WireGaugeCalc Free calculator · NEC cited · works offline
20 Amp Circuit Breaker Wire Size
20 amp circuit breaker wire size is fast to confirm with WireGaugeCalc.
The calculator checks the NEC 310.16 ampacity for copper or aluminum at your termination temperature, so you size the conductor before you pull it.
You get a defensible gauge in seconds instead of guessing at the panel.
Open the calculatorSmallest conductor whose ampacity meets the load, NEC Table 310.16. Field aid only, verify against the current NEC and your local authority having jurisdiction.
The Minimum Wire Size For A 20 Amp Circuit
A 20 amp breaker generally needs 12 AWG copper, since 14 AWG is rated for 15 amps and is too small to protect at 20 amps.
The calculator reads the 60 or 75 degree C column to match your terminals, so the gauge stays inside the NEC ampacity table.
Aluminum runs larger because its ampacity is lower for the same gauge, and WireGaugeCalc adjusts for that automatically.
How WireGaugeCalc Sizes The Conductor
Enter the load and the calculator works through the inputs that drive the answer:
- Breaker rating set at 20 amps
- Conductor material, copper or aluminum
- Termination temperature, 60 or 75 degrees C
- NEC 310.16 ampacity for the chosen gauge
- Derating for ambient heat or bundled conductors
One app for every NEC calculation
WireGaugeCalc keeps the calculations you reach for most in one place:
- Voltage drop and wire sizing for any run
- Conduit fill for EMT, PVC, and RMC
- Box fill and junction box sizing
- Ampacity and temperature derating
- Motor circuit and load calculations
- Conduit bend offsets and saddles
Built for the field, works offline
The whole app runs on your phone, so it keeps working in a basement, an attic, or a job site with no signal.
There is no account to create and nothing to load. Open it, run the number, and get back to work.
- No signup and no signal needed
- Answers in a tap, not a spreadsheet
- Same tool on phone, tablet, and desktop
Every result cites the NEC article
Each answer shows the table or formula it came from, so you can check the method and learn the code as you go.
That makes the app useful on the job and during exam prep, since the reasoning is right next to the number.
Switch the code year your job runs on
Jurisdictions adopt the NEC at different times, so you can match the calculation to the code in force:
- NEC 2017, 2020, and 2023 tables
- Copper and aluminum conductors
- 60, 75, and 90 degree C terminations
- Single-phase and three-phase systems
Run the number, then get back to work
Stop flipping through a paper book or hopping between calculator sites. Enter your values, read the code-cited answer, and move on. Free to use, no signup.
Open the calculatorFrequently asked questions
Is 14 AWG ever allowed on a 20 amp breaker?
No, 14 AWG copper is rated for 15 amps and must be protected by a 15 amp breaker. A 20 amp circuit needs 12 AWG copper at minimum.
What is the minimum wire size for a 20 amp circuit in aluminum?
Aluminum typically needs 10 AWG to carry 20 amps, since its ampacity is lower than copper for the same gauge. Confirm the result against the current NEC and your local authority having jurisdiction before you wire it.
Does a long run change the gauge for a 20 amp breaker?
It can, because voltage drop on a long run may push you to upsize past the ampacity minimum. The calculator flags when distance matters.
Is WireGaugeCalc free to use?
Yes. Every calculator is free to run with no signup. A paid tier adds saved projects, PDF reports, and extra code years, but the core math stays free.
Does it work without internet?
Yes. The app runs on your device, so it keeps working with no signal on a job site, in a basement, or in an attic.
Are the results code accurate?
Results follow published NEC tables and standard formulas, and each answer shows the article it came from. It is a field aid, not a stamp of approval, so verify against the current code and your local authority before you wire anything.
Which NEC code year does it use?
You can switch between NEC 2017, 2020, and 2023, since jurisdictions adopt the code at different times. Pick the year your job runs on.