A guide to selection of electrical cable Last modified: Fri Jul 9 14:18:02 2004
©2001 Kevin Boone, all rights reserved. No part of this publication may be reproduced in material form (including photocopying or printing), or transmitted in electronic form, without the express permission of the copyright holder, except for the purposes of private study, review, and criticism, or in accordance with the provisions of the Copyright, Designs and Patents Act (1988). So there.
Overview This article describes the selection of cables for `difficult' domestic electrical installations. By `difficult' I mean installations where cables are unusually long, cu rrents abnormally high, or shock protection requirements especially rigorous. I have written this article for competent, sensible DIY enthusiasts who may already be doing electrical work, but bu t lack the theoretical and regulatory knowledge to tackle these more difficult jobs. I have assumed that the reader has a basic understanding of electrical theory and is prepared to do some simple arithmetic. Cable selection is guided by two main principles. First, the cable should be able to carry the current load imposed on it without overheating. It should be able to do this in the most extreme conditions of temperature it will encounter during its working life. Second, it should offer sufficiently sound earthing to (i) limit the voltage to which people are exposed to a safe level and (ii) allow the fault current to trip the fuse or MCB in a short time. To meet these requirements requires consideration of the circuit load current, the ambient temperature, installation technique, cable thickness and length, and the over-current protection device. In some cases you may need to consider factors that are outside your control, like the external earth loop impedance. Typical `worst-case' values for these factors are given in the article.
Scope of this article In most domestic wiring scenarios, the principles and techniques described in this article are simply not relevant. The materials and equipment currently cu rrently available are designed to simplify installation, and common sense and the ability ab ility to read the manufacturer's instructions are all that is required. Ordinary domestic power and lighting circuits do not require any special skills or knowledge to install, beyond what you would find in a DIY handbook. This article covers the issues that DIY books steer clear of, like running long cables to outbuildings, installing supplies for electrical showers, and electrical wiring in bathrooms. It assumes that the reader has sufficient time and enthusiasm to get to grips with the theory, which can be rather technical in places. This article is intended for readers in the UK, and in places where UK practices and regulations are followed.
How this article is organized This article has three chapters and an appendix. Chapter 1 describes de scribes the theory of over-current
