# Wire Size Calculator

Calculate the right wire gauge (AWG) for your circuit. Enter amps, distance, and voltage to find the minimum wire size that meets NEC voltage drop limits.

## What this calculates

Choosing the right wire size prevents overheating, voltage drop problems, and code violations. Enter your circuit's amperage, distance, and voltage, and this calculator tells you the smallest AWG gauge that keeps voltage drop within your chosen limit. It covers copper and aluminum wire in both single-phase and three-phase configurations.

## Inputs

- **Load Current** (A) — min 0 — Current the circuit will carry in amperes.
- **Source Voltage** (V) — min 0 — Supply voltage (e.g., 120V, 240V, 480V).
- **One-Way Distance** (ft) — min 0 — One-way wire length from source to load.
- **Max Voltage Drop** — options: 2% (strict), 3% (NEC branch), 5% (NEC total) — Maximum acceptable voltage drop percentage.
- **Wire Material** — options: Copper, Aluminum — Copper has lower resistance; aluminum is cheaper.
- **Phase** — options: Single Phase, Three Phase — Single phase multiplier = 2; three phase = √3.

## Outputs

- **Recommended Wire Gauge** — formatted as text — Minimum AWG gauge to meet your voltage drop limit.
- **Voltage Drop with Recommended Wire** (V) — Actual voltage drop using the recommended gauge.
- **Voltage Drop %** — formatted as percentage — Voltage drop as percentage with recommended gauge.
- **Voltage at Load** (V) — Voltage delivered to the load.

## Details

**How it works:**

The calculator checks each wire gauge starting from 14 AWG and finds the first one where the voltage drop stays within your selected limit (2%, 3%, or 5%).

**Voltage drop formula:**
- Single phase: Vdrop = 2 × I × R × L/1000
- Three phase: Vdrop = √3 × I × R × L/1000

Where I is current in amps, R is resistance per 1,000 feet for the wire gauge, and L is the one-way distance in feet.

**NEC guidelines:**
- **3% max** for branch circuits (outlets, lights, appliances)
- **5% max** for the combined feeder and branch circuit

These are recommendations, not hard code requirements, but following them avoids problems like dimming lights, slow-starting motors, and wasted energy.

**Copper vs. aluminum:**

Aluminum has about 61% of copper's conductivity, so it needs to be about 1.6 times thicker (roughly two gauge sizes larger) for the same performance. Aluminum is lighter and less expensive per foot, making it popular for long runs and service entrance cables.

**Example:** A 20A circuit on 120V with a 150-foot run needs at least 8 AWG copper to stay under 3% drop. Using 12 AWG on that run would result in nearly 6% drop.

## Frequently Asked Questions

**Q: How do I determine what wire size I need?**

A: You need three things: the current (amps) the circuit will carry, the one-way distance from the panel to the load, and the supply voltage. This calculator checks each AWG gauge and finds the smallest one that keeps voltage drop within your chosen limit.

**Q: What is the 80% rule for wire sizing?**

A: The NEC requires that continuous loads (running 3+ hours) not exceed 80% of the conductor's ampacity. This calculator focuses on voltage drop, which is a separate concern. You should verify that your chosen wire gauge also meets the ampacity requirements for your circuit.

**Q: When should I use aluminum wire instead of copper?**

A: Aluminum makes sense for long runs, service entrance cables, and large feeder circuits where the cost savings outweigh the need for a larger gauge. For branch circuits inside walls, copper is more common because it is smaller and easier to work with.

**Q: Does distance really matter that much?**

A: Yes. Voltage drop is directly proportional to distance. A 50-foot run might be fine with 12 AWG, but the same circuit at 200 feet could need 6 AWG. This is why long runs to detached garages, barns, or outbuildings often require much heavier wire than you would expect.

---

Source: https://vastcalc.com/calculators/physics/wire-size
Category: Physics
Last updated: 2026-04-08