---
title: "Selecting size and number of lifts"
subject: "Industrial Power Distribution and Illumination"
chapter: 5
key: "semester_6/industrial_power_distribution_and_illumination:lift-size-number-selection"
source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/lift-size-number-selection
site: BCT Engineering
---

# Selecting size and number of lifts

Industrial Power Distribution and Illumination · Chapter 5: Electrification of Industrial Building

- Asked in: 2072 Asoj, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/lift-size-number-selection
- Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-5

## Question

Explain the methods of selection of size and number of lift for office building.

## Short answer

Selection based on peak traffic handling: determine population from area, calculate 5-minute demand (12-15% of population), choose car capacity/speed per building height, compute round trip time, derive handling capacity per car, then number of lifts = demand/capacity; interval = RTT/lifts (25-40 s target).

## Answer

The size (capacity and speed) and number of lifts for an office building are chosen so that the peak traffic (morning arrival or evening departure) is handled within an acceptable time and waiting interval.

#### Steps
1. **Population**: net rentable area ÷ area per person (about 8–12 m²/person in offices).
2. **Peak demand**: persons to be moved in the critical 5 minutes (handling capacity 12–15 % of population in 5 min for offices), or the whole population in a given time (e.g. emptying in 60 min).
3. **Choose car capacity and speed**: capacity 8–20 persons (68 kg each); speed rises with building height (about 1 m/s up to 5 floors, 1.5–2.5 m/s for 10–20 floors, higher for taller).
4. **Round trip time (RTT)** = running time up and down (2H/v) + door and stop time (number of probable stops × time per stop) + loading time at ground floor.
5. **Handling capacity per car** = car capacity × (time period / RTT).
6. **Number of lifts** = required persons per period ÷ capacity of one car; round up.
7. **Interval (waiting time)** = RTT / number of lifts; should be about 25–40 s for offices.
8. **Motor size** of each lift from P = (unbalanced mass) × g × v / η.
9. Provide at least one fire/service lift, and connect lifts to standby supply.

**Example in brief**: 2000 persons, 10-person cars, RTT = 162 s → each car moves 10 × 3600/162 ≈ 222 persons/h; to move 2000 in an hour, 2000/222 = 9.01 → 10 lifts.

## Check yourself

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

1. What does RTT consist of?
   A. Running time only
   B. 2H/v plus stop time of probable stops plus loading time at ground floor
   C. Interval only
   D. Door time at the top floor
   Answer: B. RTT includes running up and down, door/stop time, and ground floor loading. (Key point: Round trip time)

2. How is the number of lifts found?
   A. Persons per period / capacity of one car, rounded up
   B. Floors / 2
   C. Car capacity / population
   D. RTT / persons
   Answer: A. Persons to be handled per period divided by one car's capacity per period, rounded up. (Key point: Number of lifts)

3. What is the waiting interval and its target for offices?
   A. RTT x number of lifts, 100 s
   B. RTT / number of lifts, about 25-40 s
   C. RTT / 2, 5 minutes
   D. Number of lifts / RTT, 1 s
   Answer: B. Interval = RTT / N should be about 25-40 s in offices. (Key point: Interval)

## Related in Industrial Power Distribution and Illumination

- [Number of lifts for office building](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Alift-number-office-building) — Industrial Power Distribution and Illumination, chapter 5
- [Selecting lift motor size](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Alift-motor-size) — Industrial Power Distribution and Illumination, chapter 5
- [Selecting distribution transformer size using Ku and Ks](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Atransformer-size-ku-ks) — Industrial Power Distribution and Illumination, chapter 4, asked 3×

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