Essential Basic Electrical Engineering Formulas – A Practical Guide for Technicians & Engineers

Whether you're a fresh technician, an engineering student, or an experienced professional, understanding key electrical formulas is a must for fieldwork and project execution. This guide will help you revise and apply the most commonly used electrical formulas in daily practice.

1. Ohm’s Law (The Foundation of Everything)

Formula:

V = I × R

Where:

V = Voltage (Volts)

I = Current (Amperes)

R = Resistance (Ohms)

Use:

Ohm’s Law is the most basic and important formula in electrical work. It helps determine current flow, voltage drop, and resistance in any circuit.

Example:

If a bulb has 60 ohms resistance and is connected to 230V, the current is:

I = V / R = 230 / 60 = 3.83 A

2. Power Formula (Single Phase & Three Phase)

Single Phase Power:

P = V × I × PF

(Where PF = Power Factor, usually 0.8 to 1)

Three Phase Power:

P = √3 × V × I × PF

Use:

Used to calculate power consumed or supplied by a device.

Example (3 Phase):

If V = 415V, I = 10A, PF = 0.9

P = 1.732 × 415 × 10 × 0.9 = 6470 Watts ≈ 6.47 kW

3. Energy Consumption (kWh Formula)

Formula:

Energy = Power × Time

kWh = (Watt × Hours) / 1000

Use:

Used to calculate energy bills or load analysis.

Example:

A 2kW motor running for 5 hours:

Energy = 2 × 5 = 10 kWh

4. Horsepower to kW Conversion

Formula:

1 HP = 0.746 kW

1 kW = 1.34 HP

Use:

Used to convert motor ratings between HP and kW.

Example:

5 HP motor = 5 × 0.746 = 3.73 kW

5. Cable Current Carrying Capacity (Thumb Rule)

Formula (Approx.):

1 sq.mm copper wire = 6 to 9 Amps

1 sq.mm aluminium wire = 4 to 6 Amps

Use:

To estimate the size of cable needed for a certain current.

Example:

For a 20A load, use 3.5 sq.mm copper wire.

Note: Always check actual values from cable datasheet or IS standards.

6. Voltage Drop Formula

Formula:

Vdrop = (√3 × I × L × R) / 1000 (3 Phase)

Where:

I = Current

L = Cable length (one-way)

R = Resistance per km

Use:

Important for long cable runs to ensure voltage remains within limits.

7. Power Factor Correction Formula

kVAR = kW × (tan φ1 – tan φ2)

Where:

φ1 = existing PF angle

φ2 = desired PF angle

Use:

Helps size capacitor banks for improving PF in industrial installations.

Tips for Using These Formulas in the Field

Always check units (V, A, W, etc.) before calculating.

Keep a printed chart of cable sizes and kW to HP conversion.

Use clamp meter and multimeter readings to plug into formulas.

Always consider safety margin while choosing wire size.

Practice these calculations before job interviews or site tests.

Also read: How to Test Earthing Resistance

Conclusion

Understanding and applying basic electrical engineering formulas can help technicians and engineers troubleshoot faster, design safer systems, and save money. Whether you're sizing cables, selecting a breaker, or calculating motor load, these formulas are your toolbox essential.