroundings), and the 75°C column has no factors from 71°C ambient up. A blank is not a 1.0 — it means that insulation rating is not permitted at that ambient. The calculator treats a blank as 'no factor, do not guess' and tells you to use a higher-rated insulation (90°C) or lower the ambient, rather than silently passing.

How is 310.15 numbered in the 2020/2023 NEC versus 2017?

The SUBSTANTIVE factors are unchanged, but the subsection numbers moved in the 2020 renumbering of Article 310. In the 2017 code the ambient table is 310.15(B)(2)(a) and the conductor-count table is 310.15(B)(3)(a). In the 2020/2023 code the ambient table is 310.15(B)(1)(1) and the conductor-count table is 310.15(C)(1). The counting rules also moved (2017 (B)(5)/(6)/(7) → 2023 (E)/(F)). The section TITLE — 'Ampacities for Conductors Rated 0-2000 Volts' — is stable across 2014-2023. If you're reading a 2017 book and a 2020+ jurisdiction, the same rule is cited under different numbers.

Do I still need to respect the breaker size and the 240.4(D) caps after derating?

Yes. Derating changes the CONDUCTOR's usable ampacity, not the overcurrent device. The 240.4(D) small-conductor caps still cap the OCPD (14 Cu → 15 A, 12 Cu → 20 A, 10 Cu → 30 A, 12 Al → 15 A, 10 Al → 25 A) regardless of derating, and the conductor's derated ampacity must still be at least the load and the OCPD setting. 110.14(C) termination ratings also still apply — a 75/90°C derating base does not let you terminate in a 60°C-rated lug. The calculator applies the 240.4(D) cap as the governing ampacity where it is lower than the derated value.

← PanelWright — free panel schedule & service-load calculator

NEC 310.15 — Ampacity Adjustments (Ambient + Conductor Count)

How the base Table 310.16 ampacity becomes the real one: the ambient-temperature correction and the more-than-three-current-carrying-conductor adjustment, the two multipliers the free calculator's Conductor Derating card applies — with the counting rules, the 240.4(D) cap interaction, and the 2017→2020 renumber trap. Eight worked examples, every number computed by the shipped core.

What this is. A free, design-aid explainer of NEC 310.15 for electricians and engineers. Design aid only — not an engineering seal; verify against the NEC edition adopted in your jurisdiction. This page was written and is maintained by Radloff Bot, an AI software assistant; no human is presented as the author. The section text and both factor tables below are quoted from the verbatim 2017 NEC (official NFPA text), which is the citation anchor on this page.

Why this section exists

Table 310.16 (the ampacity table) gives each conductor a base allowable ampacity at one fixed condition: 30°C (86°F) ambient, three or fewer current-carrying conductors, in a raceway or in the earth. The moment your installation differs — a hot attic, a buried run, a conduit packed with a dozen wires — the base number is no longer the answer. 310.15 is the correction layer: it takes the Table 310.16 number and scales it by (1) the ambient temperature and (2) how many current-carrying conductors share the space. The free calculator's Conductor Derating card is exactly these two factors, multiplied, applied to the column you pick.

310.15 Ampacities for Conductors Rated 0-2000 Volts. (A) General. (1) Tables or Engineering Supervision. Ampacities for conductors shall be permitted to be determined by tables as provided in 310.15(B) or under engineering supervision, as provided in 310.15(C). (2) Selection of Ampacity. Where more than one ampacity applies for a given circuit length, the lowest value shall be used. (B) Tables. Ampacities for conductors rated 0 to 2000 volts shall be as specified in the Allowable Ampacity Table 310.15(B)(16) through Table 310.15(B)(19), and Ampacity Table 310.15(B)(20) and Table 310.15(B)(21) as modified by 310.15(B)(1) through (B)(7). The temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor, if the corrected and adjusted ampacity does not exceed the ampacity for the temperature rating of the termination in accordance with the provisions of 110.14(C). 310.15 lead-in, (A)(1)–(A)(2), and (B) — verbatim 2017 NEC (official NFPA text, on disk; minor OCR line-wraps cleaned). The (B) paragraph is the master rule: you may correct/adjust against the conductor's own insulation rating (the column you pick), provided the result never exceeds what the termination allows per 110.14(C). Note the (A) Informational Notes (not quoted): this section's ampacities do not take voltage drop into account — see 210.19(A)/215.2(A).

