T/F: thermal limit design urban, losses rural

TRUE.

Thermal limit in urban areas:

• Urban areas have high load density (many kW per km²), so feeders are short but carry large currents.
• Over a short length, the voltage drop ($\Delta V \propto I \cdot l$) stays within limits even at high current. The limiting factor is the current-carrying (thermal) capacity of the conductor or cable: overheating, sag and insulation damage.
• So urban feeders are designed by the thermal (ampacity) limit.

Higher losses in rural areas:

• Rural loads are scattered, with low load density, so feeders (11 kV and LT) are very long for small loads. They are designed by the voltage drop limit, not the thermal limit.
• Power loss $= 3I^2R$, and $R$ is proportional to length. For the same delivered power, long feeders with small conductors give a much higher percentage loss.
• Rural distribution transformers are lightly loaded for most of the day (low load factor), so no-load (core) losses form a large share of the energy supplied.
• Poor maintenance, unbalanced single-phase loading, long service drops and theft add to the losses.

Example: a 10 kW load at the end of a 0.5 km LT feeder has only one-quarter of the loss of the same load at the end of a 2 km feeder (loss ∝ length for a fixed current).

Thus urban feeders reach their thermal limit first, while rural feeders, though lightly loaded, have a higher percentage loss than urban feeders.