Cable sizing for mechanical plant begins with a coordinated equipment selection. Confirm running current, starting method, protective-device requirements and route length, then check current-carrying capacity, voltage drop, fault-loop performance and installation conditions against applicable standards. An equipment schedule based on assumed fan duty is insufficient while system resistance remains unresolved.
A sports-centre smoke-exhaust review showed why mechanical and electrical designs must be coordinated. Detailed duct-pressure calculations produced a fan selection with maximum running current of 32.5 A rather than the scheduled 15 A. Over a 50 m circuit, the indicated multicore cable increased from 2.5 mm² to 6 mm², affecting isolators, VSDs, protection and switchboard capacity.
Final cable selection must reflect grouping, ambient temperature, thermal insulation, installation method and any required fire-resistance classification. For motor circuits, coordinate overload and short-circuit protection with manufacturer data and starting characteristics. Check voltage drop during both normal running and starting where torque or control stability could be affected.
The lesson is commercial as well as technical: late fan reselection on that project contributed to an approved variation of about $50,000. Completing pressure-loss calculations and preliminary fan selections during design gives the electrical team dependable loads and greatly reduces construction-stage changes.
The original review found that the consultant's generic pressure allowance differed from the calculated duct-system resistance by more than 100%. Detailed static-pressure calculations should therefore be completed for every ventilation system during design, with preliminary fan selections checked so the later construction selection remains within a reasonable design tolerance; TPCE's article recommends targeting ±20%.
TPCE's HVAC wire-sizing calculator is intended as a quick coordination tool for equipment loads and references AS/NZS 3008.1.1:2017, including the applicable table used for the result. It is useful for early cross-checking, but the final design still requires an electrical engineer to confirm installation conditions, protection, voltage drop, fault performance and all other project-specific requirements.