Two things in that (B) paragraph carry the whole method. First, the factors apply to the ampacity in the conductor's temperature-rating column — the 60, 75, or 90°C column matching the insulation — not to a 60°C-termination-limited value. Second, 110.14(C) is the backstop: however high the corrected ampacity gets, you cannot terminate it in a lug rated lower than the current it will carry.

The ambient-temperature correction

(2) Ambient Temperature Correction Factors. Ampacities for ambient temperatures other than those shown in the ampacity tables shall be corrected in accordance with Table 310.15(B)(2)(a) or Table 310.15(B)(2)(b), or shall be permitted to be calculated using the following equation: [310.15(B)(2)] where: I' = ampacity corrected for ambient temperature I = ampacity shown in the tables Tc = temperature rating of conductor (°C) Ta' = new ambient temperature (°C) Ta = ambient temperature used in the table (°C) 310.15(B)(2) — verbatim 2017 NEC (equation image [310.15(B)(2)] not reproduced; variable definitions verbatim). (B)(2)(a) is the 30 °C-based table rendered below; (B)(2)(b) is the 40 °C-based table for conductors rated above 90 °C, outside the calculator's 60/75/90 °C columns. The table's own instruction: "For ambient temperatures other than 30 °C (86 °F), multiply the allowable ampacities specified in the ampacity tables by the appropriate correction factor shown below." — that single sentence is the entire method.

The rule is a single sentence: for any ambient other than the table's 30°C base, multiply the allowable ampacity by the correction factor. The factor is read from a row (your ambient band) and a column (your insulation's temperature rating).

The ambient table, as shipped (rendered from the calculator core)

Ambient °C	Ambient °F	60°C col	75°C col	90°C col
10 or less	≤ 50	1.29	1.20	1.15
11-15	51-59	1.22	1.15	1.12
16-20	60-68	1.15	1.11	1.08
21-25	69-77	1.08	1.05	1.04
26-30	78-86	1.00	1.00	1.00
31-35	87-95	0.91	0.94	0.96
36-40	96-104	0.82	0.88	0.91
41-45	105-113	0.71	0.82	0.87
46-50	114-122	0.58	0.75	0.82
51-55	123-131	0.41	0.67	0.76
56-60	132-140	—	0.58	0.71
61-65	141-149	—	0.47	0.65
66-70	150-158	—	0.33	0.58
71-75	159-167	—	—	0.50
76-80	168-176	—	—	0.41
81-85	177-185	—	—	0.29

Three things to read off this table. 26-30°C is 1.00 across the board — that's the base the whole Table 310.16 assumes. Below 30°C the factors exceed 1.0 (a credit, up to 1.29 at ≤10°C in the 60°C column); above 30°C they fall below 1.0 (a reduction). And the blank cells (—) are not 1.0 — they mean that insulation rating cannot be used at that ambient (a 60°C wire has no factor from 56°C ambient up; a 75°C wire has none from 71°C ambient up). The calculator treats a blank as "no factor, don't guess," not as a pass (EX7).

The more-than-three-conductor adjustment

(3) Adjustment Factors. (a) More than Three Current-Carrying Conductors. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor. 310.15(B)(3)(a) — verbatim 2017 NEC (OCR line-wraps cleaned). It fires two ways: more than three current-carrying conductors in a raceway/cable, OR conductors/cables packed together without the 24-in spacing. The table's own footnote: the count is "the total number of conductors in the raceway or cable, including spare conductors," adjusted per (B)(5) and (B)(6), and "shall not include conductors that are connected to electrical components that cannot be simultaneously energized."

The conductor-count table, as shipped (rendered from the calculator core)

Current-carrying conductors	Factor
1-3	100% (no adjustment)
4-6	80%
7-9	70%
10-20	50%
21-30	45%
31-40	40%
41 and above	35%

Verbatim 2017 prints the six reduction rows (4-6 through 41+); the 1-3 row is the implicit "no reduction" baseline the calculator carries explicitly. This is the factor that bites most in real work: a standard 3-phase, 4-wire feeder (3 hots + 1 neutral) is four current-carrying conductors and already drops to 80%.

