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.

Example: A 16A continuous load requires wire sized for 20A minimum.

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.

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.

Mistake #7: Not Planning for Future Expansion

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

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

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