For a 25 amp circuit, use 4. 0mm² copper cable under BS 7671 Table 4D5A, rated at 36A clipped direct. For cable runs exceeding 25m, check voltage drop. This comprehensive guide provides step-by-step instructions for sizing electrical cables in accordance with Australian Standard AS/NZS 3008. Whether you're an electrical engineer, contractor, or student, this resource will help you master the essential calculations for selecting the. For a 25 amp circuit, use 4. Values. Hence it is important to choose the type of wiring and the size carefully. Understanding Electrical Cables Types and Applications 3. How to. Calculate recommended cable size from amps, voltage, phase, one-way cable length, conductor material, voltage drop, and ampacity.