# Voltage Drop Calculator

Calculate voltage drop across electrical wire runs. Enter wire gauge, length, current, and material to check NEC compliance for branch and feeder circuits.

## What this calculates

Figure out how much voltage you lose across a wire run before it reaches your load. Enter the wire gauge, length, current, and material to get the voltage drop, percentage, and NEC compliance status. Useful for sizing wire in residential and commercial electrical work.

## Inputs

- **Source Voltage** (V) — min 0 — Supply voltage at the source (e.g. 120V or 240V).
- **Load Current** (A) — min 0 — Current drawn by the load in amperes.
- **One-Way Wire Length** (ft) — min 0 — Distance from source to load (one direction only).
- **Wire Gauge (AWG)** — options: 14 AWG, 12 AWG, 10 AWG, 8 AWG, 6 AWG, 4 AWG, 2 AWG, 1 AWG, 1/0 AWG, 2/0 AWG, 3/0 AWG, 4/0 AWG — American Wire Gauge size.
- **Wire Material** — options: Copper, Aluminum — Copper has lower resistance; aluminum is lighter and cheaper.
- **Phase** — options: Single Phase, Three Phase — Single phase uses a factor of 2; three phase uses sqrt(3).

## Outputs

- **Voltage Drop** (V) — Voltage lost across the wire run.
- **Voltage Drop %** — formatted as percentage — Voltage drop as a percentage of source voltage.
- **Voltage at Load** (V) — Voltage delivered to the load after the drop.
- **NEC Compliance** — formatted as text — Whether the voltage drop meets NEC recommended limits.
- **Wire Resistance (round trip)** (Ω) — Total resistance of the wire run.

## Details

**How voltage drop works:** When current flows through a wire, the wire's resistance causes a voltage loss between the source and the load. Longer runs and thinner wire mean more resistance and a bigger drop.

**The formulas:**
- Single phase: Vdrop = 2 x I x R x L/1000
- Three phase: Vdrop = sqrt(3) x I x R x L/1000

Where I is current in amps, R is resistance per 1000 feet for the wire gauge, and L is the one-way wire length in feet. The factor of 2 for single phase accounts for both the hot and neutral conductors.

**Wire material matters:** Aluminum wire has about 1.6 times the resistance of copper. It is lighter and less expensive, but you typically need to go up a gauge or two to match copper's performance on a given run.

**NEC recommendations:** The National Electrical Code recommends no more than 3% voltage drop for branch circuits and no more than 5% total (feeder plus branch). These are recommendations, not hard limits, but staying within them helps avoid flickering lights, overheating motors, and other problems.

**Practical example:** A 20-amp circuit on 12 AWG copper wire running 100 feet will drop about 7.92V on a 120V single-phase circuit, which is 6.6%. That exceeds NEC limits, so you would want to upsize to 10 AWG or shorten the run.

## Frequently Asked Questions

**Q: What is an acceptable voltage drop?**

A: The NEC recommends a maximum of 3% voltage drop for branch circuits and 5% total for feeder plus branch circuits combined. For a 120V circuit, that means keeping the drop under 3.6V for the branch and under 6V total.

**Q: How do I reduce voltage drop?**

A: You have three main options: use a larger (thicker) wire gauge, shorten the wire run, or increase the source voltage. Going from 12 AWG to 10 AWG roughly cuts the resistance by 37%, which proportionally reduces the drop.

**Q: Why is aluminum wire resistance higher than copper?**

A: Aluminum has about 61% of copper's conductivity, so it has roughly 1.6 times the resistance for the same gauge. To compensate, electricians typically use aluminum wire that is one or two sizes larger than the copper equivalent.

**Q: Does this calculator account for temperature?**

A: The resistance values used here are standard NEC values at 75 degrees C (167 degrees F), which is the rated temperature for most common wire insulation types like THWN. Actual resistance varies slightly with temperature.

**Q: What is the difference between single phase and three phase voltage drop?**

A: Single phase circuits use a multiplier of 2 because current travels through two conductors (hot and neutral). Three phase circuits use a multiplier of sqrt(3), or about 1.732, because the phases share the return path. This means three phase circuits have a lower voltage drop for the same current and wire length.

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