Calculators
Electrical calculators that show their work
Every calculator explains its answer step by step, names the code section it relies on, and works offline once your phone has loaded it. NEC and IEC side by side, free and with no account.
NEC field calculators
Wire size, ampacity, voltage drop, conduit and box fill, load calculations and grounding, edition by edition from 2017 to 2026.
- Residential Load CalculatorWork out a one-family dwelling’s calculated load and minimum service size by the standard or optional method, edition by edition, from NEC 2017 to the new 2026 Article 120.
- Wire Size CalculatorThe conductor, breaker and equipment ground for a load, with the 125 % rule, derating, terminal temperatures and voltage drop worked out step by step for each NEC edition.
- Ampacity CalculatorThe ampacity of a conductor as installed: table value, ambient and fill derating, rooftop heat, the terminal limit and the largest breaker, for NEC 2017 to 2026.
- Voltage Drop CalculatorVoltage drop in volts and percent, the voltage at the load, the longest run within your limit and the smallest conductor that meets it, by the NEC, IEC 60364 or BS 7671.
- Ground Wire Size CalculatorEquipment grounding conductors from Table 250.122, including the proportional increase when phase conductors are upsized and the rules for parallel runs, plus grounding electrode conductors from Table 250.66.
- Conduit Fill CalculatorCheck any mix of conductors, including compact and bare ones, in 14 raceway types, find the smallest trade size that works, or list the most of one conductor each trade size takes, edition by edition.
- Box Fill CalculatorAdd up the volume allowances for every conductor, device, clamp, fitting and grounding conductor, compare them with a standard or marked box plus rings, and see how many more wires fit, under the edition you work to.
- Available Fault Current CalculatorFind the bolted fault current at a transformer and at the end of a feeder by the impedance method, compare it with equipment ratings, and get the number for the NEC 110.24 label.
IEC and BS 7671 calculators
Cable sizing with installation methods and correction factors, earth fault loop impedance, CPC sizing and fault current.
- Cable Size CalculatorFind the smallest cable that carries the load as installed and keeps the voltage drop in limit, from the BS 7671 Appendix 4 or IEC 60364-5-52 tables, with every factor shown.
- Earth Fault Loop Impedance CalculatorWork out Zs for a circuit, the maximum its protective device allows for automatic disconnection, the longest run that complies, and the most a test should read.
- CPC Size CalculatorCheck that a protective conductor survives the fault energy until the device disconnects, with k worked out from first principles and the simplified table as a second answer.
- Prospective Fault Current CalculatorTurn loop impedance readings into prospective short-circuit and earth fault currents, estimate three-phase levels, and check the breaking capacity of the protective devices.
Electrical power and conversions
Ohm’s law, amps, watts and kVA, three-phase power and transformers. The physics is the same everywhere.
- Ohm’s Law CalculatorEnter any two of voltage, current, resistance and power. The other two appear instantly, with the working shown step by step.
- Amps to Watts CalculatorWatts = amps × volts. On AC, multiply by the power factor too, and for three-phase by √3 as well. Enter the current and the voltage to get watts, kilowatts and volt-amperes, with every step shown.
- Watts to Amps CalculatorAmps = watts ÷ volts. On AC, divide by the power factor too, and for three-phase by √3 as well. Enter the power and the voltage to get the current, with the working and a breaker check.
- Three-Phase Power CalculatorEnter the line-to-line voltage and either the line current or the load in kW or kVA. You get kW, kVA, kVAR and line current, with the working.
- Transformer CalculatorFind primary and secondary full-load amps and the turns ratio, size a transformer for a load, or estimate the secondary fault current from %Z.
Electronics calculators
Resistor and capacitor codes, LED resistors, voltage dividers and the 555 timer.
- Resistor Color Code CalculatorPick the bands to get the resistance and tolerance, or type a value to see its colours. Works for 3, 4, 5 and 6-band resistors.
- SMD Resistor Code CalculatorPick the characters printed on a chip resistor to get its value, or enter a resistance to see how it is marked. Covers 3-digit, 4-digit, R notation and EIA-96 codes, with package sizes and power ratings.
- Capacitor Code CalculatorPick the three digits printed on a capacitor, plus any tolerance letter, voltage code or dielectric, to get the value in pF, nF and µF. Or enter a value to find its code.
- Voltage Divider CalculatorFind the output of a resistor divider, or the resistor values for a target voltage, with the effect of the load, the current, the power in each resistor and the nearest standard values.
- LED Resistor CalculatorEnter the supply voltage, the LED colour and the current you want. Get the exact resistor, the next standard value up, the current it really gives and the wattage to buy, for one LED or several.
- 555 Timer CalculatorWork out a 555’s astable frequency and duty cycle, a monostable pulse width, or the R1 and R2 for a target frequency, with standard values and every step shown.