Free Electrical Calculators — NEC Sizing, Protection & Cost
Size a circuit end to end without opening the code book: conductor gauge from ampacity and voltage drop, the breaker that protects it, the conduit and box that hold it, and the grounding conductors that go with it. Convert whatever the nameplate gives you — watts, horsepower, kVA — into the amps the sizing tools need. Run the whole-house load calculation that decides whether the service can take it. Then price the job. Every tool is free, instant, and explains its formula in plain English with the NEC article cited.
Code baseline: NEC 2023. NEC 2026 was published in August 2025, but most states still enforce the 2020 or 2023 edition — and local amendments override the model code either way. Results are for estimating and planning; a licensed electrician and your AHJ have final say on anything installed.
Conductors, Raceway & Protection
Size a circuit end to end — conductor gauge from ampacity and voltage drop, the breaker that protects it, the conduit and box that hold it, and the grounding conductors that go with it.
Wire Size Calculator
The AWG your circuit actually needs — sized by NEC Table 310.16 ampacity and by voltage drop, then governed by whichever one demands the larger conductor.
Answers: "What size wire do I need for 50 amps?"
Open calculatorVoltage Drop Calculator
Voltage drop in volts and percent for any gauge, length, and load — plus the longest run that still lands inside the 3% recommendation.
Answers: "How much voltage drop on a 100-foot run?"
Open calculatorAmpacity & Derating Calculator
Derated ampacity after ambient temperature correction and conductor bundling — then capped by the 75°C termination rule and the small-conductor limits.
Answers: "How many amps can 10 AWG carry at 40°C with 6 conductors?"
Open calculatorBreaker Size Calculator
The overcurrent device your load requires — continuous loads at 125%, rounded up to the next standard NEC rating, with the conductor that must match it.
Answers: "What size breaker do I need for a 40 amp load?"
Open calculatorService & Feeder Wire Size Calculator
Dwelling service and feeder conductors from NEC Table 310.12 — the 83% allowance that lets 4/0 aluminum carry a 200 amp service, with the neutral and GEC to match.
Answers: "What size wire for a 200 amp service?"
Open calculatorGround Wire Size Calculator
Equipment grounding conductor from the breaker rating and grounding electrode conductor from the service conductors — two different tables, plus the electrode exceptions.
Answers: "What size ground wire for a 200 amp service?"
Open calculatorConduit Fill Calculator
NEC Chapter 9 fill percentage for EMT, PVC, RMC, and IMC — how full the raceway is, how many more conductors fit, and the next trade size up.
Answers: "How many #12 wires fit in 1/2 inch EMT?"
Open calculatorBox Fill Calculator
Cubic inches required by NEC 314.16 with conductors, clamps, devices, and grounds each counted the way the code actually counts them.
Answers: "What size box do I need for 6 wires and a receptacle?"
Open calculatorLoad & Service Sizing
Run the whole-house load calculation, turn it into a service rating, prove what the service already draws from measured demand, and size backup power — before anyone quotes a panel upgrade.
Residential Load Calculator
Whole-house calculated load in VA and amps by the NEC 220.82 optional method — general lighting, small appliance, laundry, appliances, and the largest HVAC term.
Answers: "How do I do a residential load calculation?"
Open calculatorService Size Calculator
Turn a calculated load into a service rating — and see exactly how much headroom is left for an EV charger, a heat pump, or an induction range.
Answers: "What size electrical service do I need?"
Open calculatorExisting Load Calculator
The NEC 220.87 method — twelve months of measured utility demand at 125% establishes the existing load, so you can prove a service has room for a new one.
Answers: "Can I add an EV charger to a 100 amp service?"
Open calculatorCommercial Load Calculator
The Article 220 Part III standard method for non-dwellings — general lighting by area, receptacles at 180 VA per strap, show window and track per foot, each with its own demand factor.
Answers: "How do I calculate a commercial electrical load?"
Open calculatorRange & Dryer Demand Factor Calculator
Table 220.55 and Table 220.54 worked properly — why a 12 kW range counts as 8 kW, why every dryer has a 5,000 VA floor, and how hard both scale in multifamily.
Answers: "How much does a 12 kW range count as?"
Open calculatorPanel Spaces & Tandem Breaker Calculator
How many more poles a panel can legally take — spaces, the listed device limit under NEC 408.54, and which of them will actually accept a tandem on a Class CTL panelboard.
Answers: "My panel is full — can I use tandem breakers?"
