Guide to Electrical Load Calculator Info graphic

The Ultimate Guide to Electrical Load Calculation

Demystifying the essential formulas and concepts needed to plan a safe and efficient electrical system for your home.

1. What is Real Power? (P)

Real Power is the energy that does the actual work—powering your lights, heating your water, and running your appliances. It's the sum of the wattage of all your devices.

P_total = P₁ + P₂ + ... + Pₙ

2. Understanding Apparent Power (S)

Apparent Power is the total power in a circuit, including Real Power and Reactive Power (used by motors, etc.). It's crucial for sizing transformers and generators. The key is the Power Factor (PF).

S = P / PF

A lower Power Factor means you need more Apparent Power to do the same amount of work.

This chart shows how the required Apparent Power (kVA) increases as the Power Factor drops for a fixed 10kW Real Power load. A Power Factor of 1.0 is most efficient.

3. Calculating Total Current (I)

This is the most critical value for safety. It determines the size of your wires and circuit breakers. It's calculated from the Total Apparent Power and the system's Voltage.

I_total = S_total / V_max

4. Sizing Your System: Wires & Breakers

Based on the total current, you can select the correct wire and circuit breaker sizes. Wires are sized by their cross-sectional area (mm²) and their ability to handle current (ampacity).

Wire Ampacity Comparison

Copper is a better conductor than aluminum, meaning it can carry more current for the same size.

Choosing a Breaker

Always select the next standard-sized breaker that is equal to or greater than your total current. This protects the circuit from overloads.

If your total current is:

18.5 A

Your standard breaker size is:

20 A

5. Final Check: Voltage Drop (Vd)

Voltage decreases over the length of a wire. Too much drop can damage appliances. The calculation depends on cable length, current, and wire properties.

Vd = (2 × L × I × ρ) / A

Factors Affecting Voltage Drop

📏

Longer Cable (L)

Increases voltage drop

⚡

Higher Current (I)

Increases voltage drop

🔬

Smaller Wire (A)

Increases voltage drop

Calculation Workflow Summary

1. Sum Real Power
(Watts)

➡️

2. Calc Apparent Power
(using PF)

➡️

3. Calc Total Current
(using Voltage)

➡️

4. Size Wire & Breaker
(using Current)

This infographic provides simplified estimations for educational purposes. Always consult a qualified electrician and adhere to local electrical codes for any real-world installation.