Common Wire Sizing Mistakes and How to Avoid Them

Mistake #1: Ignoring Voltage Drop

The most common and dangerous mistake. Many installers only check ampacity and ignore voltage drop, leading to:

• Dim lights and poor performance
• Motor damage from low voltage
• Wasted energy and higher bills
• Potential code violations

Solution: Always calculate voltage drop for runs over 50 feet. Use our Voltage Drop Calculator.

Mistake #2: Not Accounting for Continuous Loads

NEC requires continuous loads (3+ hours) be calculated at 125% of actual current.

Mistake #3: Forgetting Temperature Derating

• Attic installations can exceed 120°F
• Bundled conductors generate heat
• Multiple circuits in conduit require derating

Solution: Apply NEC temperature correction factors from Article 310.

Mistake #4: Improper Aluminum Terminations

Using standard devices with aluminum wire causes:

• Loose connections over time
• Overheating and fire hazards
• Oxidation and high resistance

Solution: Always use CO/ALR rated devices and anti-oxidant compound.

Mistake #5: Oversizing Breakers

Wire must be protected by properly sized overcurrent devices. Never install a larger breaker to "solve" tripping issues.

Mistake #6: Mixing Wire Sizes

All current-carrying conductors in a circuit must be the same size unless specifically designed otherwise. Using different sizes creates weak points where smaller wire overheats.

Mistake #7: Not Planning for Future Expansion

Installing slightly larger wire initially costs little more but provides flexibility for future upgrades. Consider:

• Future load increases (EV chargers, home additions)
• Equipment upgrades requiring more power
• Code changes that may require larger minimums

Mistake #8: Incorrect Grounding

• Ground wire must be sized per NEC Table 250.122
• Oversized phase conductors require larger ground
• Separate grounding for sub-panels in detached structures

Mistake #9: Using Wrong Wire Type for Location

Different environments require different wire types:

• Wet locations: Use THWN or UF cable
• Direct burial: Use UF-B or cables rated for direct burial
• High temperature: Use THHN or higher-rated insulation
• Conduit in sunlight: Account for temperature rise

Mistake #10: Ignoring Local Code Requirements

Local codes may have stricter requirements than NEC:

• Some jurisdictions require 12 AWG minimum for all circuits
• AFCI/GFCI requirements vary by location
• Conduit requirements may be stricter
• Always verify with local authority having jurisdiction (AHJ)

How to Avoid These Mistakes

1. Use proper calculation tools and tables
2. Account for all derating factors
3. Follow NEC requirements strictly
4. When in doubt, size up one gauge
5. Have work inspected by qualified electrician
6. Use our Wire Gauge Calculator for accurate sizing

Consequences of Improper Wire Sizing

Using incorrect wire sizes can result in:

• Fire hazards: Overheated wires can ignite insulation and building materials
• Equipment damage: Low voltage harms motors and electronics
• Failed inspections: Code violations must be corrected before approval
• Voided insurance: Non-code-compliant work may void coverage
• Liability issues: Property damage or injury liability

Conclusion

Proper wire sizing is not optional—it's essential for safety and code compliance. By understanding these common mistakes and using proper calculation methods, you can ensure your electrical installations are safe, efficient, and code-compliant. When in doubt, always consult with a licensed electrician.