FAQ
Electrical calculator FAQs
Straight answers to the wiring, load, conversion, and code questions people actually ask — each grounded in the 2023 NEC, with the right calculator to run your own numbers.
Using the calculators
What is Electrical Calcs, and are the calculators free?
Yes — completely free. Electrical Calcs is a set of 11 electrical calculators — wire and breaker sizing, voltage drop, conduit fill, load, generator, solar, and unit conversions — all built on the 2023 NEC. There's no account, no ads, and no paywall, and every tool shows the formula behind its result and cites the code so you can check the work. Browse all 11 calculators →
Which NEC edition do the calculators use?
All tools follow the 2023 National Electrical Code (NFPA 70), and each result names the specific article it's based on. Where the 2026 edition changes a rule, we flag it on the relevant page. One caveat: jurisdictions adopt the NEC on different schedules, so confirm which edition your local AHJ (Authority Having Jurisdiction) currently enforces.
Are the results accurate enough for a real installation or permit?
They're built for planning, not to replace a licensed electrician. The calculators follow NEC formulas exactly, and every worked example is verified in code before it ships — but a permit or inspection needs a licensed pro and your local AHJ's sign-off. Use these to get the numbers right, then confirm the install with a professional. See how we build and verify each tool →
Do I need an account or an app?
No — nothing to sign up for or install. There's no account, no email, and no download. Open a calculator in your browser, on desktop or the phone you're holding on the job, and use it. Everything runs on the page.
Which calculator should I use for my project?
It depends on the job. To size a conductor, use the Wire Size Calculator; to check a run's drop, the Voltage Drop Calculator; to size protection, the Circuit Breaker Calculator; to fit wires in a pipe, the Conduit Fill Calculator. For a home's service, use the Electrical Load Calculator; for backup power, Generator Size; for solar, Solar Panel; and to convert between watts, amps, and kVA, the Watt-to-Amp or Ampere calculators. See the full directory →
Wiring & circuits
What size wire do I need for a 30, 40, 50, or 100 amp circuit?
At standard 75 °C terminations: 30 A → 10 AWG copper, 40 A → 8 AWG, 50 A → 6 AWG, 60 A → 6 AWG, and 100 A → 3 AWG. (For a 100 A dwelling service, NEC 310.12 permits 4 AWG copper.) Two things move these: continuous loads — anything running 3+ hours, like an EV charger — need the wire to carry 125% of the load, which is exactly why a continuous 50 A circuit takes 6 AWG rather than the bare-minimum 8 AWG; and long runs, where voltage drop can force an upsize. Aluminum runs about one size larger. Run your exact case in the Wire Size Calculator.
What size breaker do I need for a water heater, dryer, or 12-gauge wire?
The breaker is set by the wire and the load — you don’t pick it freely. By conductor (NEC 240.4(D)): 14 AWG copper → 15 A, 12 AWG → 20 A, 10 AWG → 30 A. By appliance: a standard 4,500 W / 240 V water heater draws 18.75 A, ×1.25 for continuous use = 23.4 A → a 30 A breaker on 10 AWG; a typical electric dryer is also 30 A / 10 AWG. For an air conditioner, don’t calculate it — use the MCA and MOCP printed on the nameplate. The Circuit Breaker Calculator handles all three.
How many watts can a 15, 20, or 30 amp circuit handle?
Multiply amps × volts, then take 80% for anything continuous. A 15 A/120 V circuit handles 1,800 W (1,440 W continuous); a 20 A/120 V circuit handles 2,400 W (1,920 W continuous); a 30 A/240 V circuit handles 7,200 W (5,760 W continuous). The 80% rule (NEC 210.20) keeps loads that run 3+ hours from sitting right at the breaker’s limit. See the Circuit Breaker and Electrical Load calculators.
How do I calculate voltage drop, and what’s the NEC limit?
For a single-phase run, voltage drop = 2 × K × I × L ÷ CM — K is 12.9 for copper (21.2 for aluminum), I the current, L the one-way length in feet, and CM the conductor’s circular-mil area. The NEC recommends keeping branch-circuit drop ≤ 3% and total feeder-plus-branch ≤ 5% (informational notes to 210.19 and 215.2 — guidance, not a hard rule). Exceed it and you get dimming and wasted energy; long runs are the usual cause. The Voltage Drop Calculator does it both by the K-method and NEC impedance.
What size conduit do I need for my wires?
The NEC caps fill at 40% for three or more conductors (53% for one, 31% for two) — Chapter 9, Table 1. Rather than work out cross-sectional areas by hand, the code’s Annex C gives the count directly: for example, nine 12 AWG THHN conductors fit in ½-inch EMT. Fill limits stop conductors overheating and keep the pull physically possible. The Conduit Fill Calculator covers every trade size and insulation type.
Loads & backup power
What size generator do I need for my house?
Add the running watts of everything you’ll power at once, then add the single largest starting surge — motors like a well pump, AC, or fridge briefly pull several times their running watts. For essentials (fridge, furnace, sump, some lights) many homes land near 5,000–7,500 W; whole-house standby units run 15–30 kW+. Size to the calculated load with ~20% headroom — and never back-feed a generator without a proper transfer switch. The Generator Size Calculator uses running plus the largest surge, the right way to size it.
