Size electrical wire by ampacity and voltage drop. Shows both AWG and metric mm² recommendations for copper or aluminum conductors.
Circuit parameters
Conductor & installation
Common circuit presets
Recommended wire size
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Voltage drop
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Drop (V)
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Ampacity
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Area (mm² / cmil)
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Formula
Single phase: Vdrop = 2 × I × R × L
Three phase: Vdrop = √3 × I × R × L
where I is current (A), R is wire resistance per unit length, and L is one-way distance.
Round trip factor (2 or √3) and length conversion are applied automatically.
AWG ampacity table (NEC 310.16)
AWG
mm²
60 °C
75 °C
90 °C
V-drop %
Ampacities shown for the selected conductor material. Row highlighted in green is the recommended size.
Frequently asked questions
AWG (American Wire Gauge) is the US/Canadian standard — a logarithmic numbering where a smaller number means a thicker conductor. Metric sizing uses the nominal cross-section in square millimetres (mm²) and is used in most of the world. Common equivalents: 14 AWG ≈ 2.08 mm², 12 AWG ≈ 3.31 mm², 10 AWG ≈ 5.26 mm², 8 AWG ≈ 8.37 mm², 6 AWG ≈ 13.3 mm². This calculator shows both in the recommendation so you can pick whatever is sold locally.
Copper has about 61 % higher conductivity per unit area, so copper is typically two AWG sizes smaller than aluminum for the same load. Aluminum is lighter and cheaper per ampere but requires anti-oxidation compound at terminations and approved AL-rated lugs. Copper is standard for branch circuits inside homes; aluminum is common for service entrance feeders and large sub-panel runs where the weight and cost savings matter. Never mix copper and aluminum at a termination without a listed connector.
Ampacity tells you the wire can carry the current safely without melting the insulation. Voltage drop is a different problem: on a long run, enough voltage can be lost in the wire that the load at the far end runs on reduced voltage. Motors overheat, incandescent lights dim, electronic power supplies work harder, and LED drivers may misbehave. The NEC recommends keeping the drop under 3 % on a branch circuit and 5 % total (feeder + branch). This calculator enforces whichever limit you enter.
These are the maximum operating temperatures of the wire’s insulation. Higher-rated insulation tolerates more heat, so the same AWG can carry more current. Common insulation types: TW, UF → 60 °C; THW, THWN, USE → 75 °C; THHN, XHHW-2 → 90 °C. Important: even if the wire is rated 90 °C, terminations at panels and breakers are usually only rated 60 or 75 °C, and NEC 110.14(C) requires you to use the lower column for sizing. This calculator gives you all three columns so you can match the limiting factor.
For single-phase circuits the current travels down one conductor and back, so the voltage drop is 2 × I × R × L. For balanced three-phase, the return path is shared between phases and the effective round-trip factor becomes √3 ≈ 1.732 instead of 2. That means a three-phase run has about 13.4 % less voltage drop than a single-phase run of the same length, current and wire size. The calculator applies this automatically when you select Three phase.
No. This calculator is for estimation and study. Real installations must follow the applicable electrical code (NEC in the US, CSA in Canada, IEC/BS in Europe) and consider correction factors for ambient temperature, number of current-carrying conductors in the same raceway, continuous loads (80 % rule), and motor starting currents. Always have safety-critical work inspected by a licensed electrician.
Values follow NEC Table 310.16 for 30 °C ambient, not more than three current-carrying conductors. No correction factors applied.
The Wire Size Calculator determines the correct conductor gauge for a residential or commercial circuit based on load current, voltage, one-way distance and a target voltage drop percentage. It checks both ampacity (NEC Table 310.16) and voltage drop in the same pass, so the recommended size always satisfies both limits. Select copper or aluminum, single-phase or three-phase, and an insulation temperature rating of 60, 75 or 90 °C. Results are returned in American Wire Gauge and the next standard metric mm² size, along with actual voltage drop, ampacity and cross-section. Example: a 20 A, 120 V branch circuit over 75 ft of copper with a 3 % drop target sizes to 10 AWG (≈ 6 mm²). Presets cover common residential feeds: 15 A lighting, 20 A outlet, 30 A dryer, 50 A range, 60 A sub-panel and 200 A service.