Electrical Wire Sizing Calculator
Determines the minimum wire cross-sectional area needed to keep voltage drop within safe limits for a given load current and run length. Use it when wiring outlets, motors, or solar systems to avoid overheating and energy loss.
About this calculator
Wire resistance causes a voltage drop proportional to current and conductor length. The required conductor cross-section (in circular mils or mm²) is found by rearranging Ohm's Law for distributed resistance. The formula used here is: conductor area = (2 × ρ × current × distance) / (voltage × allowable voltage drop fraction). The factor of 2 accounts for the round-trip path of current through both the hot and return conductors. Conductor resistivity (ρ) differs between copper (≈10.4 Ω·cmil/ft) and aluminum (≈17 Ω·cmil/ft). A lower allowable voltage drop percentage demands a larger, lower-resistance wire. NEC guidelines typically limit voltage drop to 3% for branch circuits and 5% for feeder plus branch combined.
How to use
Suppose you're running a 20 A circuit at 120 V over 75 feet of copper wire, with a maximum 3% voltage drop (0.03). Allowable drop = 120 × 0.03 = 3.6 V. Copper resistivity constant = 10.4. Area = (2 × 10.4 × 20 × 75) / (120 × 0.03) = 31,200 / 3.6 = 8,667 circular mils. A standard #10 AWG copper wire (~10,380 cmil) comfortably exceeds this, so #10 AWG is the minimum safe choice for this run.
Frequently asked questions
What happens if I use a wire gauge that is too small for the circuit current?
Undersized wire has higher resistance, which causes excessive voltage drop and heat generation. Over time this can degrade insulation, trip breakers, or—in severe cases—start an electrical fire. The NEC mandates minimum wire sizes specifically to prevent these hazards. Always select a gauge whose ampacity exceeds your load current and whose resistance keeps voltage drop within acceptable limits.
How does one-way distance affect wire sizing for long cable runs?
Because current must travel out to the load and return through the neutral or ground conductor, the total resistive path is twice the one-way distance. This means doubling the run length doubles the voltage drop for the same wire size and load. Long runs to outbuildings, motors, or EV chargers often require upgrading one or two AWG sizes beyond what a short run would need, even if the load current is modest.
Why do copper and aluminum wires need different sizes for the same current?
Aluminum has roughly 61% of the conductivity of copper, meaning it has higher resistance per unit cross-section. A copper #10 AWG wire can safely carry currents that would require an aluminum #8 AWG to match. Aluminum is lighter and cheaper per foot, making it common for large feeders and utility service entrances, but connections must use anti-oxidant compound and aluminum-rated terminals to prevent corrosion-related resistance increases.