Which NEC table does this use?

Base ampacities come from NEC Table 310.16, the standard table for conductors in raceways and cables. Temperature correction uses Table 310.15(B)(1) and bundling adjustment uses Table 310.15(C)(1).

Why does the result mention a 75°C termination limit?

NEC 110.14(C) caps the usable ampacity at the temperature rating of the weakest termination, which is 75°C on most modern breakers and lugs. Even if you use 90°C wire, you generally cannot load it beyond its 75°C column value.

Do I count the neutral as a current-carrying conductor?

Usually no. A neutral that carries only the unbalanced current of a balanced multiwire circuit is not counted, and equipment grounding conductors never count. Neutrals on systems with significant harmonic or single-phase loads may count — see NEC 310.15(E).

Does it handle continuous loads at 125%?

No. Enter the already-adjusted design current. For continuous loads, multiply the load by 1.25 before entering it, per NEC 210.19 and 215.2, so the conductor and breaker are sized for the continuous-duty requirement.

Is copper or aluminium better?

Copper carries more current per size and is preferred for small branch circuits and terminations. Aluminium is lighter and cheaper for large feeders but needs one or two sizes larger for the same ampacity and antioxidant compound at terminations.

Wire Gauge Selector — Gera Tools

Sizing a conductor the way an electrician actually does it

Picking a wire gauge is not just looking up an amp value in a single table. The published ampacity in NEC Table 310.16 is a base figure measured at a 30°C ambient with no more than three current-carrying conductors bundled together. Real installations are hotter and more crowded than that, so the base value must be derated before you can trust it. This selector does the full chain: it reads the base ampacity for your material and insulation rating, multiplies by the temperature and bundling factors, and then walks up the table until it finds the smallest standard size whose derated ampacity still covers your load.

How it works

The corrected ampacity of any conductor is:

corrected = base_ampacity × temp_factor × bundling_factor

The base ampacity comes from NEC Table 310.16 for your conductor material (copper or aluminium) and insulation temperature column (60, 75, or 90°C). The temperature factor comes from Table 310.15(B)(1) — it is 1.0 at 30°C and falls as the ambient rises. The bundling factor comes from Table 310.15(C)(1): 0.8 for 4–6 conductors, 0.7 for 7–9, 0.5 for 10–20, and so on. The tool needs corrected >= load, so internally it computes the required pre-derate ampacity as load / (temp_factor × bundling_factor) and selects the first table row that meets it.

One important subtlety it also surfaces: NEC 110.14(C). Terminations on standard equipment are rated 75°C, so the usable ampacity is capped at the 75°C column value for that size even when 90°C insulation gives a higher number. You may derate from the 90°C column, but you cannot load the wire above its 75°C value.

Example and notes

A 40 A continuous-rated feeder of copper THWN (75°C) at 30°C ambient with three conductors needs no derating: 8 AWG copper at 50 A in the 75°C column covers it. Move that same run into a 46°C rooftop conduit with eight conductors and the factors become 0.82 × 0.70 = 0.574 — the 8 AWG now derates to about 29 A and you must step up to 4 AWG. This is exactly the kind of mistake a quick table lookup misses, and why the derating chain matters. Always confirm against the code edition adopted in your jurisdiction and have an installation verified by a licensed electrician.