Wire Gauge (AWG) & Voltage Drop Calculator

Conductor sizing is one of the most consequential decisions in electrical installation design. A wire that is too small for its load will overheat, degrade its insulation, and eventually create a fire hazard. Even when the conductor can carry the current without overheating, the resistance of a long run produces a voltage drop that can starve the load of the voltage it needs to operate correctly. Wire gauge selection therefore has two independent requirements: adequate ampacity and acceptable voltage drop.

This calculator returns the physical dimensions of AWG and metric conductors and computes the two-way voltage drop for DC and single-phase AC circuits.

Governing Formulas

Voltage drop in a two-wire DC or single-phase AC circuit is calculated from Ohm's law applied to the round-trip conductor length:

V_drop = 2 × I × L × R_{per unit length}

where I is the load current in amperes, L is the one-way run length, and R_{per unit length} is the conductor resistance per unit length (for example Ω/m or Ω per 1,000 ft). The factor of two accounts for current flowing out to the load and back. The percentage drop is then:

V_drop% = (V_drop / V_source) × 100

Conductor resistance itself is derived from the resistivity of the material and the geometry:

R = ρ L / A

where ρ is the resistivity (approximately 1.72 × 10⁻⁸ Ω·m for annealed copper at 20 °C) and A is the cross-sectional area of the conductor. Because resistance is inversely proportional to area, doubling the conductor area halves the voltage drop.

AWG and Metric Sizing

The American Wire Gauge system is logarithmic: increasing AWG number by 6 approximately halves the conductor area, and decreasing AWG number by 3 approximately doubles it. AWG 10 has roughly twice the area of AWG 13, and AWG 4 roughly twice the area of AWG 7. Metric conductor sizes (mm²) are simple geometric areas, which makes them easier to compare directly. Common approximate equivalents include AWG 14 ≈ 2.08 mm², AWG 12 ≈ 3.31 mm², AWG 10 ≈ 5.26 mm², and AWG 8 ≈ 8.37 mm².

Input Parameters

• Gauge standard: AWG or metric cross-sectional area. Choosing the wrong standard changes the resistance dramatically.
• System voltage: The nominal source voltage (for example 12 V, 24 V, 120 V, 240 V). Lower voltage systems are far more sensitive to voltage drop, because the permitted drop is a fixed percentage of a small number.
• Load current: The maximum steady-state current the load draws. Motor starting currents and inrush must be evaluated separately for short-duration drop.
• Run length: One-way distance from source to load. The calculator applies the factor of two for the return path.
• Conductor material: Copper or aluminium. Aluminium has roughly 60% of the conductivity of copper, so a larger cross-section is needed for the same resistance.

Engineering Interpretation

Ampacity and voltage drop address different failure modes. Ampacity is about the thermal limit of the insulation: exceeding it overheats the conductor and can start a fire. Voltage drop is about performance: excessive drop causes motors to stall or run hot, electronics to reset, and lights to dim. The National Electrical Code recommends that voltage drop in branch circuits not exceed 3%, with a total of 5% for the combined feeder and branch circuit. Meeting the ampacity requirement does not automatically satisfy the voltage drop requirement, and vice versa; both must be verified.

Units and Conversion

• 1 AWG step changes area by a factor of approximately 1.261
• Circular mils (CM) = (diameter in mils)², used in US conductor tables
• 1 mm² ≈ 1,973 circular mils
• Copper resistivity at 20 °C ≈ 1.72 × 10⁻⁸ Ω·m
• Aluminium resistivity at 20 °C ≈ 2.82 × 10⁻⁸ Ω·m

Alternative and General Cases

This calculator handles DC and single-phase AC circuits. Three-phase circuits require a different voltage drop formula in which the factor of two is replaced by √3 (approximately 1.732) for the line-to-line case, because the return current is shared among the phases. The calculation also assumes steady-state conditions and unity power factor; in AC circuits with significant reactive load, the impedance rather than the resistance governs the drop, and both resistance and inductive reactance must be considered, especially for large conductors and long runs. Skin effect, proximity effect, and temperature rise above 20 °C further increase resistance and are not modelled here.

Assumptions and Limitations

• Steady-state DC or single-phase AC operation.
• Unity power factor; reactive impedance is neglected.
• Conductor temperature assumed near 20 °C.
• No skin effect, proximity effect, or mutual heating.
• No derating for conduit fill, ambient temperature, or bundling.
• Load current assumed constant; motor starting currents are not considered.
• The result does not constitute a code compliance or protection study.

Practical Engineering Use

Wire gauge and voltage drop calculations appear in virtually every electrical installation: residential branch circuits, industrial motor feeders, off-grid solar arrays, battery banks, marine and RV wiring, and telecom power distribution. They are used to select conductor size for a given load and distance, to verify that an existing run can accept additional load, and to diagnose performance problems such as motors running hot or equipment resetting under load. Conductors should always be selected from the tables in the applicable code or standard, using the ampacity and voltage drop criteria together, and installation should be verified by measurement on commissioning.