WireGaugeCalc Free calculator · NEC cited · works offline
Home Generators Size Calculators
Home generators size calculators are simple with WireGaugeCalc, since you list your loads and the app totals the running and starting watts.
The app adds steady watts and the largest motor surge so you can read the generator capacity you need.
So you can pick a unit that carries the loads without tripping on startup.
Open the calculatorEnter any two values and read the rest.
Ohm’s law: V = I × R and P = V × I. Field aid only, verify against the current NEC and your local authority having jurisdiction.
Sizing a backup generator by watts
You add the running watts of every load you want on at once, since that is the steady demand.
Then you add the starting surge of the single largest motor load, because motors draw extra current as they spin up.
The sum sets the minimum generator output, so a margin above it keeps the unit from running flat out.
What the standby generator wattage tool uses
Enter the loads you want backed up and the tool totals them:
- Running watts of each appliance
- Starting watts of the largest motor
- Heating and cooling loads
- Well pump or sump pump surge
- A capacity margin above the total
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
Why are starting watts higher than running watts?
Motors in pumps, compressors, and air conditioners draw a brief surge as they start, often several times their running watts. You size the generator for that peak plus the other running loads.
Does a generator size calculator replace an electrician?
No. The calculator gives a working estimate, but transfer switch sizing and the connection must follow the current NEC and your local authority having jurisdiction.
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.