Wire Size Calculator (NEC Ampacity and Voltage Drop)

Background.

Run a 20 A, 120 V circuit to a workshop 150 ft from the panel and 12 AWG copper satisfies the ampacity table. At full load, though, the 300 ft of wire out and back drops about 11.85 V — nearly 10 % of the supply — and the saw at the far end sees roughly 108 V. Holding the drop to 3 % takes 6 AWG, three sizes heavier, still on a 20 A breaker.

That is the whole problem in miniature. A conductor can be too small for two unrelated reasons. One is heat: current warms the copper, the copper cooks its insulation, and the National Electrical Code's ampacity rules exist to stop that. Those rules are enforceable. The other is voltage: every foot of wire has resistance, so part of the supply is spent getting to the load. Nothing burns, but motors run hot and slow, and the code only recommends a limit. Each constraint gets its own minimum size here, and the larger one is the answer.

The heat side is several rules stacked on Table 310.16 of the 2023 NEC. A load that runs three hours or more needs 125 % of its current in ampacity. Hot surroundings and crowded raceways shrink what a conductor may carry. Section 110.14(C) caps the result at the temperature rating of the breaker or lug the wire lands on. And 240.4(D) limits the breaker on 14, 12 and 10 AWG whatever the table says. There is also a separate path for a one-family dwelling's service or main feeder, which 310.12 lets you size at 83 % of its rating.

This is AC premises wiring only. Low-voltage DC systems and boats follow different rules, and local amendments override the national tables in many places. Use the result to plan and price the job; a licensed electrician must confirm the conductor, the terminations and the overcurrent device before anything is installed.

What is wire size calculator?

Wire size in North America is a conductor's cross-sectional area, named on the American Wire Gauge scale up to 4/0 and in thousands of circular mils (kcmil) above that. The gauge runs backwards, so 14 AWG is thin and 4/0 is thick, and every six gauge steps roughly quadruples the area: 14 AWG is 4,110 circular mils and 8 AWG is 16,510. A circular mil is the area of a circle one thousandth of an inch across.

Ampacity is the current a conductor can carry continuously without exceeding its insulation's temperature rating, and it belongs to the conductor as installed rather than to the wire on the reel. One 8 AWG THHN copper conductor is 55 A in Table 310.16's 90 °C column, 50 A once it lands on 75 °C terminals, 45.1 A run through a 50 °C attic, and 38.5 A sharing a raceway with six to eight others. Voltage drop is the loss along the length, set by resistance and current alone; bundling and ambient temperature do not enter it, and run length does not enter ampacity.

How to use this calculator.

1. 01Choose the sizing basis. The dwelling option covers only the service or main feeder carrying a whole one-family dwelling's load, single-phase 120/240 V, rated 100–400 A — enter that rating as the current. Everything else, three-phase included, is a branch circuit or feeder.
2. 02Enter the load's real running current, not the breaker size, and mark it continuous if it runs three hours or more, as EV charging and electric heat do. Set the nominal voltage and the phase.
3. 03Choose the material and the insulation rating printed on the jacket; most building wire is 90 °C THHN/THWN-2. Set the termination rating from the labels on the breaker and lugs. If you cannot check them, automatic assumes 60 °C on circuits of 100 A or less.
4. 04Enter the hottest ambient temperature along the route and count every current-carrying conductor in the raceway or cable, other circuits included.
5. 05Measure the one-way length along the cable's real path, drops and the panel loop included, and set the drop budget: 3 % for a branch circuit, 5 % for feeder and branch circuit together.
6. 06Read both minimum sizes and which one governs. Then confirm your jurisdiction's adopted code edition and amendments, and have a licensed electrician sign off before buying or pulling wire.

The formula.

The ampacity path runs in a fixed order. First comes the required current: the load itself, 1.25 × the load if it is continuous (NEC 210.19(A)(1)), or 0.83 × the rating on a one-family dwelling service or feeder (310.12). Next, each conductor size's Table 310.16 figure is taken from the column matching its insulation and multiplied by two factors: the ambient correction from Table 310.15(B)(1), which is 1.00 at the table's 30 °C basis and falls to 0.82 at 46–50 °C for 90 °C wire, and the bundling adjustment from Table 310.15(C)(1) — 0.80 for four to six current-carrying conductors, 0.70 for seven to nine, 0.50 for ten to twenty, then 0.45, 0.40 and 0.35. The ambient bands are whole degrees, so 30.1 °C already counts as the 31–35 °C band.

Section 110.14(C) then caps the derated figure at the termination column's ampacity. On automatic, the termination is taken as 60 °C when the circuit's breaker would be 100 A or less and 75 °C above that; the dwelling path always uses 75 °C. Finally, 240.4(D) rules out 14, 12 and 10 AWG copper when the required ampacity exceeds 15, 20 or 30 A, and 12 and 10 AWG aluminium above 15 or 25 A. The smallest size that survives all of that is the ampacity minimum.

