Wire Gauge Calculator

Cable sizing decides whether an installation is compliant, safe and economic over its 25-year life. Picking the conductor cross-section from the current draw, the run length and the acceptable voltage drop is what BS 7671 asks the design engineer to do. Undersize and the cable runs hot, the insulation degrades, and the install fails the next periodic inspection. Oversize and the capital cost of copper is higher than it needs to be, eating into the payback on the project that triggered the new circuit in the first place.

The calculation matters most in long cable runs: warehouses with 200-metre runs to remote loading bays, EV charging installations spread across a depot, factory expansions that feed new lines from an existing substation across the site. In all three cases, the voltage drop limit (3% on lighting, 5% on power under BS 7671) is the constraint that drives the conductor size, not the current-carrying capacity. A 32A circuit run 150 metres at 400V three-phase will need 16mm squared, not the 6mm squared the current rating alone would suggest.

Purely Energy works with mid-market clients adding capacity across multi-site portfolios. We connect facilities and operations teams with the design engineers and contractors who sign off the install against BS 7671, and we make sure the supply contract reflects the new load profile once the cabling work is complete. That coordination across procurement, capacity and on-site design is what we mean when we say we manage energy as a strategic cost, not a commodity purchase.