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Electrical

Ohm's law, electrical power, cable sizing and other electrical calculations.

Ohm's law calculatorWork out voltage, current and resistance.Electrical power calculatorSingle-phase and three-phase power.kVA to kW converterApparent and real power with power factor.Cable size calculatorSize conductors by current and cable run length.Capacitor calculatorWork out capacitance, charge and stored energy.Resistor calculatorTotal resistance of series and parallel combinations.

Electrical calculations are needed by an electrician designing an installation and by a homeowner working out whether the existing cable will carry a new appliance. The six calculators below cover three areas: the basic relationships between voltage, current and resistance, sizing the cable run, and the conversions around a supply connection.

If you are not sure which one you need, start from what you already know. You know an appliance's power rating and want the current it draws — Ohm's law, or the electrical power calculation. You know the current and the cable length and want the conductor size — cable sizing. Your supply agreement quotes apparent power in kVA and you need real power in watts — the kVA to kW converter. The remaining two work out capacitor capacitance and the combined resistance of resistors in series or parallel.

The result is always indicative. For real work, an electrician has to sign off the conductor size and the protective device against the wiring regulations, because installation method and ambient temperature come into it too — and the calculator knows nothing about your building.

Electrical calculation guides
ConversionkVA to kW: what the difference actually meansA generator rated 100 kVA doesn't give you 100 kW. The gap between the two numbers is the power factor, and getting it wrong is the single most common reason people undersize or oversize a generator, UPS or transformer.6 min read
BasicsOhm's law explained: U = R x I, with worked examplesOhm's law is one equation – U = R × I – but it answers three different questions depending on which of the three values you already know. Here's how to use it, and how it connects to power.7 min read
TechniqueHow to size a cable: current, length and voltage dropA cable that's too thin for its length doesn't just risk overheating – it can lose enough voltage over the run that equipment at the far end underperforms or trips out. Here's the calculation method BS 7671 is built around, and where its limits are.8 min read