WireGaugeCalc Free calculator · NEC cited · works offline
40 Amp Wire Size
40 amp wire size is fast to find with WireGaugeCalc, since the calculator reads the NEC 310.16 ampacity table for you.
You pick copper or aluminum and the termination temperature column, so the result matches how the breaker and lugs are actually rated.
You get the minimum gauge in seconds, so you can buy and pull the right conductor the first time.
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.
What Size Wire For 40 Amp Circuits
A 40 amp breaker wire size is usually 8 AWG copper at the 75 degree C column, since that conductor carries 50 amps and covers the 40 amp setting.
Aluminum runs larger, so 6 AWG aluminum is common when you choose that material.
The calculator confirms the gauge against the table, so you are not guessing from memory on site.
How The 40a Wire Size Is Decided
The result follows a few clear inputs:
- Conductor material, since copper carries more current per gauge than aluminum
- Termination temperature, because most breakers and devices are rated 75 degrees C
- The 40 amp breaker rating, which the conductor ampacity must equal or exceed
- NEC 310.16 column values for the chosen 40 amp wire gauge
- Whether the load is continuous, since that can push the size up
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
Can I use 10 AWG copper on a 40 amp breaker?
No, 10 AWG copper tops out at 30 amps and is not rated for a 40 amp breaker. Use 8 AWG copper at the 75 degree C column for a standard 40 amp circuit.
What gauge cable for a 40 amp breaker if I use aluminum?
Aluminum is one size larger, so 6 AWG aluminum is typical for 40 amps at 75 degrees C. Verify the size against the current NEC and your local authority having jurisdiction before you wire it.
Does a 40 amp range or oven change the wire size?
A 40 amp breaker still calls for 8 AWG copper, though derating or long runs can change it. Check the appliance nameplate and the breaker rating together.
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.