Electrical Installation Calculations

Basic

Christopher Kitcher

• Publisher: Routledge
• Year: 2022
• ISBN:
  9781000588118
• Edition: 10
• Topic: Technology & Engineering
• Subtopic: Construction & Architectural Engineering
• Index: Technology & Engineering

Now in its 10th edition, Electrical Installation Calculations: Basic has been updated to include any changes required to bring it in line with the 18th edition of the IET electrical wiring regulations (BS7671: 2018).

Electrical calculations required for exams can prove difficult to master, but for more than 40 years, this book series has proved very helpful to students and professional electrical engineers studying for electrical qualifications. It covers all the calculations required for Level 2 electrical qualifications, along with other useful calculations that may be used in the electrical industry but may not feature in the syllabus of some exams.

Although the calculations in this book are referred to as 'basic', they form the foundation of all calculations carried out in the electrical industry, which have been set out simply with worked examples along with additional questions and answers. Key terms are explained in a glossary, which can be used to assist with the reader's understanding.

1 Use of Calculators

DOI: 10.4324/9781003258735-1

Throughout the ‘Basic’ and ‘Advanced’ books the use of a calculator is encouraged. Your calculator is a tool, and like any tool, practice is required to perfect their use. A scientific calculator will be required, and although they differ in the way the functions are carried out, the end result is the same.

The examples are given using a Casio scientific calculator.

The figure printed on the button is the function performed when the button is pressed.

To use the function in small letters above any button the shift button must be used.

Practice is important.

A syntax error will appear when the figures are entered in the wrong order.

x² will multiply a number by itself, i.e. 6 × 6 = 36. On the calculator this would be 6 x² = 36.

When a number is multiplied by itself it is said to be squared.

x³ will multiply a number by itself and then the total by itself again.

For example, when we enter 4 on calculator x³ = 64. When a number is multiplied in this way it is said to be cubed.

$\sqrt{}$: will give you the number which achieves your total by being multiplied by itself, i.e. $\sqrt{36}=6$. This is said to be the square root of a number, and is the opposite of squared.

$\sqrt[3]{}$ will give you the number which when multiplied by itself three times will be your total, i.e. $3\times 3\times 3=27$. This is said to be the cube root.

X ^–1 will divide 1 by a number, i.e. $\frac{1}{4}=0.25$. This is the reciprocal button and is useful in this book for finding the resistance of resistors in parallel and capacitors in series.

EXP is for the powers of 10 function, i.e. 25 × 1000 = 25 × 10³ = 25000.

Enter into calculator 25 EXP 3 = 25000. (Do not enter the X or the number 10.)

If a calculation shows 10–³ i.e.: 25 × 10–³ enter 25 EXP − 3 = (0.025). (When using EXP if a minus is required use the button (-).)

Brackets should be used to carry out a calculation within a calculation.

EXAMPLE

Calculation: $\frac{3\times 4}{2}=65.46$

Enter into calculator 32 ÷ (0.8 × 0.65 × 0.94) =

Remember Practice makes Perfect.

2 Simple Transposition of Formulae

DOI: 10.4324/9781003258735-2

To find an unknown value:

1. ■ The subject must be on the top line and must be on its own.
2. ■ The answer will always be on the top line.
3. ■ To get the subject on its own values must be moved.
4. ■ Any value that moves across the = sign must move from above the line to below line or from below the line to above the line.

EXAMPLE 1

$\frac{3\times 4=2\times 6}{3\times 4=2\times ?}$

Transpose to find? $\frac{3\times 4}{2}=6$

EXAMPLE 2

$\frac{2\times 6}{?}=4$

Step 1: $2\times 6=4\times ?$

Step 2: $\frac{2\times 6}{4}=?$

Answer: $\frac{2\times 6}{4}=3$

EXAMPLE 3

$5\times 8\text{}\times \text{}6=3\text{}\times \text{}20\text{}\times ?$

Step 1: move 3 × 20 away from unknown value; as the known values move across the = sign they must move to bottom of equation $\frac{5\text{...}}{}$