What Electrical Calculators Cover
Electrical work starts with numbers: how much power a home needs, what size wire can safely carry it, how large the breaker must be, and what it will all cost. The electrical calculators in this collection handle each of those questions, so you can plan a project before you ever open a panel. Use them to size wires, check voltage drop on long runs, estimate total dwelling load, convert between amps, watts, and volts, choose breaker sizes, size conduits, plan solar arrays and generators, and budget the full rough-in cost.
Each calculator focuses on one step of the planning process. Together they form a complete workflow — from load estimate to wire size to cost — that matches how electricians and builders actually scope residential electrical work. The results are planning estimates: always have a licensed electrician review final designs and confirm compliance with your local code and Authority Having Jurisdiction (AHJ), as the National Electrical Code (NEC) or its local amendments apply in most U.S. jurisdictions, and international projects follow their own standards (e.g., IEC 60364).
All Electrical Calculators
- Wire Size Calculator — Suggested copper wire gauge from amperage.
- Voltage Drop Calculator — Voltage drop for a copper run.
- Conduit Size Calculator — Suggested EMT conduit size for wire count.
- Electrical Load Calculator — Dwelling service size from square footage & appliances.
- Amperage Calculator — Amps from watts and volts.
- Watts Calculator — Watts from volts × amps.
- Breaker Size Calculator — Recommended breaker size (125% of load).
- Solar Panel Calculator — Panel count from daily kWh & sun hours.
- Generator Size Calculator — Generator size from running & starting watts.
- Electrical Cost Calculator — Total electrical rough-in cost by circuit count.
When You Need These Calculators
Different projects call for different calculations. Here are the most common scenarios and which tools each one needs:
New Home Build
A new build starts with the Electrical Load Calculator to establish the service size (100, 200, or 400 amp) from square footage and appliances. From there, the Wire Size Calculator, Breaker Size Calculator, and Conduit Size Calculator plan every branch circuit, and the Electrical Cost Calculator builds the rough-in budget for the bid.
Remodel or Addition
Adding rooms, a kitchen, or an EV charger changes the load picture. Run the load calculator first to confirm the existing service can handle the new demand — if it can’t, you need a panel upgrade before the remodel can proceed. The Voltage Drop Calculator is especially important for additions, since new circuits often mean long wire runs to a remote subpanel.
Panel or Service Upgrade
Upgrading from a 100-amp to a 200-amp service is driven by load: two HVAC systems, an induction range, an EV charger, and a hot tub will push a smaller panel past its limit. The load calculator quantifies total demand, the breaker calculator sizes protection for new circuits, and the cost calculator prices the upgrade including the panel, circuits, permits, and utility coordination.
Solar Sizing
The Solar Panel Calculator converts your daily energy use (kWh) and local peak sun hours into a panel count. Pair it with the load and watts calculators to understand which loads the array needs to cover — and note that net metering rules, panel orientation, and shading vary by region, so treat the result as a starting point for a solar designer’s quote.
Backup Power
The Generator Size Calculator sizes a generator from running watts plus the highest starting (surge) watts — critical because motors in refrigerators, well pumps, and air conditioners draw 2–3× their running watts at startup. Knowing total load first prevents the most common generator mistake: buying a unit that’s fine for running loads but stalls on startup surges.
How to Use Them
The calculators work best in a specific order that mirrors professional planning. Follow this workflow:
- Estimate the load first. Use the Electrical Load Calculator to total dwelling demand in VA from square footage, small-appliance and laundry circuits, and major appliances. This is the foundation every other decision rests on.
- Convert and size circuits. Use the Amperage Calculator and Watts Calculator to move between units, then the Wire Size Calculator and Breaker Size Calculator to size conductors and protection for each circuit. Breakers are sized at 125% of the continuous load per the NEC continuous-load rule (see below).
- Check long runs and raceways. Run the Voltage Drop Calculator on any run longer than about 30 m (100 ft), and the Conduit Size Calculator whenever you route multiple conductors through conduit — NEC Chapter 9 limits conduit fill to 40% for three or more conductors.
- Price the project. Feed circuit counts and wire/conduit requirements into the Electrical Cost Calculator for a rough-in budget. Add a 10–15% contingency for change orders, which are normal on every electrical job.
