Don't Make this Mistake in Electrical Design! What’s the first step in designing a circuit? When designing an electrical circuit it's common practice to go piling into calculations to figure out breaker ratings, cable sizes and similar. However there's some important information to gather before you go down that road. We take a look into the various books that give advice on electrical design, including BS 7671 The Wiring Regulations, the On-Site Guide and The Electrical Installation Design Guid