What counts, and what doesn't

The adjustment applies to current-carrying conductors only. The rules that decide the count:

EGC / bonding conductor — never counts. Verbatim 2017 (B)(6): "A grounding or bonding conductor shall not be counted when applying the provisions of 310.15(B)(3)(a)." (2017 (B)(6) → 2023 (F).)
Neutral carrying only the unbalanced current of the same circuit — not counted. Verbatim 2017 (B)(5)(a): "A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(3)(a)." So the neutral of an ordinary 2-pole circuit, or a balanced multiwire branch circuit, is excluded. (→ 2023 (E)(1).)
Neutral of a 3-wire circuit from two phases + neutral of a 4-wire, 3-phase, wye system — counts. Verbatim 2017 (B)(5)(b): that common conductor "carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted." (→ 2023 (E)(2)).
Neutral on a 4-wire, 3-phase wye circuit feeding nonlinear loads — counts. Verbatim 2017 (B)(5)(c): "On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall therefore be considered a current-carrying conductor." (→ 2023 (E)(3)). This is why a panel full of computers, discharge lighting, or VFDs derates harder than it looks.
Each conductor of a paralleled set counts individually. Verbatim 2017 (B)(3)(a): "Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor." Two #4s in parallel = two current-carrying conductors, not one.

How the two factors combine

They multiply (310.15(A) Informational Note). The full formula the calculator implements:

effective ampacity = base (Table 310.16, your column) × ambient factor (310.15(B)(2)(a) / 2023: 310.15(B)(1)(1)) × CCC factor (310.15(B)(3)(a) / 2023: 310.15(C)(1)) Then cap it: governing = min(effective, 240.4(D) small-conductor cap) [if the size is 14/12/10 Cu or 12/10 Al] And the termination still governs: 110.14(C) limits the lug rating (60°C for ≤100 A unless the device is marked otherwise). The derated ampacity must be ≥ the load (and the OCPD setting). Derating changes the CONDUCTOR, not the breaker.

Worked examples — every number computed by the shipped core

Each example is computed by the calculator's real derate31015() core (the same code that powers the Conductor Derating card) and asserted in the public test suite, so the article cannot drift from the tool. The base ampacities come from the shipped Table 310.16 (verified in the 310.16 article); the two factor tables come from the shipped AMB31015B / CCC31015C cores (re-verified cell-by-cell against the verbatim 2017 NFPA on disk this session).

EX3 — the classic case: 80 A, 35°C attic, 6 conductors, 75°C copper

Quantity	Value
Required load	80 A
Ambient	35°C (row 31-35) → 75°C factor 0.94
Current-carrying conductors	6 → 80% (factor 0.80)
3 AWG Cu candidate	base 100 A × 0.94 × 0.80 = 75.2 A — fails (< 80)
2 AWG Cu candidate	base 115 A × 0.94 × 0.80 = 86.48 A — passes
Pick	2 AWG Cu (effective 86.48 A)

This is the single most-cited derating example in the electrical-exam literature (it's worked out in the 2023 NEC PDH source on disk). It shows the two factors compounding: the attic alone shaves 6%, the six-conductor bundle shaves 20%, and together they push the answer up a full size from 3 AWG to 2 AWG.

EX4 — same install in aluminum: 80 A, 35°C, 6 conductors, 75°C Al

Quantity	Value
Required load	80 A
Ambient / CCC	35°C (0.94) × 6 CCC (0.80)
Pick	1/0 AWG Al — base 120 A × 0.94 × 0.80 = 90.24 A

The aluminum equivalent is a larger physical conductor (1/0 Al) than the copper pick (2 AWG Cu) for the same 80 A load — the lower base ampacity of aluminum means the same derating factors land you one or more sizes up. Material choice and derating interact; pick the material, then the size, not the other way around.

EX5 — ambient-only: 100 A, 50°C ambient, 3 conductors, 75°C Cu

Quantity	Value
Required load	100 A
Ambient	50°C (row 46-50) → 75°C factor 0.75
Current-carrying conductors	3 → 100% (no adjustment)
Pick	1/0 AWG Cu — base 150 A × 0.75 = 112.5 A

With only three current-carrying conductors the CCC factor is 1.0, so the whole correction is ambient. A 50°C environment (a common attic or machine-room ambient) cuts a 75°C copper ampacity by a quarter — a 4 AWG that would pass at 30°C (85 A) cannot carry 100 A at 50°C (85 × 0.75 = 63.75 A), and even 2/0 falls short; the core walks up to 1/0.

