Wire Sizing and Amperage Calculation

Introduction

• Instructor: Ralph, Electrical Instructor at NTI
• Purpose: Help understand wire sizing and amperage calculation
• Common query: Whether a wire size is appropriate for the required amperage

Accessing the Electrical Code

• Free electronic copy available at NFPA.org
• PDF format of NEC (National Electrical Code)
• Limitations: Cannot print or copy

Basic Concepts

Power Triangle

• Part of Ohm's Law
• Equations:
  • P (Power in watts) / V (Voltage) = I (Current in amps)
  • P / I = V (Voltage)
  • V x I = P (Power in watts)
• Objective: Solve for amperage to size wires

Voltage Standards

• Residential: 120/240 volts
• Commercial: Can vary
• Voltage info can be found on unit nameplate or with a multimeter

Calculating Amperage

Steps

1. Determine wattage in kilowatts and convert to watts (multiply by 1,000)
2. Divide wattage by voltage to find amperage
3. Use NEC (Table 310.16) or Google to find wire size that supports the amperage

Examples

• Example 1: 8,800 watts / 240 volts = 36.66 amps → Use wire for 40 amps
• Example 2: 11.8 kW (11,800 watts) / 240 volts = 49.16 amps → Use wire for 50 amps
• Example 3: 23 kW (23,000 watts) / 480 volts = 47.91 amps → Use wire for 50 amps
• Example 4: 10 kW (10,000 watts) / 208 volts = 48.07 amps → Use wire for 50 amps

Wire Selection Based on Environment

• Wire types need to support specific temperature ratings
• Consider exposure to elements like direct sunlight
• Higher temperature wires (e.g., 75°C or 90°C) for outdoor/extreme environments

NEC Table 310.16

Key Points

• Two sections: Copper (left) and Aluminum Copper Clad (right)
• Aluminum wires in commercial settings; may need larger size than copper
• Consider environment where wire will be used:
  • Inside: 60°C (140°F)
  • Outside/High Sun: 75°C (167°F) or 90°C (194°F)

Wire Types

• TW, RHW, THHW, etc.
• Article 310 provides detailed descriptions and specifications

Determining Wire Size

• Use amperage to find wire size (AWG or KC Mills)
• Check compatibility of wire type with its intended use

Additional Considerations

• Circuit breakers protect wire, not other equipment
• Smaller conductors restrict breaker sizes
• Reverse engineering: Use existing wire size/type to determine amperage capacity

Conclusion

• Power triangle key to solving wire sizing issues
• Always refer to NEC and verify wire capabilities against required ampacity
• Contact instructors for further help and clarification

Use these notes as a reference guide for understanding wire sizing and amperage calculations, ensuring safe and proper electrical work.