- Review with a licensed electrician. Planning estimates are not installation drawings. Have a licensed electrician finalize the design, pull permits, and confirm AHJ requirements before work begins.
Key Formulas & Concepts
You don’t need an engineering degree to plan electrical work, but these fundamentals will make every calculator result easier to interpret:
Ohm’s Law and the Power Formula
The two core equations are P = V × I (power in watts equals voltage times current) and V = I × R (voltage equals current times resistance). These explain why a 2,400 W space heater draws 20 A at 120 V but only 10 A at 240 V — higher voltage means lower current for the same power, which is why high-demand appliances run on 240 V circuits.
The NEC 80% / 125% Continuous-Load Rule
Under NEC 210.19 and 210.20, a circuit supplying a continuous load (one expected to run 3+ hours, such as lighting or an EV charger) must be sized so the load doesn’t exceed 80% of the breaker rating. Equivalently, size the breaker and conductors at 125% of the continuous load — a 16 A continuous load needs a 20 A breaker. The Breaker Size Calculator applies this rule automatically.
Voltage Drop
Wire has resistance, so voltage falls along the length of a run. The basic relationship is Vdrop = 2 × I × R × L for a single-phase circuit (the “2” accounts for the out-and-back path through both conductors), where I is current in amps, R is conductor resistance per unit length, and L is one-way length. NEC fine print notes recommend keeping branch-circuit drop under 3% (5% combined feeder + branch). Long runs at 120 V lose percentage faster than at 240 V — a common reason to feed outbuildings or additions at 240 V.
Ampacity
Ampacity is the maximum current a conductor can carry safely under its conditions of use — determined by wire gauge, copper vs. aluminum, insulation temperature rating (60°C, 75°C, or 90°C), ambient temperature, and how many current-carrying conductors share a raceway (derating). Always size to the lowest temperature rating in the chain, which is typically the 75°C (or 60°C) termination rating, not the 90°C wire rating.
Conduit Fill: the 40% Rule
NEC Chapter 9, Table 1 limits conduit fill to 40% of the interior cross-section for three or more conductors (53% for one, 31% for two). This leaves room for heat dissipation and future pulls. The conduit calculator applies this limit against the actual cross-sectional areas of your conductor sizes.
Dwelling Service Load: VA per Square Foot
NEC 220.12 assigns a general lighting and receptacle load of 3 VA per square foot (about 32 VA/m²) of dwelling area. Small-appliance branch circuits add 1,500 VA each (minimum two) and laundry adds 1,500 VA, before major fixed appliances are counted. This is the basis the load calculator uses for its service-size estimate.
For deeper dives, see Voltage Drop Calculator: Complete Guide and Wire Size Calculator: Common Mistakes.
Cost Factors
Electrical costs vary widely by region, labor market, and material prices. The Electrical Cost Calculator gives planning-range estimates — treat them as budgeting tools, not quotes. The main cost drivers are:
| Cost factor | What drives it | Planning note |
|---|---|---|
| Labor | Electrician hourly rates, job complexity, accessibility | Largest line item on most jobs; urban markets run notably higher than rural ones |
| Copper wire | Commodity copper prices, gauge, run lengths | Copper is volatile — wire prices swing with global markets; budget with a contingency |
| Panel & service upgrades | Panel amperage, meter/main combo, utility coordination | A service upgrade adds the panel, breakers, grounding, and often utility-side work |
| Permits & inspections | Local AHJ fee schedules, project scope | Non-negotiable cost; unpermitted work creates insurance and resale problems |
| Regional variation | Labor markets, code amendments, material availability | The same panel upgrade can cost nearly double between markets — get local quotes |
As a rough planning band (U.S., 2026): a panel/service upgrade typically runs $2,000–$4,500; whole-house rough-in for new construction runs $8,000–$15,000 for a mid-size home; dedicated 240 V circuits (EV charger, hot tub) run $300–$1,200 each depending on run length and panel capacity. Always get 2–3 licensed quotes for real numbers.
Pro Tips
- Size wire to the terminations, not just the load. Breaker terminals and device ratings often limit you to 60°C or 75°C ampacity — the wire’s 90°C rating doesn’t apply if the weakest link is cooler. See common wire-sizing mistakes.
