Amp to Wire Size Calculator — AWG Wire Gauge from Amperage
Enter your circuit load current and select the conductor material and insulation temperature rating to find the smallest AWG or kcmil wire size that safely carries the load under NEC 2020 rules, including the 125% upsize factor for continuous loads.
A
Conductor material
Insulation temperature rating
Continuous load?
12 AWG — minimum AWG/kcmil that meets NEC ampacity
How does this calculator work?
Find the minimum wire size (AWG/kcmil) by multiplying load amps by 1.25 for continuous loads, then picking the first conductor in NEC Table 310.15(B)(16) whose ampacity meets or exceeds that value. A 20 A continuous load on 75 °C copper needs at least 12 AWG (25 A rated).
Formula
How this is calculated
The NEC (National Electrical Code) rates every conductor size with a maximum continuous ampacity that depends on the insulation temperature rating and conductor material. This calculator uses NEC 2020 Table 310.15(B)(16), which covers insulated copper and aluminum conductors installed in conduit or a raceway — the most common scenario for building wiring.
For continuous loads (circuits energised for three or more consecutive hours, such as HVAC, lighting, or EV chargers), the NEC requires the conductor and overcurrent device to be sized at 125% of the full-load current (per sections 210.19(A) and 210.20(A)). For non-continuous or intermittent loads, no upsize factor is applied. The calculator applies this factor to the raw amperage before searching for the first wire size whose table ampacity meets or exceeds the design current.
Important limitations: these are baseline ampacities without derating for conduit fill, ambient temperature above 30°C, or bundled cables. If your installation involves more than three current-carrying conductors in a conduit, high-temperature environments, or buried cables, additional derating applies and the required wire size may be larger. Always verify the final design with a licensed electrician and check your local jurisdiction's adopted code version.
Frequently asked questions
AWG stands for American Wire Gauge. Counterintuitively, a higher gauge number means a smaller, thinner wire — 14 AWG is thinner than 10 AWG. For very large conductors the system switches to kcmil (thousands of circular mils), where a larger number does mean a larger wire.
THHN wire is physically rated to 90 °C, but NEC 110.14(C) requires that the ampacity used be limited by the temperature rating of the terminations — usually 75 °C on breakers and equipment. You can use the 90 °C column when every component in the circuit (conductors AND terminations) is rated for 90 °C, which is uncommon in most residential and commercial installations.
Copper is preferred for small branch circuits (14–10 AWG) because aluminum requires anti-oxidant compound and special connectors rated for aluminum use. Aluminum is widely used and cost-effective for large feeders and service entrances (6 AWG and above) where the cost and weight savings are significant, provided proper installation practices are followed.
TG we-Calculate Editorial Team. (2026). Amp to Wire Size Calculator — AWG Wire Gauge from Amperage [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/amp-to-wire-size-calculator
TG we-Calculate Editorial Team. "Amp to Wire Size Calculator — AWG Wire Gauge from Amperage." TG we-Calculate. 2026. https://we-calculate.com/calculator/amp-to-wire-size-calculator.
TG we-Calculate Editorial Team, "Amp to Wire Size Calculator — AWG Wire Gauge from Amperage," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/amp-to-wire-size-calculator
@misc{wecalculate_amp_to_wire_size_calculator, title = {Amp to Wire Size Calculator — AWG Wire Gauge from Amperage}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/amp-to-wire-size-calculator}}, year = {2026}, note = {TG we-Calculate} }
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