Temperature Derating and Voltage Drop: The Complete Guide
Understanding how ambient temperature affects conductor ampacity, resistance, and voltage drop calculations for hot environments.
Temperature affects electrical conductors in two important ways: it changes the allowable current-carrying capacity (ampacity) and it changes the actual resistance of the conductor. Understanding both effects is essential for accurate voltage drop calculations in hot environments.
Ampacity Derating vs Resistance Change
These are related but distinct concepts that are often confused:
Ampacity Derating
Reduces the allowable current when ambient temperature exceeds 30°C, preventing conductor overheating. Per NEC 310.15(B).
Resistance Change
Increases actual resistance as temperature rises. Affects voltage drop directly. Based on conductor temperature, not ambient.
Temperature Coefficient of Resistance
Conductor resistance increases with temperature according to:
For copper, every 10°C increase above the reference temperature (75°C for NEC tables) increases resistance by approximately 3.93%. At 90°C, resistance is about 6% higher than table values.
Practical Applications
- Rooftop Installations: Conduit exposed to direct sun can reach 60-70°C. Significant ampacity derating and resistance increase required.
- Industrial Environments: Near boilers, furnaces, or hot processes. May require high-temperature insulation and adjusted calculations.
- Cold Environments: Refrigerated warehouses actually improve voltage drop performance due to lower resistance.
Calculate with Temperature Adjustment
Use our voltage drop calculator to verify your designs account for temperature effects properly.
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