Find the maximum current capacity for any wire gauge per NEC Table 310.16. Supports copper and aluminum, all temperature ratings, ambient correction, and conduit fill derating.
Ampacity — short for "ampere capacity" — is the maximum continuous current a conductor can safely carry without exceeding its insulation temperature rating. It is one of the most fundamental concepts in electrical wiring design and is governed primarily by NEC (National Electrical Code) Table 310.16, which provides base ampacity values for conductors rated 0-2,000 volts in raceways, cables, or direct buried configurations. Understanding ampacity is critical for electricians, engineers, and anyone involved in electrical installation because undersized wiring creates fire hazards, while oversized wiring wastes money. This guide covers the complete ampacity determination process including base values, temperature correction, conduit fill derating, and practical application rules.
NEC Table 310.16 organizes ampacity data in a grid format. The rows represent wire gauges from 14 AWG to 2000 kcmil. The columns represent three insulation temperature ratings: 60°C (types TW, UF), 75°C (types THW, THWN, XHHW, USE), and 90°C (types THHN, THWN-2, XHHW-2, USE-2). Each combination of wire gauge, material (copper or aluminum), and temperature rating yields a specific base ampacity value. The 75°C column is the most commonly used for general-purpose wiring because most modern wire insulation is rated for 75°C, and most equipment terminations are rated for 75°C as well.
The base ampacity values in NEC Table 310.16 assume an ambient temperature of 30°C (86°F). When the surrounding air temperature exceeds this baseline, the conductor's ability to dissipate heat decreases, and the ampacity must be reduced using correction factors from NEC Table 310.15(B)(1). This is particularly important in hot climates, attics (which can reach 50°C+ in summer), and near heat-producing equipment. For a 75°C-rated conductor at 40°C ambient, the correction factor is 0.88 — meaning a 12 AWG copper wire that normally carries 25 amps is reduced to 22 amps. In an attic at 50°C, the factor drops to 0.75, reducing the same wire to only 18.75 amps. Conversely, cooler environments (below 30°C) actually allow higher ampacity, though this is rarely applied in practice.
When multiple current-carrying conductors share a raceway (conduit, cable tray, or similar enclosure), they generate heat that compounds and reduces the ability of each conductor to dissipate thermal energy. NEC Table 310.15(C)(1) provides adjustment factors based on the number of current-carrying conductors. With 4 to 6 conductors, ampacity is reduced to 80% of the base value. With 7 to 9 conductors, it drops to 70%. The most severe derating occurs with 10 to 20 conductors at 50% — cutting the ampacity in half. It is important to note that only current-carrying conductors count for this calculation. Equipment grounding conductors are never counted. Neutral conductors carrying only unbalanced current from line-to-neutral loads are not counted in three-phase systems (but are counted in single-phase three-wire systems when the neutral carries the full unbalanced current).
The choice between copper and aluminum significantly affects ampacity. Copper has approximately 61% higher conductivity than aluminum, which translates to roughly 25-30% higher ampacity for the same wire gauge. For example, 4 AWG copper carries 85 amps at 75°C, while 4 AWG aluminum carries only 65 amps — a 23% reduction. To achieve equivalent ampacity, aluminum conductors must be upsized by one to two gauge sizes: 2 AWG aluminum (90A) roughly matches 4 AWG copper (85A). While aluminum is significantly cheaper per foot and lighter, it requires special installation considerations including anti-oxidant compound at all connections, AL/CU or CO/ALR rated devices and connectors, and proper torque specifications to prevent loosening due to aluminum's higher thermal expansion rate. For residential feeders and large commercial runs, aluminum is the cost-effective choice. For branch circuits under 100 amps, copper is universally preferred due to its superior reliability and easier installation.
One of the most commonly misunderstood aspects of ampacity is the termination temperature limitation. Even if a conductor has 90°C insulation, the actual ampacity may be limited by the temperature rating of the device it connects to. Per NEC 110.14(C), for circuits under 100 amps, conductors must be sized using the 60°C column unless the equipment is listed and marked for 75°C terminations. For circuits 100 amps and above, the 75°C column is used. However, when applying derating factors (temperature or conduit fill), the 90°C column value may be used as the starting point, and the result is then limited to the applicable 60°C or 75°C column value. This "use 90°C for derating, limit to 75°C" approach is a critical technique for maintaining adequate ampacity in harsh conditions.
In real-world installations, the ampacity determination process follows a specific sequence. First, identify the required current from the load calculation. Next, select a conductor gauge from NEC 310.16 that provides at least that much ampacity in the appropriate temperature column. Then apply any applicable derating factors for ambient temperature and conduit fill. If the derated ampacity falls below the required current, upsize the conductor until it satisfies the derated requirement. Finally, verify that the conductor size is compatible with the overcurrent protection device (circuit breaker or fuse) per NEC Article 240 and that the voltage drop at the installation distance is within acceptable limits (3% for branch circuits, 5% for feeders plus branch circuits combined). This systematic approach ensures safe, code-compliant installations that will perform reliably for decades.
Several installation scenarios require additional ampacity considerations beyond the basic NEC 310.16 table. Rooftop installations are subject to significant temperature adders — conduits on rooftops in direct sunlight can add 33°C to ambient temperature per NEC Table 310.15(B)(3)(c), meaning a roof conduit at 35°C ambient effectively operates at 68°C, dramatically reducing ampacity. Underground installations use NEC Table 310.60 instead of 310.16, with different ampacity values based on soil thermal resistivity and burial depth. Bundled cables (multiple cables run together without maintaining spacing) require the same conduit fill derating as conductors in a raceway. Continuous loads — those operating for three hours or more — must not exceed 80% of the conductor's derated ampacity per NEC 210.20(A), effectively adding another 1.25× multiplier to the required conductor sizing. Understanding these special cases is what separates a competent electrical designer from one who only knows the basic table lookup.