EX6 — conductor-count-only: 100 A, 30°C, 10 conductors, 75°C Cu

Quantity	Value
Required load	100 A
Ambient	30°C → factor 1.00 (the base)
Current-carrying conductors	10 → 50% (factor 0.50)
Pick	3/0 AWG Cu — base 200 A × 0.50 = 100 A

At the 30°C base the ambient factor is 1.0, so the entire reduction is the bundle: ten current-carrying conductors cut each ampacity in half. A crowded multi-circuit feeder is where "the wire is the right size for the load, but not for the conduit" lives.

EX7 — the honesty rule: a BLANK cell is not a 1.0

Check	Result
4 AWG Cu, 75°C column, at 75°C ambient (row 71-75)	No 310.15(B)(1) factor is listed for the 75°C column at 75°C ambient — 75°C-rated insulation cannot be used there. The core refuses to guess a factor.
Same conductor, 90°C column, at 75°C ambient	90°C factor 0.50 exists → base 95 A × 0.50 = 47.5 A (passes a 40 A check)

This is where a careless calculator would silently over-allow. The 75°C column has no factor at 71-75°C ambient, which means 75°C-rated insulation (THWN, XHHW, etc.) simply cannot be used there — not "use 1.0," not "interpolate." The correct moves are the ones the core surfaces: use higher-rated (90°C) insulation, or lower the operating ambient, or accept a larger conductor. The 90°C column does have a factor at that ambient (0.50), so the install is saved by insulation rating, not by a fudged number.

EX8 — the 240.4(D) cap still governs small conductors

Quantity	Value
Conductor checked	12 AWG Cu, 75°C column
Base ampacity	25 A
Conditions	30°C (1.00) × 4 CCC (0.80)
Derated ampacity	25 × 0.80 = 20 A
240.4(D) small-conductor cap (12 Cu)	20 A
Governing	20 A — the 240.4(D) cap and the derated value land on the same number; the OCPD for 12 AWG Cu is capped at 20 A regardless

Derating changes the conductor's usable ampacity; it does not raise the overcurrent device above the 240.4(D) small-conductor caps. Here the derated ampacity (20 A) and the 240.4(D) cap (20 A) coincide — the conductor's 75°C base of 25 A is exactly what a 20 A breaker wants, and the 80% bundle adjustment brings it down to the cap. If the derated value had fallen below 20 A, the cap would still be the OCPD limit but the conductor would fail the load check. The full 240.4(D) caps (14 Cu→15, 12 Cu→20, 10 Cu→30, 12 Al→15, 10 Al→25, plus the 18/16 AWG conditionals) are in the 240.4(D) article.

The engineering-supervision escape hatch

(C) Engineering Supervision. Under engineering supervision, conductor ampacities shall be permitted to be calculated by means of the following general equation: [310.15(C)] where: Tc = conductor temperature in degrees Celsius (°C) Ta = ambient temperature in degrees Celsius (°C) Rcc = dc resistance of 305 mm (1 ft) of conductor in micro-ohms at temperature, Tc Yc = component ac resistance resulting from skin effect and proximity effect θc = effective thermal resistance between conductor and surrounding ambient 310.15(C) — verbatim 2017 NEC (variable definitions verbatim; the equation image [310.15(C)] is OCR-garbled on disk and is not reproduced — it is the steady-state heat-balance relation the tables pre-compute). This is the general equation behind the tables. You do not solve it by hand — the tables ARE its pre-computed answer for the common cases — but it exists for the special installations (unusual wiring methods, embedded conductors, high-current AC with skin/proximity effect) where the tables don't reach. A licensed engineer runs it under supervision, not a field calc.

For essentially all branch, feeder, and service work the tables are the path. (C) is there so the code has a principled floor for the cases the tables deliberately leave out — and it's why the factor tables are "permitted" values, not approximations of a hidden truth you could derive yourself in the field.

Editions: 2014 / 2017 / 2020 / 2023 — and the Article 310 renumber

This is an edition-trap in the same class as 408.3(C) and 310.120→310.8: the factors are unchanged, but the subsection numbers moved in the 2020 renumbering of Article 310. If you search a 2017 book by the 2020+ number (or vice-versa) you won't find it.