How many solar panels do I need to power my home?
Roughly: annual kWh ÷ (peak sun-hours × 365 × system losses) ÷ panel wattage. For a typical US home that’s about 15–22 modern 400 W panels — but it swings widely with your electricity use, your location’s sun-hours, and available roof space. The honest number needs your real usage and local sun data. The Solar Panel Calculator uses NREL sun-hour data and your kWh to size it.
How do I calculate my home’s total electrical load?
NEC Article 220 gives two methods — the Standard method (Part III) and the Optional method (220.82) — and you’re allowed to use whichever yields the smaller service. You total the general lighting and receptacle load, add the appliances (range, dryer, water heater), apply the code’s demand factors, and take the larger of heating vs. AC. Divide the result by 240 V for your minimum service amperage. The Electrical Load Calculator runs both methods side by side.
How many amps does a refrigerator (or typical appliance) use?
A household refrigerator draws roughly 3–6 running amps at 120 V (about 100–800 W), with a brief startup surge several times higher when the compressor kicks in. That surge is why generator and circuit sizing care about starting watts, not just running watts. For any appliance, amps = watts ÷ volts. Convert yours with the Ampere or Watt-to-Amp calculator.
Conversions & formulas
How many amps is 1000, 1500, or 2000 watts?
Amps = watts ÷ volts, so the voltage decides the answer. At 120 V: 1,000 W = 8.33 A, 1,500 W = 12.5 A, 2,000 W = 16.67 A. At 240 V those halve (4.17 A, 6.25 A, 8.33 A). That’s for DC or a purely resistive load; an AC motor adds power factor, and three-phase adds a √3 factor. The Watt-to-Amp Calculator handles all of them.
How do I convert watts to amps (and amps to watts)?
A = W ÷ V, and W = A × V. For AC you multiply by the power factor (A = W ÷ (V × PF)); for three-phase you add √3: A = W ÷ (√3 × V × PF). So 1,500 W at 120 V is 12.5 A resistive, and a little more for a motor once PF is in. Do it both directions in the Watt-to-Amp or Ampere calculator.
What is Ohm’s Law and how do I use it?
Ohm’s Law is V = I × R — voltage equals current times resistance — and it pairs with the power equation P = V × I. Rearranged, I = V ÷ R and R = V ÷ I, so given any two of voltage, current, and resistance you can find the rest (and the power). It’s the foundation under most electrical math. The Ohm’s Law Calculator solves for whichever two you’re missing.
How do I calculate amps from volts and watts?
Divide watts by volts: A = W ÷ V. A 1,200 W device on 120 V draws 10 A. For an AC load with a power factor, divide by (V × PF); for three-phase, by (√3 × V × PF). The Ampere Calculator moves between amps, kW, and kVA for you.
Accuracy, NEC & safety
What is a good power factor?
Power factor runs 0 to 1, and closer to 1 is better: 0.95–1.0 is excellent, 0.85–0.95 moderate, and below 0.85 poor. It’s the ratio of real power (kW) to apparent power (kVA) — a low PF means you’re pulling more current than the actual work requires, and many utilities penalize commercial customers below about 0.90–0.95. Capacitors correct it. The Power Factor Calculator finds your PF and the correction needed.
Why does the calculator tell me to upsize wire for a long run?
Because ampacity isn’t the only limit — voltage drop is. A conductor big enough to carry the current can still lose too much voltage over distance, so on long runs the tool sizes up to keep drop within the recommended 3% branch / 5% total. That’s why a 30 A circuit at 120 ft may need a larger wire than the ampacity table alone suggests. The Voltage Drop and Wire Size calculators account for both.
What is the 80% rule (continuous-load rule)?
A continuous load — one drawing current for 3 hours or more — must not exceed 80% of the breaker’s rating; equivalently, the breaker and wire are sized to at least 125% of that load (NEC 210.20 and 210.19). It’s why a 50 A continuous load wants a conductor rated for about 62.5 A. The rule leaves thermal margin so nothing runs hot right at its limit. The Circuit Breaker and Electrical Load calculators apply it automatically.
Do these results replace a licensed electrician?
No — they’re planning and estimating tools. Every calculator follows NEC formulas and is verified in code, but real installations need a licensed electrician and your local AHJ’s approval, and local amendments can differ from the base code. Use these to get the numbers right and understand the “why,” then have the work done and inspected properly. See how we build and verify each tool.
How these answers are checked
- Grounded in the 2023 NEC. Every code-based answer names its article — Table 310.16 for ampacity, 240.4(D) for small conductors, Chapter 9 for conduit fill, 210.20 for the 80% rule.
- Verified in code. The numbers here are checked against those references before publishing — a wrong figure is a safety issue, not a typo.
- Reviewed by a licensed electrician. Answers are checked against the code by a licensed pro, so they match how the work is actually done.
A planning aid, not a permit. These answers are general and for estimating. Real installations depend on your exact conditions and on local amendments — always confirm with a licensed electrician and your local AHJ before you buy materials or start work.
Stop guessing — calculate it
Every answer above has a tool behind it. Start with the most-used three, or open the full set.