WireGaugeCalc Free calculator · NEC cited · works offline
400 Amp Wire Size
400 amp wire size is simple with WireGaugeCalc, since the calculator reads NEC 310.16 ampacity for your conductor material and termination temperature.
You enter copper or aluminum and the rated terminal temperature, and the tool returns a conductor that carries 400 amps.
So you can spec the service or feeder conductor without flipping through the ampacity table by hand.
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.
Reading the 310.16 Ampacity Table
Conductor material matters, since aluminum carries less current than copper of the same size and needs a larger gauge for 400 amps.
Termination temperature sets the column you use, so a 75C lug limits you to the 75C ampacity even when the conductor insulation is rated higher.
What Sizes a 400 Amp Conductor
A few details decide the conductor you pull:
- Conductor material, copper or aluminum
- Termination temperature rating, usually 75C on lugs
- Whether parallel sets are used to reach the ampacity
- Ambient temperature and any derating
- Number of current-carrying conductors in the raceway
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
What size copper wire do I need for a 400 amp service?
Sizing follows NEC 310.16 at the lug termination temperature, commonly 75C, and large services are often run as parallel sets. Enter your material and termination temperature and the calculator returns a conductor for 400 amps.
Can I use aluminum for a 400 amp feeder?
Yes, aluminum is allowed, but it needs a larger gauge than copper since its ampacity is lower at the same size. Confirm the result against the current NEC and your local authority having jurisdiction before ordering.
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.