Open calculatorEV Charger Circuit Calculator
EVSE circuit sized as a continuous load per NEC 625 — breaker, conductor, ground, voltage drop, and whether the existing service can actually take it.
Answers: "What size wire and breaker for a 48 amp EV charger?"
Open calculatorGenerator Sizing Calculator
Standby generator kW from the loads you need to keep running — checked against both running capacity and the motor-starting surge that usually governs the size.
Answers: "What size generator do I need for my house?"
Open calculatorFormulas & Conversions
The arithmetic behind every circuit — watts to amps and back, Ohm's law, apparent versus real power, three-phase √3 math, power-factor correction, and running cost.
Watts to Amps Calculator
Convert watts to amps for DC, single-phase, and three-phase circuits — then get the breaker and conductor that current needs.
Answers: "How many amps is 1,500 watts?"
Open calculatorAmps to Watts Calculator
Convert amps to watts and volt-amperes, with power factor handled properly so real and apparent power are never confused.
Answers: "How many watts is 20 amps?"
Open calculatorOhm's Law Calculator
Enter any two of voltage, current, resistance, and power — the other two are solved instantly, with the formula used shown.
Answers: "What is the current if V is 120 and R is 10?"
Open calculatorElectrical Unit Converter
Convert between watts, kilowatts, horsepower, BTU per hour, volt-amperes, and amps — the units that appear on nameplates and never match.
Answers: "How many watts is 1 horsepower?"
Open calculatorkVA Calculator
Apparent power, real power, and reactive power from line current or load — single-phase and three-phase, with the next standard transformer size.
Answers: "How many kVA is 100 amps at 480 volts?"
Open calculatorThree-Phase Power Calculator
Line current, kW, kVA, and kVAR for balanced three-phase systems — from load, from amps, or from motor horsepower with efficiency.
Answers: "What amps does a 50 kW three-phase load draw?"
Open calculatorPower Factor Correction Calculator
Capacitor kVAR needed to move from your current power factor to a target — with the line current saved and the demand charge it avoids.
Answers: "How much kVAR to correct 0.75 power factor to 0.95?"
Open calculatorkWh Cost Calculator
What any appliance or circuit costs to run — daily, monthly, and annually — from its wattage, run time, and your electricity rate.
Answers: "What does a 1,500 watt heater cost to run?"
Open calculatorMotors, Transformers & Fault Current
Commercial and industrial work, where the rules change — table full-load current rather than the nameplate, breakers deliberately larger than the conductor, transformer inrush headroom, and the interrupting rating the equipment actually needs.
Motor Full Load Amps Calculator
The NEC Table 430.248 / 430.250 full-load current that conductors and the breaker must be sized from — plus the nameplate figure the overload device uses instead.
Answers: "What is the full load amps of a 10 HP motor?"
Open calculatorMotor Circuit Calculator
The complete Article 430 branch circuit — conductor at 430.22, protective device from Table 430.52, overload from the nameplate, and the EGC that follows the oversized breaker.
Answers: "How do I size a motor branch circuit?"
Open calculatorTransformer Sizing Calculator
Standard kVA from the connected load and a growth margin, primary and secondary full-load current, and the Table 450.3(B) overcurrent ceilings that carry magnetising inrush.
Answers: "What size transformer do I need?"
Open calculatorAvailable Fault Current Calculator
Symmetrical fault current at a transformer secondary and at the end of a feeder by the point-to-point method, with the interrupting rating NEC 110.9 requires at each point.
Answers: "How much fault current is available at my panel?"
Open calculatorConductor Resistance Calculator
Chapter 9 Table 8 resistance with temperature correction and Table 9 reactance, combined into the effective impedance that actually causes voltage drop — plus the I²R heat.
Answers: "What is the resistance of a conductor run?"
Open calculatorDevices & Lighting
Lay out what goes on the wall and the ceiling — how many outlets a circuit may serve, and the can count and spacing a room needs.
Receptacle & Circuit Calculator
How many outlets or fixtures one circuit may serve — 180 VA per strap in commercial work, no NEC count limit at all in dwellings, plus the 210.23(A) load limits.
Answers: "How many outlets can I put on a 20 amp circuit?"
Open calculatorRecessed Lighting Calculator
Can count, grid layout, and spacing for any room — worked both by the ceiling-height spacing rule and by the lumens the room's use actually needs.
Answers: "How many recessed lights do I need?"
Open calculatorSolar & Interconnection
Getting an array onto an existing service — the 705.12 busbar check that decides whether the panel can take the interconnection at all, and the temperature extremes that bound a string.
