# Wire Resistance Calculator

Calculate wire resistance, voltage drop, and ampacity by AWG gauge, length, and material. Copper and aluminum support. Free online electrical calculator.

## What this calculates

Wire resistance depends on the conductor material, length, and cross-sectional area (determined by the AWG gauge). This calculator computes the resistance of a wire run, the voltage drop at a given current, and the maximum safe current capacity according to standard ampacity ratings.

## Inputs

- **Wire Gauge (AWG)** — options: 10 AWG, 12 AWG, 14 AWG, 16 AWG, 18 AWG, 20 AWG, 22 AWG, 24 AWG — American Wire Gauge size.
- **Wire Length** (ft) — min 0 — Total length of the wire run in feet.
- **Wire Material** — options: Copper, Aluminum — Conductor material.
- **Current (for voltage drop)** (A) — min 0 — Current flowing through the wire (used to calculate voltage drop).

## Outputs

- **Wire Resistance** (Ω) — Total resistance of the wire.
- **Voltage Drop** (V) — Voltage drop across the wire at the given current.
- **Max Current Capacity** (A) — Maximum recommended current for this wire gauge (NEC ampacity).
- **Wire Diameter** (mm) — Conductor diameter for the selected AWG gauge.

## Details

Wire resistance is calculated using R = ρL/A, where ρ is the resistivity of the conductor material, L is the wire length, and A is the cross-sectional area. Copper has a resistivity of 1.724 × 10-8 Ω·m, making it the most common conductor. Aluminum has higher resistivity (2.65 × 10-8 Ω·m) but is lighter and cheaper, often used for power distribution.

The American Wire Gauge (AWG) system defines wire diameters using the formula d = 0.127 × 92(36-AWG)/39 mm. Lower AWG numbers mean thicker wire with lower resistance. Common household wiring uses 14 AWG (15A circuits) and 12 AWG (20A circuits).

Voltage drop is a critical concern in long wire runs. The NEC recommends no more than 3% voltage drop for branch circuits and 5% total from service entrance to the farthest outlet. Excessive voltage drop wastes energy as heat and can cause equipment to malfunction. To reduce voltage drop, use thicker wire (lower AWG) or shorter runs.

## Frequently Asked Questions

**Q: What AWG wire should I use for a 15-amp circuit?**

A: 14 AWG copper wire is rated for 15 amps per the NEC. For longer runs where voltage drop is a concern, consider upsizing to 12 AWG.

**Q: How does wire length affect resistance?**

A: Resistance is directly proportional to length. Doubling the wire length doubles the resistance and the voltage drop. For long runs, use a thicker gauge to compensate.

**Q: Why is aluminum wire cheaper but requires thicker gauge?**

A: Aluminum has about 61% the conductivity of copper, so it needs a larger cross-section (lower AWG number) to carry the same current safely. However, aluminum is lighter and less expensive per foot.

**Q: Does this account for round-trip wire length?**

A: No. Enter the total conductor length. For a circuit, the current must travel to the load and back, so the effective length is typically twice the physical distance.

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Source: https://vastcalc.com/calculators/technology/wire-resistance
Category: Technology
Last updated: 2026-04-21