Convert amperage to wire gauge using NEC ampacity tables. Copper and aluminum, temperature ratings, conduit fill, and ambient temperature derating.
Selecting wire gauge based on amperage is the most fundamental task in electrical wiring. The National Electrical Code (NEC) Table 310.16 specifies the maximum current each wire size can safely carry, depending on conductor material, insulation temperature rating, and installation conditions.
However, simply looking up the base ampacity is not enough. The NEC requires derating when multiple conductors share a conduit (reducing heat dissipation) or when ambient temperature exceeds the standard 30°C. These derating factors can reduce the effective ampacity by 20-50%, often bumping you up to the next wire size.
This calculator applies NEC ampacity tables with both conduit fill and temperature derating, showing the minimum wire gauge that safely carries your required current. It displays a complete table so you can evaluate options and understand the margin at each gauge. Check the example with realistic values before reporting. Use the steps shown to verify rounding and units. Cross-check this output using a known reference case.
Looking up NEC tables and applying derating factors is a multi-step process that is easy to get wrong. Forgetting conduit derating or ambient temperature correction can result in an undersized conductor — a code violation and safety hazard.
This calculator automates the entire process: select your conditions and instantly see the minimum compliant wire gauge with full derating applied. The comparison table shows every gauge and its derated ampacity, so you can also evaluate the cost/benefit of upsizing for future capacity.
Derated Ampacity = Base Ampacity × Conduit Factor × Temperature Factor. Select the smallest wire whose derated ampacity ≥ required amps. Conduit factors: 1-3 conductors = 100%, 4-6 = 80%, 7-9 = 70%, 10+ = 50%.
Result: 10 AWG copper
10 AWG copper at 75°C has 35A ampacity. With 3 conductors at 30°C, no derating applies. 35A > 30A with a 17% margin.
NEC Table 310.16 is the most-referenced table in electrical work. It lists the allowable ampacity of insulated conductors from 14 AWG to 2000 kcmil for copper and aluminum at three temperature ratings: 60°C, 75°C, and 90°C. These values assume no more than 3 current-carrying conductors in a raceway at an ambient temperature of 30°C.
When conditions differ from these assumptions, the NEC requires adjustment. Table 310.15(C)(1) provides derating factors for more than 3 conductors in a raceway, and Table 310.15(B)(1) provides correction factors for ambient temperatures above 30°C. Both factors are multiplicative with the base ampacity.
In practice, wire selection involves more than just ampacity. You must also consider voltage drop (especially for long runs), physical size (will it fit in the conduit and boxes), flexibility (stranded vs. solid), and the installation environment (wet, dry, underground, exposed to sunlight). Ampacity sets the minimum gauge, but these other factors may dictate a larger wire.
Aluminum conductors have about 78% of the ampacity of copper at the same gauge — roughly equivalent to copper two sizes smaller. Aluminum is lighter and cheaper, making it preferred for large feeders and service entrance cables. For branch circuits (15-20A), copper is standard because aluminum requires special connectors and is more susceptible to connection problems at small gauges.
10 AWG copper at 75°C (35A ampacity). For aluminum, 8 AWG (40A). If you have more than 3 conductors in conduit or high ambient temperature, you may need to upsize.
Different insulation types have different maximum operating temperatures. THHN wire is rated for 90°C, THW for 75°C, and TW for 60°C. Higher temperature rating allows more current for the same gauge.
NEC requires derating when more than 3 current-carrying conductors share a conduit. Neutral conductors carrying only unbalanced current are not counted. Ground wires are never counted.
You can use 90°C wire, but the ampacity must be limited to the 75°C column because the terminals are only rated for 75°C. However, the 90°C rating does help when derating — you apply derating factors to the 90°C base, then verify the result does not exceed the 75°C column.
NEC requires minimum 14 AWG for 15A circuits and 12 AWG for 20A circuits in residential wiring. Many local codes require 12 AWG minimum for all general-purpose circuits.
Not directly — ampacity is about heat from current flow, not voltage. However, higher voltage allows less current for the same wattage, so 240V circuits use smaller wire than 120V circuits for the same power.