Solar 120% Rule Calculator
The NEC 705.12(B)(3)(2) busbar check — main breaker plus PV backfed breaker against 120% of the busbar rating, and the largest inverter the panel will actually accept.
Answers: "Can my panel take a solar interconnection?"
Open calculatorSolar String Sizing Calculator
Maximum and minimum modules per string — cold-weather Voc against the inverter's ceiling per NEC 690.7, hot-cell Vmp against the bottom of the MPPT window.
Answers: "How many solar panels per string?"
Open calculatorCost & Pricing
Price the work once the circuit is designed — labor, material, overhead, and the margin that has to survive all of it.
Electrical Estimate Calculator
Turn labor hours, material, and fixtures into a quoted price at your target gross margin — and see what using markup instead of margin costs you.
Answers: "How do I price an electrical job?"
Open calculatorPanel Upgrade Cost Calculator
What a service or panel upgrade actually costs — panel, meter socket, service conductors, mast, permit, and labor, built up part by part.
Answers: "How much does a 200 amp panel upgrade cost?"
Open calculatorEV Charger Installation Cost
A Level 2 install priced line by line — conductor run, wiring method, panel work, permit, and labor — with a regional multiplier and a quoting band.
Answers: "How much does it cost to install an EV charger?"
Open calculatorStandby Generator Installation Cost
A whole-house standby priced line by line — the set, the transfer switch, the gas run, the pad, the permit, and labour, with the share the machine itself actually represents.
Answers: "How much does a standby generator cost to install?"
Open calculatorRewiring Cost Calculator
Whole-house rewiring cost by square footage, wall access, and construction age — with the dollars-per-square-foot figure to sanity-check it.
Answers: "How much does it cost to rewire a house?"
Open calculatorLabor Rate Calculator
Your true loaded cost per billable hour from wage, burden, overhead, and unbillable time — then the rate to bill at your target margin.
Answers: "What should my hourly rate be?"
Open calculatorWorking on HVAC equipment?
Sizing the circuit for a condenser, air handler, or heat pump starts from the equipment nameplate, not from a load calculation. The HVAC Amp Draw Calculator turns BTU capacity into RLA, minimum circuit ampacity (MCA), and maximum overcurrent protection (MOCP) — the numbers that decide the breaker and the wire. There are 43 HVAC calculators alongside these.
More electrical tools on the way
Motor and transformer circuits, available fault current, solar interconnection and the 705.12 120% busbar rule, generator sizing, and lighting layout are next. Calculators for these trades are also planned:
Coming soon
Which calculator do I need?
Match your question to the right tool. For a full circuit the natural order is: Load → Wire → Breaker → Conduit/Box → Ground — get the load, size the conductor, protect it, then house and bond it. If you already know the amps, start at wire size.
| You're asking… | Use this tool | The code rule it's built on |
|---|---|---|
| What gauge wire does this circuit need? | Wire Size Calculator | Table 310.16 ampacity vs VD = 2×K×I×L÷CM — larger conductor wins |
| Is this run too long for the wire I have? | Voltage Drop Calculator | 1φ: VD = 2×K×I×L÷CM · 3φ: VD = 1.732×K×I×L÷CM |
| How many amps can this conductor really carry? | Ampacity & Derating Calculator | base × ambient (310.15(B)(1)) × bundling (310.15(C)(1)), capped by 110.14(C) |
| What breaker protects this load? | Breaker Size Calculator | non-continuous ×1.00 + continuous ×1.25 → next 240.6(A) rating |
| What size wire for a 200 amp service? | Service & Feeder Wire Size Calculator | Table 310.12 — dwelling conductors at 83% of the service rating |
| What size ground wire do I need? | Ground Wire Size Calculator | EGC from Table 250.122 (by OCPD) · GEC from Table 250.66 (by service conductor) |
| How many wires fit in this conduit? | Conduit Fill Calculator | Chapter 9 Table 1 — 53% for 1 conductor, 31% for 2, 40% for 3 or more |
| What size box does this rough-in need? | Box Fill Calculator | 314.16(B) — conductors ×1, all grounds ×1, clamps ×1, each device yoke ×2 |
| What is this house's calculated electrical load? | Residential Load Calculator | 220.82 — first 10 kVA at 100%, remainder at 40%, plus the largest HVAC term |
| Does the existing service have room for more? | Service Size Calculator | amps = VA ÷ 240 → smallest standard service; flag above 80% utilisation |
