WireGaugeCalc Free calculator · NEC cited · works offline
Home Standby Generator Sizing
Home standby generator sizing is simple with WireGaugeCalc, since the tool totals the running watts of your loads and adds the starting surge.
You enter each appliance and motor, so the result reflects both steady draw and the inrush when motors start.
The payoff is a generator rating that carries the home 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.
Adding Up The Load For Standby Power
Running watts set the steady demand, so the calculator sums every appliance, light, and motor you plan to back up.
Motors like the AC compressor and well pump pull a starting surge several times their running watts, so the tool adds that inrush to the largest motor.
The result combines running watts and surge, since the generator must handle the peak the moment a motor kicks on.
What The Generator Sizing Uses
The generator rating depends on:
- Running watts of every backed-up load
- Starting surge for the largest motor
- Air conditioner or heat pump compressor draw
- Well pump or sump pump motor inrush
- Whole-house versus essential-circuit coverage
- The voltage and phase of the service
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
Do I size the generator for running watts or starting watts?
You size for both. Total the running watts of all loads, then add the starting surge of the largest motor on top. The generator must meet that combined peak so motors start without stalling the unit.
What loads usually drive home standby generator sizing?
The air conditioner, electric water heater, well pump, and electric range usually dominate the total. Motor loads matter most since their startup surge sets the peak demand.
Does the transfer switch and feeder need to match the generator?
Yes, the transfer switch and feeder conductors must be rated for the generator output and protected correctly. Confirm the wiring sizing against 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.