Temperature Effects on Voltage Drop and Conductor Sizing

Temperature and Conductor Performance

Temperature significantly affects conductor performance in two ways: it changes the resistance of the conductor material, and it affects the allowable ampacity based on insulation ratings and heat dissipation. Understanding these thermal effects is essential for accurate voltage drop calculations and safe electrical system design, particularly in extreme temperature environments or heavily loaded circuits.

Metal conductors exhibit a positive temperature coefficient of resistance—their resistance increases as temperature rises. This means a circuit operating at elevated temperature will have higher voltage drop than the same circuit at room temperature, a factor that must be considered in critical applications.

Resistance vs Temperature

The standard resistance values in NEC Chapter 9, Table 8 are given at 75°C. For conductors operating at different temperatures, resistance can be calculated using the temperature coefficient of resistance.

For every 10°C increase above 75°C, copper resistance increases by approximately 3.93%. In a heavily loaded circuit operating at 90°C, the resistance (and thus voltage drop) is about 6% higher than the standard table values suggest.

Ambient Temperature Considerations

NEC ampacity tables are based on an ambient temperature of 30°C (86°F). When conductors are installed in environments with higher ambient temperatures, their ampacity must be reduced (derated) to prevent overheating. This derating affects both ampacity and, indirectly, the actual operating temperature and resistance.

Practical Applications

Calculate with Temperature Factors

Our voltage drop calculator includes temperature considerations for accurate results in any environment. Factor in your ambient temperature and operating conditions for precise calculations.