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Wire Gauge Calculator

Find the right wire size for any circuit — enter amps, distance, and voltage to get AWG/mm² with voltage drop analysis

A
ft
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Material:
Units:

Results

Enter current, distance, and voltage to see recommendations

About This Tool

Using wire that's too thin causes voltage drop, overheating, and wasted energy. Using wire that's too thick wastes money. This calculator finds the sweet spot — the right gauge for your current, distance, and acceptable voltage drop. Enter your circuit parameters, and the calculator shows every common wire gauge with its voltage drop, power loss, and whether it passes your threshold. It supports both AWG (American Wire Gauge) and metric mm², copper and aluminum conductors, and works for AC and DC circuits from 12V landscape lighting to 240V EV chargers. The result table highlights the minimum recommended gauge and shows the NEC ampacity rating for each size, so you can verify your wire meets both voltage drop and current capacity requirements.

How to Use

1. Enter the current draw in amps 2. Enter the one-way wire distance (calculator doubles it for round-trip) 3. Set your system voltage and max acceptable voltage drop (default 3%) 4. Choose conductor material (copper or aluminum) 5. See the recommended wire gauge and full comparison table 6. Click any row to see detailed calculations

Frequently Asked Questions

What wire size do I need for 20 amps?
For a 20A circuit, NEC requires minimum 12 AWG (3.3 mm²) copper wire. However, for longer runs you may need 10 AWG to keep voltage drop under 3%. This calculator factors in distance to give you the right answer.
What is voltage drop and why does it matter?
Voltage drop is the loss of voltage as current flows through wire resistance. Over long distances, it can be significant — a 100ft run of 14 AWG carrying 15A on a 120V circuit drops about 5V (4.2%). This means your appliances get 115V instead of 120V, reducing performance and efficiency. NEC recommends keeping total voltage drop under 3% for branch circuits.
What is the difference between AWG and mm²?
AWG (American Wire Gauge) is used in North America — lower numbers mean thicker wire (14 AWG is thinner than 10 AWG). The metric system uses cross-sectional area in mm² — higher numbers mean thicker wire (2.5 mm² is thinner than 6 mm²). This calculator shows both.
Should I use copper or aluminum wire?
Copper has lower resistance and higher ampacity per size, making it standard for most residential wiring. Aluminum is cheaper and lighter, used for large feeders and service entrance cables, but requires larger gauge for the same current. Aluminum also needs special connectors to prevent oxidation issues.
How do I calculate voltage drop?
Voltage drop = 2 × Length × Current × Resistance per unit length. The factor of 2 accounts for the round trip (hot wire out, neutral back). For example, 50ft of 12 AWG copper carrying 15A drops 2 × 50 × 15 × 0.00162 = 2.43V on a 120V circuit (2.0%).
What is the 3% voltage drop rule?
NEC (National Electrical Code) recommends maximum 3% voltage drop for branch circuits and 5% total for feeders plus branch circuits combined. While not a hard code requirement, exceeding these limits can cause equipment malfunction, dimming lights, and motor overheating.

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