Calculate proper wire size per NEC ampacity tables based on load current, distance, voltage drop limits, and installation conditions.
Recommended Wire Size
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Ampacity Requirement
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Actual Voltage Drop
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Voltage Drop
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Wire Ampacity
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Max Breaker Size
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The National Electrical Code (NEC) establishes minimum safety standards for electrical installations. Proper wire sizing prevents overheating, fire hazards, and ensures adequate voltage delivery to equipment.
Load current should be obtained from equipment nameplates or manufacturer specifications. For motors, use NEC Table 430.248 for full-load current ratings rather than nameplate values.
Continuous loads operate for 3 hours or more continuously and require conductors sized at 125% of the load current per NEC 210.19(A)(1). Examples include lighting and HVAC systems.
Excessive voltage drop reduces equipment efficiency and can cause malfunction. The NEC recommends limiting voltage drop to 3% for branch circuits and 5% total for feeders plus branch circuits.
Higher ambient temperatures reduce wire ampacity. NEC Table 310.15(B)(2)(a) provides derating factors - for example, conductors in 40°C ambient must be derated to 82% of their base ampacity.
When more than 3 current-carrying conductors are in the same raceway or cable, apply derating factors from NEC Table 310.15(B)(3)(a). Neutral conductors carrying only unbalanced current don't count.
Aluminum conductors are lighter and less expensive but have higher resistance than copper. They require larger sizes to carry the same current and need special termination methods per NEC 110.14.
Use the lowest temperature rating in the circuit - either the conductor insulation, device terminals, or overcurrent protection. Most standard devices are rated 60°C or 75°C, limiting the usable conductor ampacity.