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The main criteria for selection of the size of distributor for a radial distribution system is
Voltage drop
Corona loss
Temperature rise
Capital cost
Voltage drop
In a radial distribution system, the main criterion for sizing a distributor is the permissible voltage drop at the farthest consumer end. Since the voltage at the end of the line must stay within regulated limits (e.g., ┬▒5% or ┬▒6% of nominal), the conductor cross-section is chosen to ensure the impedance drop does not exceed these statutory requirements.
In a radial distribution system, the main criterion for sizing a distributor is the permissible voltage drop at the farthest consumer end. Since the voltage at the end of the line must stay within regulated limits (e.g., ┬▒5% or ┬▒6% of nominal), the conductor cross-section is chosen to ensure the impedance drop does not exceed these statutory requirements.
╬ФV=IтЛЕRcos╧Х+IтЛЕXsin╧Х тАФ Voltage drop formula for an AC distributor
R=╧БAlтАЛ тАФ Resistance dependency on cross-sectional area A
The distributor delivers power to various tapping points along its length. The total voltage drop is cumulative, determined by the line current, resistance, and reactance of the conductor segments. If the cross-section is too small, resistance increases, leading to a voltage drop beyond the permissible limit, causing poor performance for end-line consumers.
Voltage regulation is the primary design constraint for distributors to maintain supply quality.
Current carrying capacity (thermal limit) is the primary constraint for feeders.
Distributors are sized for voltage drop, whereas feeders are sized for current-carrying capacity.
Radial systems are simple but suffer from higher voltage drops at the terminal ends.
Ensures compliance with consumer voltage standards
Prevents flickering and equipment malfunction due to under-voltage
Increases capital expenditure if long lines require thicker conductors
Does not account for future load growth without oversizing
Rural electrification networks
Low voltage residential distribution feeders
Thermal limits are typically checked after voltage drop calculations to ensure the selected conductor can handle the current without excessive heating.
Corona loss (Option B) is generally ignored in low-voltage distribution as it occurs significantly at higher transmission voltages (above 33kV).
Temperature rise (Option C) determines the maximum safe current, but in distribution systems, voltage drop almost always forces a conductor size larger than what is required for simple thermal limit.
A is correct тАФ The primary design constraint for sizing a distributor in a radial system is maintaining the voltage drop within prescribed statutory limits.
Always remember: 'Distributors' are sized by voltage drop, while 'Feeders' are sized by current-carrying capacity (ampacity).