Electrical calculations for everyday engineering work.

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These are calculation aids. Cable and breaker choices must be confirmed against your local wiring code and the manufacturer's data before use. Cable current ratings follow IEC 60364-5-52 reference method C for copper PVC cable.

Formulas and common questions

What is Ohm's law?

Voltage V = current I × resistance R. Power P = V × I = I² × R = V² ÷ R. For example, 230 V across 46 ohms gives 5 A and 1150 W.

How do I calculate current from power in single-phase and three-phase?

Single-phase: I = P ÷ (V × power factor). Three-phase: I = P ÷ (√3 × V × power factor), with V as the line-to-line voltage. A 50 kW load at 400 V and power factor 0.85 draws about 84.9 A.

How do I choose a circuit breaker size?

Work out the design current, add a margin for continuous loads if your code asks for it (25 percent is common), and pick the next standard rating above it: 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250, 320, 400 A and so on. The breaker must also not exceed the cable's current capacity.

How do I size a cable and check voltage drop?

Pick the smallest cable whose current capacity, after temperature and grouping correction, is at least the breaker rating and the design current. Then check the voltage drop: three-phase ΔV = √3 × I × L × (R cosφ + X sinφ), single-phase uses 2 instead of √3. Limits of 3 to 5 percent are typical. For a fault, the cable area must be at least fault current × √time ÷ k.

How do I calculate the capacitor needed for power factor correction?

kVAR = P × (tan φ1 − tan φ2), where φ1 and φ2 are the angles of the present and target power factor. For 250 kW going from 0.75 to 0.95 you need about 138 kVAR.

How do I calculate the electricity bill?

Energy in kWh = power in kW × hours per day × days. Cost = kWh × price per kWh plus any fixed charge and tax. Four 75 W fans for 10 hours a day over 30 days use 90 kWh.

What is demand factor in a load calculation?

Demand factor is the share of the connected load that actually runs at the same time. Demand load = connected load × demand factor. The main breaker, transformer or generator is sized on the demand, plus spare capacity for growth.