Wire and Conduit Size Calculator

What these tools do

The wire tool reproduces the standard NEC sizing path a designer walks: begin at the 75°C ampacity table (the column most terminations are rated for), correct the ampacity for the actual ambient, adjust it for how many current-carrying conductors share the raceway’s heat, and take the smallest conductor that still covers the load, reporting each factor so the arithmetic can be audited. It then states the largest standard breaker the small-conductor rule allows on the sizes where that rule bites. The fill tool sums conductor areas from Chapter 9 Table 5 against each EMT size’s internal area from Table 4, applying Table 1’s percentage for the conductor count. The drop tool runs the circular-mil identity every electrician knows and frames the result with the code’s own informational guidance rather than a verdict.

How to use them

• Continuous loads need their 125 percent first. The NEC sizes conductors for continuous loads at 125 percent of the load before these tables; multiply a continuous load yourself and enter the result, so the tool’s path stays the published one.
• Count current-carrying conductors honestly. Neutrals carrying unbalanced current count; equipment grounds do not. The adjustment factor is where bundled home runs quietly outgrow their breakers’ assumptions.
• Fill is a legal ceiling, not a pulling promise. A run at the code maximum with bends and distance is a hard pull and a hotter bundle; experienced installers upsize beyond the minimum the tool reports, and jam-ratio cautions apply on three-conductor pulls.
• Voltage drop guidance is informational. The 3 percent branch / 5 percent total figures are the NEC’s fine-print recommendations for efficiency, not enforceable limits (some energy codes and specs do enforce them); equipment with tight voltage tolerance sets its own budget.

The tables and formulas

• Ampacity: NEC Table 310.16, 75°C column. Copper: 14 AWG 20 A, 12: 25, 10: 35, 8: 50, 6: 65, 4: 85, 3: 100, 2: 115, 1: 130, 1/0: 150, 2/0: 175, 3/0: 200, 4/0: 230. Aluminium: 12: 20, 10: 30, 8: 40, 6: 50, 4: 65, 3: 75, 2: 90, 1: 100, 1/0: 120, 2/0: 135, 3/0: 155, 4/0: 180.
• Corrections: ambient factors from Table 310.15(B)(1) (75°C column); count adjustment from Table 310.15(C)(1). Allowable ampacity = table × ambient factor × count factor.
• Fill: Chapter 9 Table 1 (1 conductor 53%, 2 conductors 31%, 3 or more 40%); conductor areas Table 5 (THHN); EMT internal areas Table 4 (1/2 in 0.304 in² rising to 4 in 14.753 in²).
• Voltage drop (single phase): VD = 2 × K × I × L / circular mils, with K = 12.9 for copper and 21.2 for aluminium.

Worked examples

Derating in action. A 30 A load, copper, in a 40°C ambient with six current-carrying conductors: the factors are 0.88 and 0.80, a combined 0.704. Now 12 AWG offers 25 × 0.704 = 17.60 A and 10 AWG 24.64 A, both short; 8 AWG, at 50 × 0.704 = 35.20 A, is the smallest that carries it. The un-derated instinct (10 AWG, it is a 35-amp wire) is wrong by a full two sizes, which is why this tool exists.

Fill. Three 12 AWG THHN conductors total 3 × 0.0133 = 0.0399 in². At the 40 percent rule they need 0.0998 in² of conduit; the half-inch EMT offers 0.304 in² internally, so it passes with the conductors occupying 13.13% of the raceway. Comfortable, legal, and pullable, three different adjectives the results keep separate.

Voltage drop. 12 AWG copper, 16 A, 100 feet one-way on 120 V: VD = 2 × 12.9 × 16 × 100 / 6,530 = 6.32 V, which is 5.27%: past the 5 percent total guidance on the branch alone, and the worked case for upsizing long runs even when ampacity is satisfied.

Reading your results

Read the wire result as a floor: termination temperature ratings, the 125 percent continuous rule, voltage drop and the authority having jurisdiction can all push a size up, never down. Read the breaker cap as a separate fact from ampacity (a conductor can be ampacity-legal and breaker-capped, as 14 and 12 AWG famously are). Read fill percentages against pulling reality on long or bent runs. And read every result with its factors printed beside it: a size quoted without its derating assumptions cannot be checked, and unchecked electrical arithmetic is how warm walls happen.

Limitations

• One code, named editions. Tables are NEC 2020/2023; other editions move table numbers and occasionally values, other countries work to other codes entirely (IEC, CEC variants), and the adopted local edition plus amendments always outrank this page.
• Scope stops where the tables stop. Motor circuits (Article 430), welders, tap rules, neutral counting subtleties, 90°C-column derating paths, free-air and tray ratings, and box fill are each their own calculations and are not folded in.
• THHN and EMT are the modelled pair. Other insulations and raceways have their own Chapter 9 areas; mixing the tool’s THHN areas with another insulation’s conductors misstates fill.
• Nothing here certifies an installation. The disclaimer bears repeating in substance: design and installation belong with qualified people and the inspector’s book, and this page is an estimator with its sources printed, not an approval.

Frequently asked questions

Why the 75°C column and not the 90°C?

Because equipment terminations, not conductor insulation, usually set the ceiling: most breakers and devices are rated 75°C (smaller ones 60/75), and the NEC’s termination rule caps the usable ampacity at the column matching the weakest termination, whatever the wire’s own 90°C rating says. The 90°C column has a legitimate role as the starting point for derating calculations in some paths; the direct-selection path this page implements is the conservative, commonly applicable one, and it says so rather than silently choosing the bigger numbers.

Why did my wire size jump two sizes for a hot attic run?

The worked example is your run: heat attacks from both factors at once, ambient correction because hot insulation sheds heat worse, and count adjustment because bundled conductors heat each other. Multiplied together they consumed 30 percent of the ampacity before the load was even considered. The honest responses are the ones the trade uses: separate the bundles, move the run, or accept the larger conductor the arithmetic insists on.

Is voltage drop a code violation if it exceeds 5 percent?

Under the NEC itself, generally no: the percentages live in informational notes recommending reasonable efficiency, and inspectors do not red-tag them. But specifications, energy codes and equipment requirements can and do make them binding on a given project, and performance suffers regardless of legality (motors run hot, lights dim, electronics sulk), which is why the tool reports the figure prominently while describing its status accurately.

Does conduit fill change with mixed conductor sizes?

Yes: the rule sums actual areas, so a mixed bundle is the sum of each size’s Table 5 area against the same percentage ceiling. This tool models the common same-size case the searcher usually means; a mixed run can be priced by entering each group through the density of its area share manually, or exactly with the tables the page cites, and the jam-ratio caution for three-conductor pulls applies either way.

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Method and sources: NEC 2020/2023, Table 310.16 ampacities at 75°C, Tables 310.15(B)(1) and 310.15(C)(1) factors, 240.4(D) small-conductor caps, Chapter 9 Tables 1, 4 and 5, and the circular-mil voltage-drop identity. Last reviewed: 14 October 2026.