What wire for 20 amps?

12 AWG minimum per NEC. Upsize to 10 AWG for long runs.

How to calculate voltage drop?

V-drop = 2 × distance × resistance per ft × amps.

When to use aluminum wire?

For large feeder runs (4 AWG+) where cost savings justify it.

Science

Wire Gauge Calculator

Calculate the correct wire gauge (AWG) for your amperage and distance. Checks NEC ampacity ratings and voltage drop limits.

Quick Answer

14 AWG for 15A, 12 AWG for 20A, 10 AWG for 30A circuits. For long runs, upsize to keep voltage drop under 3%. Voltage drop = 2 × length × resistance × amps.

Circuit Details

Amperage (A)

Voltage (V)

One-Way Distance (ft)

Max Voltage Drop (%)

Results

8 AWG

Recommended

2.51V

Voltage Drop

2.1%

Drop %

117.5V

At Load

AWG Reference Table

AWG	Max Amps	V-Drop	Status
14	15A	10.10V	FAIL
12	20A	6.35V	FAIL
10	30A	4.00V	FAIL
8	40A	2.51V	OK
6	55A	1.58V	OK
4	70A	1.00V	OK
3	85A	0.79V	OK
2	95A	0.62V	OK

About This Tool

The Wire Gauge Calculator determines the correct American Wire Gauge (AWG) size for your electrical circuit based on amperage, voltage, distance, and acceptable voltage drop. It references NEC (National Electrical Code) ampacity ratings for copper conductors in standard conditions.

Voltage Drop Explained

All wire has resistance, and resistance causes voltage loss over distance. The NEC recommends no more than 3% voltage drop for branch circuits and 5% total from the panel to the load. Excessive voltage drop causes dim lights, slow motors, and wasted energy as heat in the wire.

NEC Ampacity Ratings

The NEC assigns maximum ampacity to each wire gauge based on insulation type and installation conditions. The values in this calculator use 60°C insulation ratings as a conservative baseline. Higher-temperature insulation allows more current, but the weakest link in the circuit determines the limit.

Copper vs. Aluminum

This calculator uses copper wire values. Aluminum wire has about 61% of copper’s conductivity, so you typically need to go up two gauge sizes. Aluminum is common for large feeder runs due to its lower cost per ampere. Always use the correct terminals rated for the wire material.

Safety Warning

Electrical work can be dangerous and may require permits and inspections. This calculator is for reference only. Always consult a licensed electrician and your local building department for actual installations. Undersized wire is a fire hazard.

Frequently Asked Questions

What wire gauge do I need for a 20-amp circuit?

12 AWG minimum for a 20-amp circuit per NEC code. For runs over 75 feet at 120V, consider upsizing to 10 AWG to keep voltage drop under 3%.

How do I calculate voltage drop?

Voltage Drop = 2 × one-way distance (ft) × wire resistance (ohms/ft) × amps. The factor of 2 accounts for both the supply and return conductors.

What is acceptable voltage drop?

NEC recommends max 3% for branch circuits and 5% total (feeder + branch). For sensitive electronics, keep it under 2%. Motors are particularly sensitive to low voltage.

Does voltage affect wire gauge?

Higher voltage circuits can use smaller wire for the same power because they draw fewer amps. A 240V circuit carrying the same wattage as 120V draws half the current and has half the voltage drop.

When should I use aluminum wire?

Aluminum is cost-effective for large feeder runs (4 AWG and larger). It requires special connectors and is not recommended for branch circuits due to expansion/connection issues.

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