---
title: "Define loss of load factor"
subject: "Transmission and Distribution System Design"
chapter: 6
key: "semester_8/transmission_and_distribution_system_design:define-loss-of-load-factor"
source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/define-loss-of-load-factor
site: BCT Engineering
---

# Define loss of load factor

Transmission and Distribution System Design · Chapter 6: Electrical Load Characteristics and Load Forecast

- Asked in: 2069 Bhadra (old course), 3 marks; 2067 Chaitra (old course), 2 marks (asked 2 times)
- 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/define-loss-of-load-factor
- Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-6

## Question

Define loss of load factor with respect to distribution consumer characteristics and discuss its significance on the design of distribution system.

## Short answer

Loss of load factor is the ratio of average power loss to loss at peak demand. It converts peak loss to annual energy loss (Eloss = Ploss,peak × LLF × hours) and is critical for economic conductor sizing, transformer selection, and setting loss limits.

## Answer

**Loss of load factor (LLF)**, also called loss factor, is the ratio of the average power loss in a system element over a period to the power loss at peak load during that period:

$$
LLF = \frac{\text{Average power loss}}{\text{Power loss at peak load}} = \frac{\text{Energy loss in time } T}{P_{loss,peak} \times T}
$$

Since copper loss varies as the square of the current, $LLF = \frac{1}{T}\int_0^T \left(\frac{P}{P_{max}}\right)^2 dt$. It always lies between $LF^2$ and $LF$, and an empirical relation is used in practice:

$$
LLF = 0.3\,LF + 0.7\,LF^2
$$

**Significance in distribution design:**

- Energy loss in feeders and transformer windings is found simply as $E_{loss} = P_{loss,peak} \times LLF \times 8760$ kWh/year, without a full hourly load curve.
- It is used to cost the losses over the life of the system, which decides the economic conductor size and transformer rating.
- Because LLF < LF, the % energy loss is lower than the % peak power loss ($\%E_{loss} = \%P_{loss,peak} \times LLF/LF$); designers use this to set acceptable peak loss limits.
- It shows the benefit of improving load factor (flatter load means lower loss per kWh sold).

## Check yourself

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

1. Loss of load factor is the ratio of:
   A. Average load to peak load
   B. Average power loss to power loss at peak load over a period
   C. Peak loss to average loss
   D. Energy sold to energy generated
   Answer: B. LLF = average power loss / loss at peak load. (Key point: Definition)

2. Which statement about LLF is correct?
   A. It lies between LF^2 and LF, and is often taken as 0.3 LF + 0.7 LF^2
   B. It is always greater than LF
   C. It equals LF^3
   D. It lies between 1 and 2
   Answer: A. Because copper loss varies with the square of current, LLF is between LF^2 and LF. (Key point: Range and empirical relation)

3. How is LLF used in distribution design?
   A. Energy loss = peak loss x LLF x 8760 kWh/year, used to cost losses and choose economic conductor and transformer size
   B. To find the voltage rating of insulators
   C. To find the number of consumers
   D. To set the power factor
   Answer: A. LLF gives annual energy loss without a full hourly curve, which drives economic sizing. (Key point: Design significance)

## Close variants set in other exams

- [Define coincidence factor](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Adefine-coincidence-factor) — Transmission and Distribution System Design, chapter 6, asked 2×
- [Define responsibility factor](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Adefine-responsibility-factor) — Transmission and Distribution System Design, chapter 6, asked 2×
- [Define coverage factor](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Adefine-coverage-factor) — Transmission and Distribution System Design, chapter 6, asked 2×

## Related in Transmission and Distribution System Design

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