Dwelling Unit Load Calculation Lecture Notes

Overview

• Focus on calculating the 120/240-volt service size for a single-family dwelling using the general method of calculation.
• Not applicable for multi-family, duplex, or commercial dwellings.

Example Details

• Living Area: 1452 square foot
• Electric Heat: 9.6 kW at 240 volts
• AC Unit: 5 horsepower at 240 volts
• Dishwasher: 1200 VA at 120 volts
• Disposal: Half horsepower at 120 volts
• Water Heater: 4.5 kW at 240 volts
• Clothes Dryer: 4.0 kW at 240 volts
• Range: 15 kW at 240 volts

Calculation Steps

Step 1: Calculate General Lighting Load

• Formula: 3 VA per square foot
• Calculation: 3 x 1452 = 4356 VA

Step 2: Small Appliance and Laundry

• Code Requirement: Two small appliance branch circuits, one laundry circuit
• Calculation:
  • Small Appliance: 2 x 1500 VA = 3000 VA
  • Laundry: 1500 VA

Step 3: Apply Demand Factors

• Use Table 220.42
  • First 3000 VA at 100%
  • Remainder up to 120,000 VA at 35%
• Calculation:
  • First 3000 VA kept
  • Remaining: 5856 VA x 0.35 = 2049.6 VA
• Total from Step 3: 5049.6 VA

Step 4: Compare Heat vs. AC

• Electric Heat: 9.6 kW = 9600 watts/VA
• AC: Convert 5 horsepower to VA using Table 430.248
  • 5 horsepower at 240 volts = 28 amps
  • Calculation: 28 amps x 240 volts = 6720 VA
• Result: Keep larger load, which is 9600 VA (Heat)

Step 5: Fixed Appliances

• Appliances:
  • Dishwasher: 1200 VA
  • Disposal: Convert half horsepower to VA
    • 9.8 amps x 120 volts = 1176 VA
  • Water Heater: 4500 VA

Step 6: Clothes Dryer

• Minimum Requirement: 5000 VA

Step 7: Cooking Equipment

• Range: 15 kW
• Table 220.55, Note 1:
  • Increase 5% for each kW over 12 kW
  • 3 x 5% = 15% increase
• Calculation:
  • Column C: 8 x 1.15 = 9.2 kW = 9200 VA

Step 8: Largest Motor

• Calculation: 25% of largest motor
  • Largest Motor: AC (6720 VA)
  • 6720 VA x 0.25 = 1680 VA

Step 9: Size the Service

• Total VA: Sum all VA values from previous steps = 37,405.5 VA
• Convert to Amperage:
  • Total VA / Voltage = Amperage
  • 37,405.5 VA / 240 volts = 155.85 amps (round to 156 amps)
• Service Size Based on Table 310.12:
  • Minimum: 175 amps
  • Wire Size:
    • Copper: 1/0 AWG
    • Aluminum: 3/0 AWG
• Ground Electrode Conductor (Table 250.66):
  • Copper: #6 AWG

Conclusion

• This concludes the process of calculating service size for a single-family dwelling using the specified nine-step format.
• Important: Do not confuse with multi-family or commercial applications, which have additional requirements.