The voltage-drop path is one equation solved for area: CM = M × K × I × L ÷ (V × budget % ÷ 100), where M is 2 for single phase (out and back) or √3 for three phase, L is the one-way length in feet, and K is 12.9 for copper or 21.2 for aluminium — the resistance in ohms of a one-circular-mil, one-foot conductor at 75 °C, from NEC Chapter 9 Table 8. I is the real running current, not the 125 % figure, because the extra 25 % is a heating margin rather than current that flows; on the dwelling path the 83 % figure is used. The smallest size with at least that many circular mils is the voltage-drop minimum, and the larger of the two minimums is recommended.

The breaker shown is the smallest standard 240.6(A) rating at or above the required ampacity, held to the 240.4(D) limit on small conductors; on the dwelling path it is the service rating.

The drop equation uses DC resistance at unity power factor and ignores reactance, which suits the common sizes; from about 2/0 upward inductance matters and NEC Chapter 9 Table 9 is the better tool. Table sizing also has edges: ambients beyond an insulation's column (above 55 °C for 60 °C wire, 70 °C for 75 °C wire, 85 °C for any), loads beyond a single 1000 kcmil conductor, which need engineered parallel sets, and circuits with their own rules, such as motors (Article 430) and air-conditioning equipment (Article 440).

A worked example.

Frequently asked questions.

References& sources.

1. [1]National Fire Protection Association, NFPA 70 — National Electrical Code, 2023 edition. Table 310.16, "Ampacities of Insulated Conductors with Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30 °C (86 °F)". Source of every ampacity in this calculator. The code text itself is gated behind NFPA's free-access registration; the values used here were read from two independent reproductions and cross-checked against each other. Retrieved 2026-07-29.
2. [2]NECA/IBEW Electricians, reproduction of NEC Table 310.15(B)(16) (formerly Table 310.16), "Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 60 °C Through 90 °C". Read directly from the PDF on 2026-07-29. Used as the primary transcription source for the ampacity table. This is the 2011-edition numbering; every value is identical to the 2023-edition Table 310.16.
3. [3]Conduit.site, reproduction of NEC Table 310.15(B)(1), "Ambient Temperature Correction Factors Based on 30 °C (86 °F)", 2020 NEC. Retrieved 2026-07-29. Source of all thirty-nine correction factors implemented here, across the 60 °C, 75 °C and 90 °C columns and the sixteen ambient bands from '10 °C or less' to '81–85 °C', including the points at which the 60 °C and 75 °C columns terminate and the conductor may no longer be used.
4. [4]Mike Holt Enterprises, "Voltage Drop Calculations". Retrieved 2026-07-29. Source of the K constants used here — 12.9 ohm-circular-mils per foot for copper and 21.2 for aluminium, both defined as the DC resistance of a 1000-circular-mil conductor 1000 ft long at 75 °C, derived from NEC Chapter 9 Table 8 — of the VD = 2·K·I·L/CM and √3·K·I·L/CM forms, and of the stated limit that the method is suitable up to 1/0 with self-inductance needing a Q adjustment at 2/0 and above.
5. [5]Zing², reproduction of NEC Table 310.15(C)(1), "Adjustment Factors for More Than Three Current-Carrying Conductors", noted as unchanged across the 2020, 2023 and 2026 editions. Retrieved 2026-07-29. Source of the 80 / 70 / 50 / 45 / 40 / 35 percent bands and of the rule that grounding conductors and the neutral of a balanced three-phase system are excluded from the count.
6. [6]WireRef, "NEC Article 240 — Overcurrent Protection", 2023 edition with 2026 updates noted. Retrieved 2026-07-29. Source of the 240.6(A) standard ampere ratings list (15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600 A and above), of the 240.4(D) small-conductor limits for copper (14, 12 and 10 AWG at 15, 20 and 30 A — the page does not reproduce the aluminium limits of 15 A on 12 AWG and 25 A on 10 AWG, which are stated in 240.4(D) itself), and of the 240.4(B) next-higher-standard-rating allowance.
7. [7]American Boat and Yacht Council, Standard E-11, "AC and DC Electrical Systems on Boats", excerpts published by Paneltronics. Read directly from the PDF on 2026-07-29. Cited as the scope boundary rather than as an input: E-11.14.2.6 requires 3 percent maximum drop on main feeders, bilge blowers, electronic equipment and navigation lights and 10 percent on non-critical circuits, and E-11.14.2.7 sizes conductors from ABYC's own Tables VI, IX and X. Marine wiring must be sized from those tables, not from the NEC tables this page implements.