Electrical Load Calculator

What is Load Calculation and Breaker Sizing?

Load calculation is the foundation of electrical design. Based on the total power of all electrical equipment, demand factor, power factor and other parameters, it calculates the actual current requirement and selects the appropriate circuit breaker (NFB) capacity. Correct load calculation not only ensures electrical safety, but also avoids tripping due to insufficient breaker capacity, or cost waste and protection failure due to excessive capacity.

How to Use This Calculator?

1. Add Appliances: Click "Add Appliance" or select from the appliance database
2. Enter Appliance Info: Fill in name, power (W) and quantity for each appliance
3. Select Voltage Type: 120V single-phase, 240V split-phase or 208V three-phase
4. Set Demand Factor: General residential 50-80% (not all appliances used simultaneously)
5. Set Safety Factor: NEC recommends 1.25x, high starting current appliances recommend 1.5-2.0x
6. Adjust Power Factor: Resistive loads 1.0, inductive loads ~0.8-0.9
7. View Recommended Breaker: System calculates standard breaker specification

Calculation Formulas

1. Total Load Power

Total Load (W) = Σ(Each Appliance Power × Quantity)

2. Demand Load

Demand Load (W) = Total Load × Demand Factor (%)

Demand factor accounts for not all appliances being used simultaneously; general residential 50-80%, commercial 70-90%

3. Total Current Calculation

Single Phase: I = P / (V × PF)

Three Phase: I = P / (√3 × V × PF)

Where: I = Current (A), P = Demand Load (W), V = Voltage (V), PF = Power Factor

4. Breaker Capacity

Breaker Capacity = Total Current × Safety Factor

Select the closest standard breaker rating not less than calculated value

Frequently Asked Questions (FAQ)

Q1: What is Demand Factor?

Demand factor is the ratio of actual simultaneous load to total installed load. For example, if home has 10 appliances totaling 10,000W but only 7,000W used simultaneously, demand factor is 70%. Residential 50-80%, commercial 70-90%, industrial 80-100%.

Q2: What is Power Factor?

Power factor is the ratio of real power to apparent power, reflecting electrical energy efficiency. Resistive loads (heaters, lamps, rice cookers) have power factor 1.0; inductive loads (AC, refrigerators, motors) ~0.8-0.9. Lower power factor requires more current.

Q3: Why Need Safety Factor?

Safety factor ensures breaker does not trip during equipment starting surge current. NEC (National Electrical Code) recommends 1.25x. For AC, refrigerator compressors with starting current 3-5x rated current, recommend safety factor 1.5-2.0x.

Q4: How to Select Breaker Capacity?

After calculating total current, multiply by safety factor, then select the closest standard rating not less than calculated value. Standard breaker ratings: 15A, 20A, 30A, 40A, 50A, 60A, 75A, 100A, 125A, 150A, 175A, 200A, 225A, 250A, 300A, 350A, 400A.

Q5: What is the Difference Between Single Phase and Three Phase Circuits?

Single phase (120V/240V) commonly used in residential and small commercial, lower equipment power. Three phase (208V) used for large equipment and light commercial, lower current for same power, can use smaller cable gauge. Three phase current formula: I = P / (√3 × V × PF), where √3 ≈ 1.732.

Usage Example

Example: Residential Electrical Distribution Calculation

• Equipment List:
  • 2-ton AC × 2 units: 1800W × 2 = 3600W
  • Refrigerator: 150W
  • Water Heater: 3000W
  • Microwave: 1200W
  • Washing Machine: 500W
  • LCD TV: 100W
  • LED Bulbs × 10: 10W × 10 = 100W
• Total Load: 8650W
• Voltage: 240V Split-Phase
• Demand Factor: 70% (not all appliances used simultaneously)
• Safety Factor: 1.25 (NEC standard)
• Power Factor: 0.9 (includes motor loads)
• Calculation Results:
  • Demand Load: 8650W × 70% = 6055W
  • Total Current: 6055 / (240 × 0.9) = 28.03A
  • With Safety Factor: 28.03A × 1.25 = 35.04A
  • Recommended Breaker: 40A (closest standard rating ≥35.04A)

Important Reminders

• This calculator provides reference data only; actual installation should comply with latest version of electrical codes
• All electrical work should be performed by licensed electricians
• AC, refrigerator compressor starting current ~3-5x rated current, requires special consideration
• Recommend reserving 20-30% expansion space to avoid insufficient capacity when adding equipment
• Large equipment (central AC, elevators) should be designed by professional electrical engineers