Calculators that show their work.
Every result comes with the formula, the reference it follows and a worked example you can check by hand. Free, no sign-up, and everything runs in your browser.
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Live: fill in any two values and the other two follow.
Three toolboxes
Electrical
Power, current, breaker and cable basics for single-phase and three-phase circuits.
16 calculators →Solar and EV
Panels, batteries, inverters, DC cable and home EV charging, with every assumption editable.
6 calculators →Protection and MV
Transformers, fault levels, CTs, relay curves, earthing and power factor, from a protection engineer.
8 calculators →Find the one you need
Ohm's law
Solve voltage, current, resistance or power from any two.
Amps to watts
DC, single-phase and three-phase, with power factor.
kW to amps
Line current for a known load, single or three-phase.
kVA to amps
Full-load current of transformers and generators.
Watts to amps
Current for a known load on DC, single-phase or three-phase.
kVA to kW
Real, apparent and reactive power from the power factor.
kW to HP
Kilowatts and horsepower, mechanical or metric, with motor input power.
Three-phase power
kVA, kW and kvar from voltage, current and power factor.
Voltage drop
Volts and percent lost in a cable, with optional reactance.
NECBreaker size
Minimum breaker for continuous loads using the 125 % rule.
AWG to mm²
Wire gauge to area and diameter, and back.
NECWire size
Conductor size from load, derating and voltage drop.
NECConduit fill
Smallest EMT for a mix of conductors.
NECMotor circuit
Table full-load current, conductor, overload and device maximum.
NECHP to amps
NEC table full-load current for a motor, plus a running-current estimate.
Generator size
kW and kVA from a load list, with motor starting.
Solar panel size
How many panels for your daily energy use.
Battery bank
Capacity in Ah and kWh, or runtime of a battery.
Off-grid system
Panels, battery, inverter and controller together.
Inverter size
Continuous and surge rating, including motor starts.
DC cable size
Smallest cable for a voltage-drop limit.
NECEV charger circuit
Breaker, ampacity and cable for a home charger.
Transformer full-load current
Primary and secondary current from kVA and voltage.
Short-circuit current
Fault level at a transformer secondary from %Z.
IECFault current at cable end
Source, transformer and cable impedance, with peak and minimum values.
IECCable size with derating
Tabulated rating needed after IEC derating factors.
IECCT burden
Connected burden and effective accuracy limit factor.
IECIDMT relay time
Operating time on the four IEC 60255 curves.
Earth rod resistance
One driven rod from soil resistivity and length.
Power factor correction
Capacitor kvar to reach a target power factor.
No calculator matches that search. Try a shorter word such as "cable" or "relay".
Built to be checked
The formula is on the page
No black box. Each calculator states its formula and the standard or method behind it.
A worked example you can redo
Enter the example's numbers into the calculator and compare with the hand calculation.
Tested against hand calculations
Every calculation has automated tests with expected values worked out independently. A failing test blocks the release.
Simplifications are stated
Where a method leaves something out, such as reactance or motor contribution, the page says so.
Worked in full
Voltage drop formula
DC, single-phase and three-phase, worked in mm² and AWG.
GUIDERelay setting checklist
Seven stages for a transformer-feeder overcurrent relay, with a worked example.
GUIDEHow to size a circuit breaker
Six steps under the NEC, with EV, mixed-load and water-heater examples.
WORKBOOKElectrical Engineer's Toolkit
The calculations in a spreadsheet you can keep, check and adapt.