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Wire Math

Guide

UK Cable Size Quick Reference (BS 7671 Framing)

A practical, metric-first quick reference for early cable sizing conversations in UK projects, with non-prescriptive BS 7671 framing.

Overview

This quick reference is built for UK-first sizing conversations: start in mm², validate assumptions early, and carry explicit caveats into final design checks.

How to Use This Quick Reference

Use this guide as a planning aid for quoting and early design conversations. It is not a replacement for a full project-specific check.

Keep mm² as the canonical unit in all UK documentation. Use AWG or SWG only to interpret source information from imported or legacy material.

Common Starting Anchors (Not Compliance Values)

For domestic planning notes, 1.5 mm² often appears around lighting runs, while 2.5 mm² is commonly seen around socket circuits. Higher-demand circuits are often discussed around larger areas such as 4 mm², 6 mm², or above.

These anchors help estimate scope and procurement, but final suitability depends on installation method, run length, load profile, protective devices, and project constraints.

Fast Quote Workflow

Start with expected current and one-way route length, choose a candidate mm², then check voltage-drop estimate and resistance impact.

If source paperwork is in AWG or SWG, convert immediately and record the selected mm² in your quote to prevent unit drift across teams.

Where BS 7671 Fits

BS 7671 should frame your verification process, not be inferred from a single calculator result. A green estimate is not an automatic compliance statement.

For final sign-off, use the current edition requirements and the full installation context.

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References