Wire Size Calculator - Determine Correct Wire Gauge

Selecting the correct wire size is fundamental to electrical safety and system performance. Wire sizing must account for multiple critical factors: the ampacity rating to safely carry the electrical load without overheating, voltage drop considerations to ensure adequate voltage reaches equipment, and temperature derating factors for ambient conditions and conductor bundling. The National Electrical Code (NEC) provides comprehensive tables and requirements to guide proper wire selection for all installation types.

Ampacity is the maximum current a conductor can carry continuously under specified conditions without exceeding its temperature rating. NEC Table 310.16 (now 310.15(B)(16) in newer editions) lists ampacity ratings for common conductor sizes at different temperature ratings: 60°C, 75°C, and 90°C. However, actual installation conditions often require derating these base ampacity values. Factors requiring derating include ambient temperatures above 30°C (86°F), more than three current-carrying conductors in a raceway or cable, and specific installation methods.

Beyond ampacity, voltage drop calculations significantly influence wire size selection for long circuit runs. While a wire may have adequate ampacity for a given load, excessive voltage drop can still cause problems. Professional practice involves checking both ampacity requirements and voltage drop calculations, then selecting the larger wire size if the two criteria differ. This dual-consideration approach ensures circuits operate safely while delivering proper voltage to connected equipment.