WireGaugeCalc Free calculator · NEC cited · works offline
250 Amp Wire Size
250 amp wire size is fast with WireGaugeCalc, since you set the load at 250 amps and the tool returns the matching conductor from NEC 310.16.
The calculator reads the ampacity column for your termination temperature and material, so the size matches how the lugs are rated.
You see the copper and aluminum options side by side before you commit to a feeder.
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.
Sizing a 250 amp conductor under NEC 310.16
A 250 amp circuit needs a conductor whose ampacity meets or exceeds 250 amps in the correct temperature column, so the tool checks the 75 degree C values by default.
Copper at 250 amps lands smaller than aluminum, since aluminum carries less current per AWG and steps up to a larger size.
The result tells you exactly which conductor satisfies the load so you avoid undersizing the run.
What sets the conductor size for 250 amps
The right size for a 250 amp circuit depends on these inputs:
- Conductor material, copper or aluminum
- Termination temperature, usually the 75 degree C column
- Ambient temperature, which can require a correction factor
- Number of current-carrying conductors in the raceway
- Whether the run is a feeder or a service entrance
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 copper wire size carries 250 amps?
Copper rated at 250 amps in the 75 degree C column is typically 250 kcmil, while a 90 degree C rating can allow a slightly smaller conductor. Confirm the exact ampacity against the current NEC 310.16 and your local authority having jurisdiction.
What aluminum size do I need for a 250 amp feeder?
Aluminum carries less per size, so a 250 amp feeder usually needs 350 kcmil aluminum in the 75 degree C column. WireGaugeCalc shows the aluminum option next to the copper so you can compare.
Why does termination temperature change my 250 amp wire size?
Most lugs are rated 75 degree C, so the conductor must be sized from that column even if the insulation is rated higher. Using the wrong column can leave the wire undersized at the terminals.
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.