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Electrical calculators

Ohm's law and power, single and three-phase kW, kVA and amps, energy cost, resistor and capacitor code decoding, series and parallel resistance, and the cost of losses in a cable run.

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Planning aid; verify against local code and the manufacturer's data; the qualified person on site is responsible. Nothing leaves this page.

Ohm's law and power

Fill in any two of the four boxes; the other two are calculated. (Volts, amps, ohms, watts.)

Voltage V

Current I (A)

Resistance R (Ω)

Power P (W)

V = I·R, P = V·I = I²·R = V²/R. If more than two boxes are filled, the first two in the order V, I, R, P are used.

Single-phase and three-phase power

Supply

Voltage (line-to-line for three-phase)

Power factor (0 to 1)

I know

Value (amps or kW)

Single-phase: kW = V·I·PF/1000. Three-phase: kW = √3·V·I·PF/1000. kVA = kW/PF.

Energy cost (kWh)

Power, kW

Hours per day

Days

Price per kWh (your currency)

Resistor colour code

Bands

Band 1 (digit)

Band 2 (digit)

Band 3 (digit)

Multiplier band

Tolerance band

Capacitor code (3-digit marking)

Three digits, e.g. 104 or 473

Tolerance letter (optional)

First two digits, then the number of zeros, in picofarads (pF). Colour-banded capacitors were marked differently by different makers and eras, so for those check the manufacturer's data rather than a generic table.

Resistors in series and parallel

Values (ohms), separated by commas or spaces; suffixes k and M are accepted, e.g. 4.7k 10k 220

Series: R = R1 + R2 + …. Parallel: 1/R = 1/R1 + 1/R2 + ….

Cost of losses in a cable run

Supply

Material

Conductor area, mm²

Length one way, m

Current, A

Hours of use per year

Price per kWh

Loss = (number of conductors) · I² · ρL/A, at 20 °C, assuming constant current. Real conductors run warmer, which raises the loss a little. Use the voltage drop calculator on this site for drop and size-finding.