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 (16A × 1.25 = 20A).
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.
Danger
Installing a 30A breaker on 14 AWG wire (rated for 15A) is extremely dangerous and a fire hazard. The wire will overheat before the breaker trips.
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
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.
Use our Wire Gauge Calculator to avoid these common sizing mistakes.