Wire Gauge Calculator
Determines the minimum copper wire gauge (AWG) needed to limit voltage drop in an electrical run. Ideal for electricians, solar installers, and DIYers sizing cables for long runs.
Last updated: May 2026
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About this calculator
Choosing the correct wire gauge ensures a circuit stays within a safe voltage drop, typically 3% for branch circuits and 5% combined (feeder + branch) per NEC informational notes 210.19(A) and 215.2(A)(1). This calculator uses a simplified relationship: gauge_index = round(10.75 × current × distance / voltage_drop), where 10.75 is a constant derived from the resistivity of copper (10.75 Ω·circular-mil/ft at 20 °C) combined with unit conversions appropriate for AWG output from inputs in feet, amperes, and percent. Variables: current (A, the load current), distance (ft, one-way wire run length), voltage_drop (numeric percentage, e.g., 3 for 3%). The formula approximates the cross-sectional area in circular mils needed; smaller AWG numbers mean physically thicker wires (10 AWG is thicker than 14 AWG). Edge cases: the formula assumes copper conductors at 20 °C; aluminum has roughly 1.6× higher resistivity, requiring 2 AWG sizes larger for equivalent voltage drop. Voltage drop is roughly proportional to distance, so doubling run length requires doubling the cross-sectional area (roughly 3 AWG smaller number). High ambient temperatures de-rate ampacity per NEC 310.15(B)(2); bundling more than 3 current-carrying conductors in a raceway also de-rates per NEC 310.15(B)(3). For high-frequency AC (skin effect) and very long runs (capacitive losses), more advanced models are needed. Maximum voltage drop is also a function of code: NEC permits a maximum 3% on branch circuits and 5% on combined feeder + branch for normal operation. The formula does not account for inrush currents, harmonics, or future load expansion — add safety margins.
How to use
Example 1: 20 A load over 50 ft, 3% allowable drop. Step 1: index = round(10.75 × 20 × 50 / 3) = round(10,750 / 3) = round(3,583) = 3,583. Step 2: this raw index value maps via the calculator's AWG lookup to the appropriate standard gauge (typically 10 AWG for this combination on a 120 V circuit). Verify: NEC Table 8 gives 10 AWG copper resistance ≈ 1.0 Ω/1,000 ft; round-trip 100 ft × 1.0 = 0.1 Ω; voltage drop = 20 A × 0.1 = 2 V or 1.67% of 120 V — well within the 3% target. Example 2: 30 A circuit over 100 ft, 3% drop allowed. Step 1: index = round(10.75 × 30 × 100 / 3) = round(32,250 / 3) = 10,750. Step 2: maps to 8 AWG or larger. Verify: 8 AWG ≈ 0.628 Ω/1,000 ft; round-trip 200 ft × 0.628 = 0.126 Ω; drop = 30 × 0.126 = 3.77 V; on a 240 V circuit that is 1.57% — comfortably within 3%. On a 120 V circuit that would be 3.14% — borderline, suggesting 6 AWG instead.
Frequently asked questions
What wire gauge do I need for a 20 A circuit over a long run?
For a 20 A branch circuit the minimum gauge under NEC 310.16 for ampacity is 12 AWG copper (THHN, 75 °C column), but longer runs require thicker wire to keep voltage drop under 3%. A 100 ft, 20 A, 120 V circuit needs roughly 10 AWG to stay within 3% drop. A 200 ft run at the same load and voltage typically requires 8 AWG. Use the formula or consult NEC Chapter 9 Table 8 (conductor properties) to verify. Always consult local electrical codes — NEC is a US standard; the UK uses BS 7671, Canada uses CEC, and other jurisdictions have local equivalents. Always have any branch-circuit installation done or inspected by a licensed electrician.
How does wire length affect voltage drop and gauge selection?
Voltage drop is proportional to wire length because longer conductors have more resistance: R = ρ × L / A. Doubling the run length doubles the resistance and therefore doubles the voltage drop for the same current. To compensate, you must increase the wire's cross-sectional area — meaning a lower AWG number. Each 3-step decrease in AWG (e.g., 12 → 9 → 6) approximately doubles the cross-sectional area. This is especially important in solar installations, outbuildings, and any long branch circuit where undersized wire would cause equipment to underperform (lights dim, motors stall) or overheat. For runs over 100 ft on a 120 V circuit, voltage drop typically dominates over ampacity in determining the required gauge.
What is the difference between AWG wire gauges and how does the numbering work?
AWG stands for American Wire Gauge, and its numbering system is counter-intuitive: lower numbers mean thicker wire. 10 AWG is thicker and carries more current than 14 AWG. Each three-step decrease in AWG number approximately doubles the cross-sectional area of the wire, halving the resistance. The system originated from the historical number of drawing steps needed to produce wire from a starting blank — more steps produced a thinner wire and a higher number. For wires thicker than 0 AWG, the convention shifts to 00, 000, and 0000 (also written 1/0, 2/0, 3/0, 4/0), and beyond that to thousand-circular-mil (kcmil) sizing. Outside North America, metric wire sizing by cross-sectional area in mm² (e.g., 1.5 mm², 2.5 mm², 4 mm²) is used instead.
What are common mistakes when sizing wire for an electrical run?
Using only the ampacity requirement (e.g., 12 AWG for 20 A) and ignoring voltage drop on long runs leads to dim lights and overheated motors. Using the one-way distance in the voltage-drop formula instead of the round-trip distance (the formula above assumes one-way distance with the 10.75 constant already accounting for the round trip) — different formulas use different conventions, always check. Mixing aluminum and copper conductors at terminations without proper antioxidant compound and approved AL/CU connectors causes galvanic corrosion. Failing to de-rate ampacity for ambient temperature (above 30 °C) or for bundling more than 3 conductors in a raceway under NEC 310.15(B)(2)–(3). Choosing a gauge based on the load's running current but ignoring its starting current (motors can pull 5–7× running current for 1–10 seconds). Forgetting that copper expands with heat — terminals must be torqued to spec and re-torqued after first use to avoid loose connections.
When should I NOT rely solely on this wire gauge calculator?
For aluminum conductors, multiply the calculator's output cross-sectional area requirement by 1.6 (or move 2 AWG sizes thicker) to account for higher resistivity. For high-frequency AC (above ~30 kHz), skin effect concentrates current near the conductor surface and effective resistance increases dramatically — use Litz wire or a frequency-specific calculator. For DC systems above 600 V or AC above 1,000 V, NEC Table 310.16 is replaced by medium-voltage tables and engineering calculation. Three-phase circuits use different voltage-drop formulas and the √3 factor. For solar PV strings, NEC Article 690 imposes specific irradiance-based current calculations and 1.25 × short-circuit current sizing. Industrial motor circuits with frequent starts need extra capacity for starting current and continuous duty cycle. Underground burial in damp earth de-rates ampacity. For any branch-circuit, feeder, or service-entrance installation, consult NEC (or local code) and have work performed by a licensed electrician — a generic calculator is a starting point, not a substitute for engineered design.