T/F: ampacity vs ambient temperature

TRUE.

In steady state, the heat produced in the conductor equals the heat it loses:

$$I^2R + q_{solar} = q_{conv} + q_{rad}$$

where $q_{conv} \propto (\theta_c - \theta_a)$ and $q_{rad} \propto (T_c^4 - T_a^4)$.

The conductor temperature $\theta_c$ is limited to a fixed maximum (for example 75–80 °C for ACSR). When the ambient temperature $\theta_a$ rises, the difference $(\theta_c - \theta_a)$ falls. So less heat can be lost by convection and radiation, and a smaller $I^2R$ is allowed. Neglecting the solar term:

$$I \propto \sqrt{\theta_c - \theta_a}$$

Example: a conductor rated 400 A at 35 °C ambient with an 80 °C limit can carry only $400\sqrt{(80-40)/45} = 377$ A at 40 °C ambient. Hence ampacity decreases as ambient temperature increases. Line ratings are therefore given for summer conditions.