| What circuit does an EV charger need? | EV Charger Circuit Calculator | 625.41/625.42 — continuous load, conductors and OCPD at 125% |
| What should I quote for this job? (contractors) | Electrical Estimate Calculator | (labor + fixtures + material + overhead) ÷ (1 − margin) |
| How many amps does this wattage draw? | Watts to Amps Calculator | DC: I = P÷V · 1φ: I = P÷(V×PF) · 3φ: I = P÷(√3×V×PF) |
| How many watts is this current? | Amps to Watts Calculator | P = V×I×PF · apparent VA = V×I (or √3×V×I) |
| I know two of V, I, R, P — what are the others? | Ohm's Law Calculator | V = I×R · P = V×I · P = I²R · P = V²÷R |
| How do I convert between watts, horsepower, and BTU? | Electrical Unit Converter | 1 hp = 745.7 W · 1 W = 3.412 BTU/h · 1 ton = 12,000 BTU/h |
| How many kVA is this load? | kVA Calculator | 1φ: kVA = V×I÷1,000 · 3φ: kVA = √3×V×I÷1,000 · kW = kVA×PF |
| What line current does a three-phase load draw? | Three-Phase Power Calculator | I = kW×1,000 ÷ (√3 × V × PF) |
| How much capacitor kVAR do I need? | Power Factor Correction Calculator | Qc = P × (tan φ₁ − tan φ₂), φ = arccos(PF) |
| What does this appliance cost to run? | kWh Cost Calculator | cost = W÷1,000 × hours × days × $/kWh |
| How much does a panel upgrade cost? | Panel Upgrade Cost Calculator | panel + meter + conductors×ft + mast + permit + labor, × region |
| How much does it cost to rewire a house? | Rewiring Cost Calculator | sq ft × $/sq ft by access × age × storeys + panel + permit |
| What hourly rate do I need to charge? (contractors) | Labor Rate Calculator | (wage + burden + overhead) ÷ billable hours ÷ (1 − margin) |
| Can I add this load without upgrading the service? | Existing Load Calculator | 220.87 — measured 12-month peak × 1.25, plus the new load at 125% if continuous |
| What size standby generator do I need? | Generator Sizing Calculator | max(running × margin, running − largest motor + its starting surge) |
| How many outlets can go on one circuit? | Receptacle & Circuit Calculator | 220.14(I) 180 VA per strap in non-dwellings; 220.14(J) sets no limit in dwellings |
| How many recessed lights, and how far apart? | Recessed Lighting Calculator | spacing ≈ ceiling ÷ 2, checked against area × footcandles ÷ lumens per fixture |
| How much does an EV charger install cost? | EV Charger Installation Cost | conductor × method + hardware + panel work + permit + labor, × region |
| What is this motor's full load current? | Motor Full Load Amps Calculator | Table 430.248 / 430.250 for the circuit — nameplate only for the overload (430.6(A)(1)) |
| How do I size a motor branch circuit? | Motor Circuit Calculator | conductor 125% (430.22) · device up to 250–300% (T430.52) · overload from nameplate (430.32) |
| What size transformer, and what protects it? | Transformer Sizing Calculator | kVA × growth → standard size · primary to 250% with secondary protection (T450.3(B)) |
| How much fault current is available here? | Available Fault Current Calculator | I = FLA ÷ %Z at the secondary, then M = 1 ÷ (1 + f) down the run — checked against 110.9 |
| What is the real impedance of this run? | Conductor Resistance Calculator | Ch. 9 T8 resistance + T9 reactance → Ze = R·cos θ + X·sin θ, not √(R² + X²) |
| What load does this commercial building have? | Commercial Load Calculator | 220 Part III — area × T220.12, then T220.42; receptacles 180 VA/strap, then T220.44 |
| How much does a range or dryer really count as? | Range & Dryer Demand Factor Calculator | T220.55 Column C is kW not % (12 kW range → 8 kW) · T220.54 has a 5,000 VA floor |
| Is my panel full, and can tandems fix it? | Panel Spaces & Tandem Breaker Calculator | 408.54 — never more devices than the panel is listed for; tandems only in CTL-rated slots |
| Can this panel take a solar interconnection? | Solar 120% Rule Calculator | 705.12(B)(3)(2) — main + PV breaker ≤ 120% of busbar, PV at the opposite end |
| How many modules per string? | Solar String Sizing Calculator | cold Voc ≤ inverter max (690.7) · hot Vmp ≥ MPPT minimum |
| How much does a standby generator install cost? | Standby Generator Installation Cost | set + transfer switch + fuel run × ft + pad + permit + labor, × region |
Every electrical formula on one page
Every rule behind these tools, with the NEC article that carries it. Each calculator's page walks through its formula in plain English with worked examples — this is the cheat sheet.
