WireGaugeCalc Free calculator · NEC cited · works offline
NEC Load Calc For On Demand Electric Hot Water
NEC load calc for on demand electric hot water is simple with WireGaugeCalc, which turns the heater rating into amps and a circuit size.
You enter the wattage and voltage from the nameplate, so the tool returns the load amps and the conductor needed to feed it.
That helps you see how big a feeder or panel the tankless unit really needs.
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.
Why Tankless Hot Water Loads Are So Large
A tankless electric water heater is a continuous load, so the NEC has you size the conductors and overcurrent at 125 percent of the heating draw.
Many whole-house units pull 18 kW to 36 kW, so at 240V that is 75 amps to 150 amps before the 125 percent factor is applied.
Because the draw is so high, the calculator shows the amps first, then the upsized conductor, so you can confirm the service has room.
What The Tankless Water Heater Load Calc Uses
Give the unit details and the tool sizes the circuit:
- Nameplate wattage or kW of the heater
- Supply voltage, usually 240V single-phase
- The 125 percent continuous-load factor
- Resulting load amps and required conductor
- A quick check against your available service size
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
Does a tankless water heater count as a continuous load?
Yes, an on demand electric heater runs at full draw the whole time it heats, so the NEC treats it as continuous and you size at 125 percent. Confirm this against the current NEC and your local authority having jurisdiction.
How many amps does a 27 kW tankless heater need?
A 27 kW unit at 240V draws about 113 amps, and with the 125 percent factor the conductors and breaker are sized near 140 amps. WireGaugeCalc returns the exact figures from your nameplate values.
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.