Electrical Calculators
Practical calculators for electricians, electrical engineers and serious DIYers. Wire sizing follows NEC conductor tables; every result shows the formula, the assumptions and the limitations of the model.
Circuit Design & Protection
Wire sizing, ampacity derating, overcurrent protection and conductor properties.
- Wire Size CalculatorFind the minimum copper wire size for your load current. NEC Table 310.16 ampacity at 75 °C, AWG and metric cross-sections, with margin over load and code notes.
- Wire Derating CalculatorApply NEC 310.15 derating: temperature correction factors and conductor fill adjustments. Shows the effective ampacity after both reductions.
- Breaker Size CalculatorFind the correct breaker size for a load, including the 125% continuous-load rule and standard breaker ratings from 15 to 400 A.
- Wire Resistance CalculatorCalculate wire resistance from AWG size, length and material. Copper and aluminum, per 1000 ft and per meter, with temperature correction — NEC Chapter 9 Table 8 data.
- Cable Reactance CalculatorCalculate AC cable impedance including reactance for large conductors, where NEC Table 9 values matter. More accurate voltage drop for big wire runs.
Voltage, Current & Power
Core electrical relationships: voltage drop, Ohm's law, three-phase power, motor current and power factor.
- Voltage Drop CalculatorCalculate voltage drop, % drop, end-of-line voltage and power lost for copper or aluminum circuits. Single and 3 phase voltage drop calculator with NEC Chapter 9 Table 8 conductor resistances.
- Ohm's Law CalculatorSolve any Ohm's law variable: leave one of voltage, current or resistance blank and get it, plus power in watts. Clean, fast and with a worked explanation.
- kVA to Amps CalculatorConvert kVA, kW or horsepower to amps for single and three-phase circuits. Includes kW↔kVA power factor conversion and the exact formulas used.
- Three-Phase Power CalculatorCalculate three-phase real, apparent and reactive power from line voltage, current and power factor. The complete power triangle in one step.
- Motor Current CalculatorEstimate single or three-phase motor full-load current from hp, voltage, efficiency and power factor. Includes code notes on NEC 430 tables and 125% sizing.
- Power Factor Correction CalculatorCalculate the capacitor kVAR needed to correct power factor from current to target. Shows apparent power freed, current reduction and the tan φ method.
Networks & Transients
Series and parallel networks, current dividers and RC charging behavior.
- Series Resistor CalculatorCalculate the equivalent resistance of two to five resistors in series, with the largest voltage-share note. The series companion of the parallel calculator.
- Current Divider CalculatorCalculate how current splits between two parallel branches: branch currents, equivalent resistance and percentage share — the dual of the voltage divider.
- RC Time Constant CalculatorCalculate the RC time constant, voltage after any time, and time to reach a target percentage — the 63.2% and 5τ design milestones explained.
Components & Electronics
Resistor identification, parallel networks, reactance, dividers and LED circuit design.
- Resistor Color Code CalculatorDecode 4-band resistor values from band colors: resistance, tolerance and the acceptable range. Standard EIA color code with all twelve colors.
- Parallel Resistor CalculatorCalculate the equivalent resistance of two to five resistors in parallel, with conductance and the dominant-branch note. Instant result per EIA color values.
- Reactance CalculatorCalculate inductive reactance (Xʟ = 2πfL) or capacitive reactance (X꜀ = 1/2πfC) at any frequency, with angular frequency and the phase behavior explained.
- Capacitor Energy CalculatorCalculate the energy and charge stored in a capacitor from capacitance and voltage: joules, watt-hours, coulombs and mAh, with the V² relationship explained.
- Voltage Divider CalculatorCalculate the output voltage of a two-resistor divider, with divider current and per-resistor dissipation. Includes loading warnings for real circuits.
- LED Resistor CalculatorCalculate the current-limiting resistor for one or more LEDs, series or parallel, with power dissipation and nearest standard E12 values.
Power Systems & Energy
Transformer sizing, energy costs, EV charging and generator selection.
- Transformer Sizing CalculatorSize a single-phase transformer from load kW and power factor, with design margin and primary/secondary currents. Standard kVA ratings listed.
- Electricity Cost CalculatorCalculate electricity cost in kWh per day, month and year. Free kWh cost calculator and electricity price calculator for home appliances, devices, and power consumption.
- EV Charging Time CalculatorCalculate EV charging time from battery size and charger power, including charge window, efficiency losses, range added and electricity cost.
- Generator Size CalculatorGenerator sizing calculator: total your running watts, add the largest motor's starting surge and get the minimum kW rating, with the sizing formula explained.
Design guide
How these calculations fit together in real projects — the order to run them in, and the decisions each one answers.