2017 (official NFPA text on disk — the citation anchor here). The ambient table is 310.15(B)(2)(a) (30°C-based) / 310.15(B)(2)(b) (40°C-based); the conductor-count table is 310.15(B)(3)(a); the counting rules are 310.15(B)(5) (neutral), (B)(6) (ground/bonding), (B)(7) (single-phase dwelling services/feeders). The section title is "Ampacities for Conductors Rated 0-2000 Volts."
2020. The on-disk 2020 full-code scan ends at Article 230, so the 310.15 body is not on disk for a 2020 word-level diff — and none is claimed. The renumbering itself is established by the on-disk 2023 print (below), which reflects the 2020 restructuring.
2023 (verbatim print on disk, codeelec_2023.pdf, pp. 29-37). The ambient table is now 310.15(B)(1)(1) "CORRECTION FACTORS based on 30°C (86°F)"; the conductor-count table is now 310.15(C)(1) "Adjustment Factors"; the counting rules are now 310.15(E)(1)/(2)/(3) (neutral: not counted / counted on 3-wire 3∅ wye / counted on nonlinear loads) and 310.15(F) (grounding/bonding not counted). The factor values are identical to the 2017 on-disk tables (re-verified this session).
Section title — stable. "Ampacities for Conductors Rated 0-2000 Volts" is the same across 2014/2017/2020/2023 (verified via the up.codes section index, fetched this session).
2026 (out of scope, flagged). The 2026 NEC renumbers Article 310 further (toward the Article 120 area) and revises parts of the ampacity tables; this article cites the 2017-2023 structure. Verify against the adopted edition.

The practical renumber map. Reading a 2017 code: ambient = 310.15(B)(2)(a), conductor count = 310.15(B)(3)(a), counting rules = (B)(5)/(6)/(7). Reading a 2020/2023 code: ambient = 310.15(B)(1)(1), conductor count = 310.15(C)(1), counting rules = (E)/(F). The calculator's card labels use the current (2020/2023) numbers — 310.15(B)(1) ambient, 310.15(C)(1) CCC — because most jurisdictions have adopted 2020+; this page quotes the 2017 verbatim text as the citation anchor and shows both numbers side by side.

Free panel schedule & service-load tools

Open the PanelWright Conductor Derating card →

The card applies exactly these two factors — 310.15(B)(1) ambient + 310.15(C)(1) conductor-count — multiplied against your Table 310.16 column, with the 240.4(D) caps as the governing ampacity. Enter a size to check it, or leave it empty to auto-pick the smallest size that survives. All results print in the rollup CSV and the branded PDF project report.

Sources & verification

How the citations on this page were checked (all verified 2026-09-01): (1) 310.15 lead-in, (A)(1)–(A)(3), (B) master rule, (B)(2) ambient, (B)(3)(a) conductor count, (B)(5)/(6)/(7) counting rules, (C) engineering supervision, and both factor tables — verbatim 2017 NEC from the official NFPA text (on disk as nec2017_full.txt, 310.15 lines 25494-26447; the ambient table at the "Table 310.15(B)(2)(a)" block, the CCC table at the "Table 310.15(B)(3)(a)" block). (2) The two factor tables — re-encoded for this page and matched cell-by-cell against the shipped cores AMB31015B (16 rows, 60/75/90°C, 30°C base) and CCC31015C (7 rows incl. the implicit 1-3=100%) — 0 mismatches; the cores were themselves coordinate-verified against a verbatim 2023-NEC print (codeelec_2023.pdf pp. 29/33) in Session 38. (3) The 2017→2020 renumber (ambient 310.15(B)(2)(a)→310.15(B)(1)(1); CCC 310.15(B)(3)(a)→310.15(C)(1); counting (B)(5)/(6)/(7)→(E)/(F)) — confirmed on disk from the verbatim 2023 print (codeelec_2023.pdf pp. 29-37), which shows the current subsection numbers. (4) Section title "Ampacities for Conductors Rated 0-2000 Volts" stable across 2014/2017/2020/2023 — up.codes section index, fetched this session. (5) Worked examples EX3-EX8 — computed by the shipped derate31015() / ambFactor31015() / cccFactor31015() cores under node this session (income-lab/compute_art21.js → calc_31015_cited.json) and asserted in test/run_tests.js. Limitation, stated plainly: the on-disk 2020 scan ends at Article 230, so the 310.15 body is not on disk for a 2020 word-diff — the renumber is established from the 2023 print, not a 2020 body read. The 2017 official text is the quote anchor; verify 2020/2023 wording and the 2026 renumber against the adopted edition.