Wire Size Calculator — AWG Gauge for Any Amperage
Enter the load current, load type (continuous or not), conductor material and run length to get the minimum NEC-compliant wire gauge plus the voltage drop across the run.
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How does this calculator work?
Pick the smallest AWG or kcmil conductor whose NEC 310.15 (75 °C, in conduit) ampacity meets or exceeds the required ampacity: load current × 1.25 for continuous loads, × 1.0 otherwise. Check voltage drop with VD = 2 × L × R / 1000 × I and keep it under 3% of system voltage.
Formula
How this is calculated
Wire sizing in the United States is governed by the National Electrical Code (NEC). The primary requirement is ampacity — the maximum current a conductor can carry continuously without overheating. Ampacity depends on wire gauge (AWG or kcmil), the conductor material (copper or aluminium), the insulation temperature rating, the ambient temperature, and whether the wire runs in conduit or open air. This calculator uses the NEC Table 310.15(B)(16) values for 75 °C-rated conductors in conduit at 30 °C ambient — the standard column used for most outlet, panel and sub-panel wiring.
For loads that run continuously (three or more hours without interruption), NEC 210.19(A) requires the wire to be rated for at least 125% of the load current. For example, a 100 A continuous load needs wire rated for at least 125 A — a 2/0 AWG copper (175 A) passes; 1/0 AWG copper (150 A) also passes. The calculator finds the smallest gauge in the table that meets or exceeds the required ampacity.
Voltage drop is calculated using the round-trip formula VD = 2 × L × R / 1000 × I, where L is the one-way run length in feet and R is the conductor resistance in ohms per 1,000 feet. NEC recommends keeping voltage drop below 3% for branch circuits and 5% for the combined feeder + branch circuit. Aluminium conductors are lighter and cheaper but have higher resistance than copper for the same gauge and must be two AWG sizes larger to match copper ampacity. Values are based on NEC 2023 tables and are for informational guidance only — always consult a licensed electrician for code-compliant installations.
Frequently asked questions
A 100 A continuous feeder requires wire rated for 125 A (100 × 1.25). The minimum copper conductor is 1/0 AWG (150 A rated at 75 °C), or 3/0 AWG aluminium (155 A). Many electricians use 2 AWG copper or 1/0 AWG aluminium for the shorter 100 A runs. Check local code and confirm with a licensed electrician.
Undersized wire has higher resistance, causing a larger voltage drop along the run. Motors and appliances at the far end see a lower voltage, running hotter and less efficiently. NEC recommends keeping branch-circuit voltage drop below 3% (e.g. 3.6 V on a 120 V circuit). Longer runs often require a larger gauge than the ampacity alone demands.
AWG (American Wire Gauge) runs counterintuitively: smaller AWG number = larger wire. 14 AWG is the smallest common household wire; 4/0 AWG (0000) is very large. For wires larger than 4/0 AWG, the industry switches to kcmil (thousands of circular mils), a direct measure of cross-sectional area — 250 kcmil is larger than 350 kcmil in AWG terms.
Also known as
TG we-Calculate Editorial Team. (2026). Wire Size Calculator — AWG Gauge for Any Amperage [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/100-amp-wire-size-calculator
TG we-Calculate Editorial Team. "Wire Size Calculator — AWG Gauge for Any Amperage." TG we-Calculate. 2026. https://we-calculate.com/calculator/100-amp-wire-size-calculator.
TG we-Calculate Editorial Team, "Wire Size Calculator — AWG Gauge for Any Amperage," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/100-amp-wire-size-calculator
@misc{wecalculate_100_amp_wire_size_calculator, title = {Wire Size Calculator — AWG Gauge for Any Amperage}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/100-amp-wire-size-calculator}}, year = {2026}, note = {TG we-Calculate} }
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