Wire Size Calculator

Proper wire sizing ensures safety and efficiency.

Undersized wires can overheat, while oversized wires waste money.

Consider both ampacity and voltage drop.

Common sizes:

• 14 AWG (15A)
• 12 AWG (20A)
• 10 AWG (30A)
• 8 AWG (40A)
• 6 AWG (55A)

Aluminum requires 1-2 sizes larger than copper for the same ampacity.

To calculate wire size, you must satisfy two independent requirements and choose the larger result.

First, size for ampacity: the conductor must carry the load current continuously without overheating, using NEC Table 310.16 (or IEC 60364 in Europe).

Second, size for voltage drop with the formula Vdrop = (2 × L × I × R) / 1000, where L is one-way length in meters, I is current in amperes, and R is conductor resistance in Ω/km. The factor of 2 accounts for the round-trip (out and return) current path in single-phase circuits; three-phase circuits use a √3 (≈1.732) factor instead.

Always select the gauge that meets whichever criterion demands the heavier conductor, per the IEEE and IEC design conventions.

Wire size is expressed two ways. In North America the American Wire Gauge (AWG) scale is dimensionless and inverse — a lower number means a thicker conductor — while most of the world uses cross-sectional area in square millimetres (mm²), an SI-derived unit defined by BIPM.

Current (ampacity) is measured in amperes (A), the SI base unit for electric current per NIST. Conductor resistance is given in ohms (Ω) or, for cables, ohms per kilometre (Ω/km).

Voltage drop is measured in volts (V) and often reported as a percentage of system voltage. Power loss appears in watts (W).

For reference, 12 AWG ≈ 3.3 mm² and 10 AWG ≈ 5.3 mm², following IEC 60228 preferred sizes.

AWG converts to cross-sectional area through the wire's diameter. The diameter in millimetres is d = 0.127 × 92^((36 − n)/39), where n is the AWG number; the area is then A = π(d/2)².

For example, 12 AWG gives a diameter of about 2.05 mm and an area near 3.31 mm². Because AWG steps are geometric, each 3-gauge decrease roughly doubles the area, and a 6-gauge decrease doubles the diameter.

In practice, engineers snap the computed value to the nearest standard IEC 60228 size (1.5, 2.5, 4, 6, 10, 16 mm² and up). Encyclopaedia Britannica notes the AWG system dates to 1857, standardising the drawn-wire ratios still used today.

Voltage drop rises linearly with both current and conductor length, so long runs are the dominant reason to upsize wire beyond the minimum ampacity gauge. Because resistance is fixed per unit length, doubling the distance doubles the drop.

The NEC recommends (though does not mandate) a maximum of 3% drop on branch circuits and 5% combined for feeders plus branches, and HyperPhysics (Georgia State University) explains this simply follows Ohm's law, V = IR — the same relationship our ohms law calculator applies to solve for any missing voltage, current, or resistance value.

A 20 A load acceptable on 12 AWG at 15 m may exceed 3% at 60 m, forcing an upgrade to 10 or 8 AWG. This is why outbuildings, well pumps, and EV chargers on long feeders frequently need conductors one or two gauges larger than an ampacity table alone would suggest.

Consider a 20 A single-phase load at 240 V running 30 metres one-way in copper with a 3% voltage-drop limit.

Using 10 AWG (about 5.3 mm²) copper, the calculator returns a voltage drop of roughly 3.93 V, which is about 1.64% of 240 V — well inside the 3% target. The resistive power lost in the conductor is about 78.7 W.

Because 10 AWG copper is rated near 30 A ampacity under NEC 75°C columns, it also comfortably clears the ampacity requirement. Had we chosen 12 AWG, the higher resistance would increase both the voltage drop and I²R heating, so the larger 10 AWG conductor is the correct pick for this run length.

Wire sizing spans residential, commercial, and industrial work.

• Electricians use it to select branch-circuit conductors for outlets, HVAC units, ranges, and subpanels under the NEC.
• EV-charger installers rely on it to feed 32-48 A Level 2 chargers over long garage or driveway runs.
• Solar and battery installers size DC and AC conductors for inverters and combiner boxes, where voltage drop directly cuts harvested energy.
• Marine and RV builders size 12/24 V DC wiring, where low voltage makes drop especially punishing.
• Industrial engineers apply IEC 60364 and IEEE guidance to motor feeders and long cable trays.

In every case the goal is the same: safe ampacity, acceptable voltage drop, and code-compliant conductor selection.

Published ampacity assumes ideal conditions, so real installations must derate.

When more than three current-carrying conductors share a conduit, NEC Table 310.15(C)(1) reduces allowable ampacity (for example, 4-6 conductors are derated to 80%). High ambient temperature also lowers ampacity because the conductor has less thermal headroom before its insulation rating is reached.

Continuous loads — those drawing current for three hours or more — must be sized to 125% of the load, meaning the conductor and breaker carry only 80% of their rating. Insulation type matters too: THHN rated at 90°C tolerates more heat than 60°C THW.

Ignoring these factors is a leading cause of overheated cables, so always derate before finalising a gauge.

Copper and aluminum trade off conductivity against cost and weight.

Copper has lower resistivity (about 1.68 × 10⁻⁸ Ω·m per NIST) so it carries more current per unit area, making it the default for branch circuits.

Aluminum's resistivity is roughly 1.6 times higher, so aluminum conductors must be one to two AWG sizes larger for the same ampacity — but aluminum is far lighter and cheaper, which is why utilities and service-entrance feeders favour it. Modern aluminum wiring is safe when installed with AL-rated terminals and an antioxidant compound to prevent connection failures.

Match the calculator's material setting to your conductor, because the resistance value driving voltage drop differs substantially between the two metals.

Sizing electrical wire has several common mistakes to avoid:

• The most frequent error is sizing for ampacity only and ignoring voltage drop on long runs, leaving lights dim and motors straining.
• A second mistake is using one-way distance in a formula that expects the round-trip factor of 2 — or vice versa — which halves or doubles the calculated drop.
• Others forget derating for conduit fill, ambient heat, or the 125% continuous-load rule, then wonder why cables run hot.
• Confusing AWG direction (thinking bigger numbers mean thicker wire) leads to dangerously undersized conductors.
• Finally, mixing copper and aluminum values, or ignoring the terminal temperature rating that can cap ampacity to the 60°C column, produces non-compliant results.

Always cross-check against the NEC or IEC tables and local code before purchasing.