- Watch the startup surge. Motors (well pumps, AC compressors, refrigerators) draw 2–3× running watts at startup — size generators and dedicated circuits for the surge, not the steady state.
- Feed long runs at 240 V. An outbuilding 40 m (130 ft) away loses far less percentage voltage at 240 V than at 120 V for the same wattage — check with the Voltage Drop Calculator before trenching.
- Oversize conduit once. Going up one trade size on conduit costs little now and avoids a costly re-pull when you add circuits later — labor, not conduit, dominates the cost.
- Plan for the loads you’ll add. EV chargers, induction ranges, heat pumps, and hot tubs are the loads that force panel upgrades later. If the budget allows, size the service for them now.
- Apply the 125% rule to EV chargers. EV charging is a continuous load by definition — a 32 A charger needs a 40 A circuit. Check breaker sizing FAQs for edge cases.
- Separate solar from load planning. Size the array for annual kWh production, but size the inverter and interconnection for peak watts — the Solar Panel Calculator handles the array; your installer handles the rest.
- Get the permit before the work. Pulling permits first is cheaper than correcting failed inspections later — and most AHJs require permits for any new circuit.
More guidance: Electrical Load Calculator: Pro Tips.
FAQ
What wire size do I need for a 20-amp circuit?
Standard residential answer: 12 AWG copper (about 3.3 mm²) on a 20 A breaker. But verify with the Wire Size Calculator — long runs may need 10 AWG to control voltage drop, and continuous loads change the breaker sizing.
How do I calculate voltage drop for a long wire run?
Voltage drop = 2 × current × conductor resistance × length (single-phase). Practically, enter your amps, one-way length, and wire size into the Voltage Drop Calculator — aim for under 3% on branch circuits. If drop is too high, upsize the wire or run at higher voltage.
What size breaker do I need for a 240V appliance?
Divide watts by 240 to get amps, then size the breaker at 125% of the load (the continuous-load rule) and round up to the next standard size. A 4,800 W water heater draws 20 A → 25 A → use a 30 A breaker. Confirm with the Breaker Size Calculator.
How do I know if my home needs a panel upgrade?
Total your loads with the Electrical Load Calculator. Warning signs: frequent breaker trips, a fuse box, a 100-amp service feeding a modern home, or planned additions (EV charger, heat pump, hot tub). If calculated demand exceeds 80% of your service rating, have an electrician evaluate an upgrade.
How many solar panels do I need to power my house?
It depends on daily kWh usage and peak sun hours. A U.S. home using 30 kWh/day with 5 sun hours needs about a 7.2 kW system — roughly 18 × 400 W panels before efficiency losses. The Solar Panel Calculator does this math with derating built in.
What size generator do I need for a house?
Add up the running watts of everything you’ll power at once, then add the starting watts of the largest motor. Whole-house coverage typically needs 12–22 kW; essentials-only (fridge, furnace fan, lights, well pump) often fits a 7.5–10 kW portable. The Generator Size Calculator walks through this.
Can I do my own electrical work, or do I need an electrician?
Rules vary by jurisdiction — many places allow homeowners to do limited work on their own residence with a permit and inspection, but some require licensed electricians for everything. Electrical work can be dangerous: arc flash, shock, and fire risks are real. Always check your AHJ, pull permits, and have a licensed electrician handle service panels, mains work, and anything you’re unsure about.
How much does it cost to wire a new house?
Rough-in costs for a mid-size new home typically run $8,000–$15,000 (U.S. planning range, 2026), plus panel/service, fixtures, and trim-out. Copper prices, labor markets, and panel capacity swing the number. Build your project-specific budget with the Electrical Cost Calculator and confirm with 2–3 licensed quotes.
What is conduit fill and why does it matter?
Conduit fill is how much of a conduit’s interior cross-section conductors occupy. The NEC limits it to 40% for three or more wires so heat can dissipate and future conductors can be pulled. Overstuffed conduit overheats and fails inspection — the Conduit Size Calculator applies the fill limits automatically.
What is the 80% rule for breakers?
A breaker feeding a continuous load (3+ hours of operation) may only be loaded to 80% of its rating — so you size breakers at 125% of the continuous load. A 16 A EV charger → 20 A breaker minimum. This is NEC 210.19/210.20 and it’s baked into the Breaker Size Calculator.
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