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: September 2026
Formula below · 3 sources (omnicalculator.com, nfpa.org, Wikipedia) · Updated Sep 2026
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About this calculator
Choosing a wire size means meeting two separate limits, and this calculator returns the smallest standard copper conductor (14 AWG to 750 kcmil) that meets both. 1) Ampacity, from NEC Table 310.16: the conductor must carry the load current, multiplied by 1.25 for a continuous load (NEC 210.19(A)(1)). Required ampacities of 100 A or less are checked against the 60 °C column, the NEC 110.14(C)(1)(a) default when terminal ratings are not known, and larger ones against the 75 °C column. The NEC 240.4(D) limits also apply: 14 AWG may be protected at no more than 15 A, 12 AWG at 20 A and 10 AWG at 30 A. 2) Voltage drop: Vdrop = 2 × I × L × R / 1,000, where L is the one-way length in feet and R is the conductor resistance in ohms per 1,000 ft from NEC Chapter 9 Table 8 (stranded copper at 75 °C; 12 AWG = 1.98, 10 AWG = 1.24, 8 AWG = 0.778). Vdrop must not exceed the percentage you enter of the system voltage; 3% for a branch circuit and 5% total for feeder plus branch are the NEC informational-note targets (210.19(A) and 215.2(A)). Sizes 1/0 AWG and larger are shown as text. Assumptions: copper conductors, 30 °C ambient, no more than three current-carrying conductors together, single-phase two-wire circuit (a three-phase run drops less, so the result stays on the safe side). Higher ambient temperatures, bundling more than three conductors (NEC 310.15) or aluminum conductors need a larger size.
How to use
Example 1: 20 A continuous load, 50 ft one way, 3% drop, 120 V. Ampacity: 20 × 1.25 = 25 A, so at least 10 AWG (30 A). Voltage drop: allowed 3.6 V; 12 AWG would drop 2 × 20 × 50 × 1.98 / 1,000 = 3.96 V, and 10 AWG drops 2.48 V. Result: 10 AWG. Example 2: 30 A over 100 ft, 3% drop, 120 V. 8 AWG drops 2 × 30 × 100 × 0.778 / 1,000 = 4.67 V (too much); 6 AWG drops 2.95 V. Result: 6 AWG. The same 30 A load at 240 V allows 7.2 V, so 8 AWG (4.67 V) passes, and 8 AWG is also what the continuous-load ampacity (37.5 A) requires.
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 ampacity minimum is 12 AWG copper (NEC 240.4(D)), or 10 AWG if the 20 A load is continuous (20 × 1.25 = 25 A). Voltage drop then decides on longer runs: at 120 V and 3%, 12 AWG reaches only about 45 ft at a full 20 A, 10 AWG about 72 ft, 8 AWG about 115 ft and 6 AWG about 183 ft. So a 100 ft, 20 A, 120 V circuit needs 8 AWG and a 200 ft run needs 4 AWG. 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. Mixing up one-way and round-trip distance: this calculator asks for the one-way length and doubles it internally. Mixing aluminum and copper conductors at terminations without proper antioxidant compound and approved AL/CU connectors causes galvanic corrosion. Failing to adjust ampacity for ambient temperature (above 30 °C) or for more than three current-carrying conductors in a raceway under NEC 310.15. Forgetting the 125% rule for continuous loads. Terminals must be torqued to the manufacturer’s specification to avoid loose, overheating connections.
When should I NOT rely solely on this wire gauge calculator?
This calculator is for copper. Aluminum has about 1.6 times the resistance of copper and lower ampacity, so it typically needs a conductor two AWG sizes larger; use the aluminum columns of NEC Table 310.16 and Chapter 9 Table 8. For high-frequency AC (above ~30 kHz), skin effect raises the effective resistance — 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. For solar PV strings, NEC Article 690 imposes specific current calculations (1.25 × short-circuit current and more). Underground burial, high ambient temperature and bundling all reduce 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.