---
title: "T/F: X/R ratio, transmission vs distribution"
subject: "Transmission and Distribution System Design"
chapter: 1
key: "semester_8/transmission_and_distribution_system_design:tf-xr-ratio-transmission-distribution"
source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-xr-ratio-transmission-distribution
site: BCT Engineering
---

# T/F: X/R ratio, transmission vs distribution

Transmission and Distribution System Design · Chapter 1: Introduction

- Asked in: 2072 Asoj, 3 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-xr-ratio-transmission-distribution
- Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-1

## Question

State whether the following statement is TRUE or FALSE. Justify your answer with a brief explanation: The X/R ratio is high for transmission line while low for distribution lines.

## Short answer

True. Transmission lines have large phase spacing and low resistance, giving dominant reactance with X/R ratios of 5–20. Distribution lines use small conductors with close spacing, making resistance significant, so X/R is low (0.5–2).

## Answer

**TRUE.**

- Transmission lines use large conductors (low resistance per km) and large phase spacing (higher inductance), so $X \gg R$. Typical $X/R$ is about 5–10 for 132–220 kV lines and 10–20 or more for EHV lines with bundled conductors.
- Distribution lines (11 kV, 400 V) use small conductors with close spacing, so $R$ is comparable to or larger than $X$; $X/R$ is about 0.5–2.

Consequences: in transmission lines voltage drop depends mainly on reactive power ($\Delta V \approx QX/V$) and power flow on angle, while in distribution feeders voltage drop depends strongly on real power ($\Delta V \approx (PR+QX)/V$), and $I^2R$ loss is a major concern.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. The statement 'X/R is high for transmission and low for distribution lines' is:
   A. False, distribution has higher X/R
   B. True
   C. False, both equal
   D. True only for cables
   Answer: B. Transmission lines have X >> R while distribution lines have R comparable to or larger than X. (Key point: Statement verdict)

2. Why is X/R high in transmission lines?
   A. Small conductors and close spacing
   B. Large conductors (low R) and large phase spacing (higher inductance)
   C. Use of DC
   D. Low voltage
   Answer: B. Large conductors reduce R and wide spacing increases inductance, so X >> R. (Key point: Reason for high X/R)

3. In distribution feeders, voltage drop depends strongly on:
   A. Reactive power only
   B. Real power too, approx (PR+QX)/V
   C. Frequency only
   D. Neither P nor Q
   Answer: B. Because R is significant, PR contributes strongly to drop, and I^2R loss is a major concern. (Key point: Voltage drop)

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