Electrical Wire Sizing: Engineering Principles & Code Considerations
In-depth guide to properly sizing electrical conductors based on thermal ampacity, voltage drop limits, ambient temperature correction, and conduit bundling factors.
The Two Independent Wire Sizing Criteria
Proper conductor sizing requires satisfying two completely independent engineering constraints:
Thermal Ampacity & Temperature Ratings
Conductor insulation is typically rated for 60°C (TW, UF), 75°C (THW, THWN), or 90°C (THHN, XHHW-2). Under the National Electrical Code (NEC Article 310) and international standards (IEC 60364-5-52), standard table ampacities must be derated for:
Conductor Material: Copper vs. Aluminum
While copper offers superior electrical conductivity and mechanical tensile strength, aluminum (specifically AA-8000 series compact stranded alloy) provides significant weight and cost savings in larger feeder sizes (typically 4 AWG and larger). Aluminum requires anti-oxidant joint compound and terminals listed specifically for dual Cu/Al compatibility.
Frequently Asked Questions
Can I use the 90°C ampacity column for final breaker sizing?
Almost never for residential/commercial branch breakers. Most standard circuit breaker and equipment terminals are only rated for 75°C (or 60°C on small 15A/20A devices). You may use the 90°C rating for derating calculations, but the final ampacity cannot exceed the terminal rating.