Electrical Formula Cheat Sheet
Vision Matrix Institute | Engineer's Toolkit
1. Ohm's Law
V = I × R | I = V / R | R = V / I
V = voltage (volts), I = current (amps), R = resistance (ohms)
2. Power (DC / Resistive Circuits)
P = V × I | P = I² × R | P = V² / R
3. Single-Phase AC Power
P = V × I × cos(φ)
cos(φ) = power factor
4. Three-Phase AC Power
P = √3 × VL × IL × cos(φ)
VL, IL = line voltage and line current; √3 ≈ 1.732
5. Voltage Drop
Single-phase: Vd = 2 × I × R × L
Three-phase: Vd = √3 × I × R × L
%VD = (Vd / V) × 100
R in ohms/metre, L = one-way cable length in metres
6. Cable Short-Circuit Withstand (Adiabatic Equation)
Smin = (I × √t) / k
Smin = minimum conductor CSA (mm²), I = fault current (A), t = clearance time (s)
| Conductor / Insulation | k value |
|---|
| Copper + PVC | 115 |
| Copper + XLPE | 143 |
| Aluminium + PVC | 76 |
| Aluminium + XLPE | 94 |