The Ampacity Calculator determines the maximum current a conductor can safely carry by applying NEC temperature and conduit fill correction factors to base ampacity values. Ensures code-compliant wiring design for residential, commercial, and industrial electrical installations.
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The calculator for ampacity determines the maximum continuous current a conductor can carry without exceeding its temperature rating, applying the National Electrical Code (NEC) correction factors for ambient temperature and conduit fill that reduce the base ampacity from standard tabulated values. Proper ampacity determination is fundamental to electrical system safety and code compliance.
The starting point is the NEC Table 310.15(B)(16) ampacity at 30°C ambient with no more than 3 current-carrying conductors in a raceway:
Aluminum conductors have approximately 80% the ampacity of equivalent-gauge copper due to higher resistivity. The wire size calculator selects the appropriate conductor gauge for a given load current and distance using these same ampacity principles.
When ambient temperature exceeds 30°C, the base ampacity must be reduced. For 90°C insulation (THHN) in 40°C ambient: correction factor = √[(90−40)/(90−30)] = 0.913. In 60°C ambient (common in attics and engine rooms): correction factor = √[(90−60)/(90−30)] = 0.707 — a 29% reduction. Hot location derating is frequently overlooked in residential wiring and represents a common NEC violation in attic-run conduit. This online calculator applies all correction factors simultaneously.
When more than 3 current-carrying conductors share a raceway, NEC Table 310.15(B)(3)(a) requires derating: 4–6 conductors × 0.80; 7–9 conductors × 0.70; 10–20 conductors × 0.50. When both temperature and conduit fill corrections apply, both factors are multiplied together. A 12 AWG THHN conductor (30A base at 90°C) in a 40°C attic conduit with 6 conductors has allowable ampacity = 30 × 0.913 × 0.80 = 21.9A — effectively limiting the circuit to 20A protection.
Even when conductors have 90°C rated insulation, the NEC limits ampacity at equipment terminals to the 75°C column rating unless the equipment is explicitly listed for 90°C terminations. Most residential and commercial equipment is rated for 75°C terminations — meaning 90°C THHN conductors must be loaded only to their 75°C ampacity at connection points. The voltage drop calculator and wiring calculators provide the complete electrical sizing toolkit.
Base ampacity is looked up from NEC Table 310.12 based on AWG and insulation rating. Temperature correction: F = √((T_rated - T_ambient) / (T_rated - 30)). Conduit derating: ≤3 CCC = 1.0, 4-6 = 0.80, 7-9 = 0.70, 10-20 = 0.50, 21+ = 0.45. Adjusted ampacity = base × temp_factor × conduit_factor.
The adjusted ampacity must be greater than or equal to the load current. The protective device (circuit breaker or fuse) must not exceed the adjusted ampacity. If adjusted ampacity is too low, upgrade the wire size, reduce the number of conductors sharing the raceway, or improve ventilation to lower ambient temperature.
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AWG 10 THWN in an attic at 50°C derated to 24.7A — adequate for a 20A HVAC circuit but not for 30A without upgrading to AWG 8.
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AWG 12 THHN with 9 CCC in conduit derated to 17.5A — insufficient for a 20A circuit. Upgrade to AWG 10 or split conductors into separate conduits.
Ampacity is the maximum current in amperes a conductor can carry continuously under the conditions of use without exceeding its temperature rating. It is determined by conductor size, insulation type, and environmental conditions.
NEC Table 310.12 provides allowable ampacity for conductors rated 0-2000V, 60°C through 90°C insulation, in free air or raceways, at 30°C ambient. It is the primary reference for residential and commercial wiring ampacity.
A neutral conductor in a single-phase 2-wire circuit always counts. In a 3-phase 4-wire wye circuit with non-linear loads (computers, VFDs), the neutral carries triplen harmonic currents and counts. In a balanced 3-phase circuit with linear loads, the neutral typically does not count.
NEC 110.14(C) generally requires that conductors be sized based on 60°C ampacity for circuits ≤100A or AWG 14-1, and 75°C for larger circuits, unless terminals are marked for higher temperatures. This prevents overheating at device terminals even if conductors are rated 90°C.
Aluminum conductors have lower ampacity than copper of the same AWG. Set the conductor material to 2 (aluminum). AWG 12 aluminum = ~20A base at 75°C vs. 25A for copper. Aluminum is generally sized two AWG sizes larger than copper for equivalent ampacity.
The conductor heats above its insulation rating. Insulation becomes brittle, cracks, and may eventually fail, causing a short circuit or ground fault. Prolonged overloading is a leading cause of electrical fires.
You can use the 90°C ampacity for derating calculations (temperature and conduit fill corrections), but the final ampacity must not exceed the 75°C or 60°C rating at the terminals as required by NEC 110.14(C).
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