Electrical
Updated Jun 28, 2026

Wire Size Calculator

Calculate the correct wire gauge for electrical circuits based on amperage, distance, and voltage. Includes voltage drop analysis.

How to Size Electrical Wire

Proper wire sizing ensures safety and code compliance. Undersized wire causes voltage drop, overheating, and fire risk. The two main factors are ampacity (current capacity) and voltage drop over distance.

Voltage Drop Formula

Voltage drop increases with distance and current, and decreases with larger wire gauge. For single-phase circuits:

Vdrop=2×L×I×R1000

Where L is the one-way distance in feet, I is current in amps, and R is wire resistance in ohms per 1000 ft.

Voltage Drop Percentage

The NEC recommends keeping voltage drop below 3% for branch circuits and 5% total (feeder + branch combined):

Drop %=VdropVsource×100
Safety First
All electrical work must comply with the National Electrical Code (NEC) and local building codes. Many jurisdictions require a licensed electrician for new circuits. Always turn off power at the breaker before working on wiring. When in doubt, hire a professional.

AWG Wire Size Reference

Common residential wire sizes and their ratings for copper conductors at 75 C (NM-B / Romex):

AWGAmpacityMax BreakerTypical Use
14 AWG15A15ALighting, general outlets
12 AWG20A20AKitchen, bath, garage
10 AWG30A30ADryers, water heaters
8 AWG40A40ARanges, large appliances
6 AWG55A60ASub-panels, EV chargers
Breaker Sizing Rule
The NEC 80% rule states that continuous loads should not exceed 80% of the breaker rating. A 20A breaker on 12 AWG wire should carry no more than 16A of continuous load (loads lasting 3+ hours).

Common Circuits and Their Wire

Quick reference for the most common household circuits. Breaker size sets the wire size, not the appliance:

15Abreaker
14 AWG
Lights, general outlets
20Abreaker
12 AWG
Kitchen, bath, garage
30Abreaker
10 AWG
Electric dryer (240V)
40Abreaker
8 AWG
Electric range (240V)
50Abreaker
6 AWG
EV charger L2 (240V)
60Abreaker
6 AWG
Small sub-panel

Copper vs Aluminum

Most residential branch circuits use copper. Aluminum is cheaper and lighter but has trade-offs. The two cannot be directly connected without special connectors:

PropertyCopperAluminum
Ampacity for same gaugeHigher~60% of copper
Typical useAll branch circuitsService entry, feeders
Upsize requiredStandard sizeGo 2 sizes larger (e.g. 6 Cu = 4 Al)
Cost per ftHigher30-50% cheaper
ConnectionsAny connectorCO/ALR or AL9 rated only

Voltage Drop Allowances by Distance

On long runs (garage, workshop, outdoor), voltage drop becomes the limiting factor - not ampacity. A 20A circuit running 100 ft should upsize to 10 AWG to stay under 3%:

CircuitUp to 50 ft50-100 ft100-150 ft
15A @ 120V14 AWG12 AWG10 AWG
20A @ 120V12 AWG10 AWG8 AWG
30A @ 240V10 AWG10 AWG8 AWG
Permit & Inspection
New circuits almost always require an electrical permit in the US, Canada, UK, and EU. Rough-in inspection (wire pulled, boxes in, outlets not finished) happens before drywall. Final inspection happens after outlets are installed. Working without a permit can void home insurance after an electrical fire, even years later.
Wire Nut Sizing
Yellow wire nuts handle 14-12 AWG combos. Red handles up to 10 AWG. Tan is for 18-16 AWG lighting fixtures only. Never cram more wires into a nut than rated - a loose connection is the #1 cause of electrical fires behind the wall.

Frequently Asked Questions