Intermediate
Resistance Temperature Correction
Correct conductor resistance for temperature — calculate resistance at any temperature from known reference value.
The Formula
R₂ = R₁ × [1 + α(T₂ − T₁)]
Resistance at temperature T₂
Variable Definitions
R₁=Resistance at reference temperature T₁(ohms (Ω))
R₂=Resistance at operating temperature T₂(ohms (Ω))
α=Temperature coefficient of resistance(per °C (Cu: 0.00393, Al: 0.00403))
T₁=Reference temperature (usually 20°C)(°C)
T₂=Operating temperature(°C)
Worked Example
Copper wire has 1.21 Ω/1000ft at 20°C. Find resistance at 75°C operating temp.
- R₂ = 1.21 × [1 + 0.00393 × (75 − 20)]
- R₂ = 1.21 × [1 + 0.00393 × 55]
- R₂ = 1.21 × 1.2162 = 1.472 Ω/1000ft
Answer: 1.472 Ω/1000ft at 75°C (22% increase)
When to Use
- •Accurate voltage drop calculations at operating temperature
- •Motor winding resistance checks (hot vs cold)
- •Power system modeling with temperature-dependent impedance
- •Verifying conductor resistance during commissioning tests
Standards & References
- NEC Chapter 9, Table 8 — DC Resistance at 75°C
- NEC Chapter 9, Table 9 — AC Impedance