The most economic load on an overhead line is

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Option A: Greater than the natural load

Explanation & Concepts:
The most economic load on an overhead transmission line is typically chosen to be greater than the natural load (surge impedance loading) to ensure efficient utilization of the conductor's current-carrying capacity. While the natural load minimizes reactive power compensation requirements, economic loading considers the total cost of energy losses and capital investment, often leading to higher operating current densities.

Working Principle: The natural load, or Surge Impedance Loading (SIL), is defined as $P_{SIL} = \frac{V^2}{Z_0}$, where $Z_0 = \sqrt{\frac{L}{C}}$. At this load, the reactive power generated by the line capacitance equals the reactive power absorbed by the line inductance. Operating strictly at SIL limits the power transfer capability. Economic loading optimization, governed by Kelvin's Law, balances the cost of transmission losses against the capital investment of the line infrastructure, frequently necessitating loading beyond the SIL threshold.

Key Formulas: $P_{SIL} = \frac{V^2}{Z_0}$ — Surge Impedance Loading; $Z_0 = \sqrt{\frac{L}{C}}$ — Surge Impedance of a lossless line

Key Points: Surge Impedance Loading (SIL) occurs when reactive generation equals absorption.; Kelvin's Law dictates the economic cross-section of conductors based on energy losses and interest charges.; Operating at loads higher than SIL generally improves the economic efficiency of the transmission line investment.; Long transmission lines are often operated near SIL to maintain voltage profile stability.

Advantages: Optimized cost per unit of energy transmitted; Balanced utilization of capital expenditure

Disadvantages: Higher loading beyond SIL increases voltage drops; Increased I^2R losses compared to operating exactly at SIL

Applications: EHV and UHV transmission planning; Economic grid operation and loss minimization

Additional Info: Kelvin's Law states that the most economical size of a conductor is one for which the annual cost of energy loss is equal to the annual interest and depreciation on the capital cost of the conductor.. Option B is incorrect because operating below natural load leads to under-utilization of the line's thermal and mechanical capacity.

Answer: A is correct — The economic loading is typically greater than the natural load to balance capital costs against operational energy loss efficiency.

Exam Tip: Remember that SIL is a characteristic property dependent only on the geometry (L and C) and voltage, whereas economic load is an operational and economic target derived from total cost analysis.

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Option B: Less than the natural load

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Option C: Equal to the natural load

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Option D: None of above

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