These reproduce NEC tables and formulas, but they cannot see your ambient conditions, existing conductors, terminal ratings, or local amendments. Confirm final design with a licensed electrician and the edition of the code your jurisdiction actually enforces.
Frequently asked questions
About the calculators, which code edition they follow, and what's next.
Are these electrical calculators really free?
Yes. Every one of them is completely free, runs instantly in your browser, and requires no account or sign-up. They're planning and field-check tools built by TradesQuote, the AI estimating platform for trades businesses.
Which NEC edition are these calculators based on?
The 2023 edition of NFPA 70 (the National Electrical Code), with 2026 changes noted on the pages where a rule actually moved. NEC 2026 was published in August 2025, but most states still enforce the 2020 or 2023 edition — check which cycle your jurisdiction has adopted, because local amendments override the model code either way.
Which electrical calculator should I use first?
For a new circuit the order is Load → Wire → Breaker → Conduit/Box → Ground. If you already know the amps, start at the Wire Size Calculator. If you're sizing or upgrading a service, start at the Residential Load Calculator and follow it into the Service Size Calculator. If the job is an EV charger, the EV Charger Circuit Calculator does the whole chain including the panel-capacity check. If you only have a wattage or a horsepower rating to work from, convert it first with the Watts to Amps Calculator or the Unit Converter, then carry the amps into the sizing tools.
Can I use these results to pull a permit or pass inspection?
Treat them as planning and verification tools, not as a submittal. They apply the same NEC tables and formulas an electrician works from, but they can't see your ambient conditions, existing conductors, terminal ratings, or local amendments. A licensed electrician and your AHJ have final say on anything installed.
Is the 3% voltage drop limit actually a code requirement?
No — and this is one of the most widely repeated errors online. The 3% branch-circuit and 5% total figures appear in Informational Notes to NEC 210.19(A) and 215.2(A). Informational Notes are explanatory and not enforceable requirements. They are strong engineering practice and many jurisdictions and specifications adopt them as mandatory, so our calculators check against them by default and tell you when drop is the reason a conductor got upsized.
Do these work for commercial and industrial work?
Yes. The Commercial Load Calculator runs the NEC Article 220 Part III standard method for non-dwellings — general lighting by area, receptacles at 180 VA per strap, show-window and track lighting per foot, each with its own demand factor. The conductor, ampacity, breaker, conduit fill, box fill, and grounding calculators are code-general and work for commercial installations at the same voltages, and the three-phase, kVA, and power-factor tools are built for 208 V, 480 V, and 600 V systems. For industrial work there are dedicated tools for motor full-load amps, complete motor branch circuits, transformer sizing and NEC 450.3(B) protection, available fault current by the point-to-point method, and conductor impedance from Chapter 9 Tables 8 and 9. The residential load calculators remain dwelling-only, because the 220.82 optional method is a dwelling shortcut with no commercial equivalent.
Do you have calculators for other trades?
Yes — there are 43 free HVAC calculators covering sizing, airflow, diagnostics, efficiency, and cost. Electrical is the second set, and calculators for plumbing, carpentry, painting, and cleaning are planned next, matching the trades TradesQuote supports for AI-powered estimating.
Trusted Electrical Standards & Resources
Every ampacity, fill percentage, demand factor, and grounding size on these pages traces back to a published standard. These are the organisations that write them and the data sources behind the cost figures.
The National Electrical Code — the source for every ampacity, conduit fill, box fill, grounding, and load-calculation rule these tools apply. Free read-only access via NFPA LiNK.
Which code cycle your jurisdiction actually enforces. States routinely run one or two editions behind the current NEC, and local amendments override the model code.
Institute of Electrical and Electronics Engineers — standards behind fault-current calculation, grounding practice, and power quality.
Listing and labelling, terminal temperature ratings, and interrupting ratings (AIC) — the equipment side of what the NEC allows you to install.
National Electrical Contractors Association — publisher of the Manual of Labor Units (MLU) and the National Electrical Installation Standards used in estimating.
Energy Information Administration — the residential and commercial electricity prices per kWh behind every operating-cost and payback figure.
Calculators size the circuit. TradesQuote prices the job.
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Guides and deep-dives on NEC sizing, code compliance, and electrical job pricing.
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