NEC Table 310.16 Ampacity Table: "what size conductor for X amps?", answered

Which column: the 110.14(C) rule

The table gives you three numbers per size; 110.14(C) tells you which one you may actually use at the terminations. The controlling sentence (verbatim, NEC 2020 full-code text):

Practically: ≤100 A circuits → 60 °C column, >100 A circuits → 75 °C column (unless the equipment is listed and marked otherwise). The 90 °C column is never the termination column on an unmarked device — it exists so a 90 °C-insulation conductor can be derated from (310.15) and still land at or above the 75 °C value the terminations require. PanelWright implements this: you pick the material and the column, and the tool warns when you use the 90 °C column ("use only as a DERATING base (310.15); final ampacity must still respect the termination rating").

The two base assumptions (read the heading)

The printed values are the base ampacities. They apply as-is only when:

1. Ambient is 30 °C (86 °F). If the conductors will sit in a hotter (or cooler) ambient, apply the 310.15(A)(1) correction factor — multiply the base ampacity by the factor from Table 310.15(A)(1) for the actual ambient and the conductor's temperature rating. Hot attics and mechanical rooms are where this bites: a conductor that passes at 30 °C can fail the check at 40 °C ambient.
2. Not more than three current-carrying conductors in the raceway, cable, or earth. If you have four or more (count the ones actually carrying current — neutrals that carry unbalanced current count, two-pole and three-pole loads count on each ungrounded leg, etc.), apply the 310.15(B)(1) adjustment factor — a percentage of the (already-ambient-corrected) ampacity that drops as the conductor count climbs. The exact percentages are in Table 310.15(B)(1) of your adopted edition; this page does not reprint them (they are edition-formatted tables, and the point of the base table is that you need to know when the base is not enough).

Both adjustments are the designer's — PanelWright reports the base pick and flags that 310.15 adjustments have not been applied. When both apply, the factors multiply together (correct first, then adjust).

The 240.4(D) overcurrent cap — the 14 AWG trap

For the three small sizes, the overcurrent device may be smaller than the table ampacity would allow, and that changes the wire size you actually need for a given circuit rating. Verbatim (2023-NEC print on disk):

The trap: a 14 AWG copper conductor has a 20 A ampacity in the 75 °C column — so ampacity-wise, 20 A "fits." But 240.4(D) caps its overcurrent device at 15 A, so a 20 A circuit cannot be wired with 14 AWG; it needs 12 AWG. PanelWright surfaces this as a note on every small-conductor pick (e.g. "240.4(D): overcurrent device for this size is capped at 15 A") so the ampacity number and the breaker limit are never confused.

The boundary values (where 1 amp bumps you a size)

The table is a staircase. These are the exact edges of the most-misquoted steps — every value below is computed by the shipped pickConductor31016() core and asserted in the public test suite. Copper, 75 °C column:

Two consequences worth internalizing: (1) 200 A is a knife edge — exactly 200 A required lands on 3/0, and 200.1 A (or a 100% continuous-load requirement that pushes the required ampacity over 200) lands on 4/0; (2) the step from 4/0 to 250 kcmil is the point where "larger conductor" stops meaning "one size up AWG" and becomes a kcmil jump (230 → 255 A).

Worked examples (all computed by the shipped core, locked by the test suite)

Rows 1–3 are the same 231.88 A neutral requirement across material/column: 250 kcmil Cu (255 A) at 75 °C, 350 kcmil Al (250 A) at 75 °C, and 4/0 Cu (260 A) at the 90 °C derating base. Rows 4–6 are the same 100 A service-line requirement: 3 AWG Cu at 75 °C, 1 AWG Al at 75 °C (aluminum is two AWG sizes larger at this level), and 1 AWG Cu if the terminations are 60 °C-rated. Rows 7–10 are the small-conductor cluster where 240.4(D) is in play. Rows 11–13 show how much the column choice moves the size for one requirement. Rows 14–15 show the table's edge and what the tool does instead of guessing past it. Every one of these is an assertion in test/run_tests.js (the feature-article block added this session plus the existing 310.16 suite).

Where Table 310.16 fits (and what it is not)

• It is the ampacity table, not a breaker table. It tells you what the conductor can carry; 240.4 and 210.19/215.2 tell you what the overcurrent device may be. For continuous loads, the required ampacity is 125% of the load (210.19(A)/215.2(A)), and the standard-breaker pick follows — that is what PanelWright's breaker sizing does before this table is consulted.
• It is not a voltage-drop table. A conductor that passes the ampacity check can still drop more than the 3%/5% informational recommendations on a long run — see the voltage drop (Ch. 9 Table 8) article, which uses the resistance table, not this one.
• It feeds the service-line and neutral picks. PanelWright's 220.82 card picks the ungrounded service conductors (required ampacity = 100% of the calculated 220.82 service current, floored at the 230.79(C) 100 A disconnect minimum), and the 220.61 neutral card picks the grounded conductor at the 83% one-dwelling minimum — both from this table. See the 220.82 and 220.61 articles.
• It is base values only. 310.15(A)(1) ambient correction and 310.15(B)(1) adjustment (both discussed above) are the designer's; the tool says so on every pick.
• Over the table (past 665 A Cu / 560 A Al at 75 °C) → 310.4 parallel conductors, not a bigger row.

Editions: 2014 / 2017 / 2020 / 2023 / 2026

The ampacity values are stable across 2014 through 2023. The 2023 NEC change analysis records no change to Table 310.16 (it is referenced in section indexes only, not in the change text), and the 2020 and 2023 values match the 2014 edition row-for-row — which is how PanelWright's copy was verified (a 2023-NEC print on disk, checked cell-by-cell, plus four independent live references). 2026: a secondary change-analysis source says the 2026 NEC keeps these ampacity values; that is flagged, not independently verified, and the tool/article remain scoped to the 2017–2023 values. (The 2026 NEC also renumbers Article 310's surroundings in places; the table's values are the part that matters for picks and are unchanged.)

Free Table 310.16 conductor picker

Open the PanelWright neutral / service-line cards →

PanelWright runs this exact table automatically: the 220.61 neutral card picks the grounded conductor (83% one-dwelling minimum), the 220.82 card picks the ungrounded service-line conductors (100% of the calculated service current, 230.79(C) floor) — both with material + temperature-column selects, the 240.4(D) cap notes on small sizes, and the 310.15 "base values only" flag. The same page also covers 220.55 cooking, 220.54 dryers, 220.53 fixed appliances, 220.56 commercial kitchen, 220.42 lighting, voltage drop (Ch. 9), phase balancing, and breaker sizing.

Sources & verification