True Partners Consulting Engineers

HVAC WIRE SIZING - A COSTLY LESSON LEARNED

Attention building industry professionals! We’re sharing a critical lesson learned on a recent project involving smoke exhaust fans, highlighting the importance of accurate system static pressure calculations and it's impact on the Electrical system components.

The Scenario:

A builder client requested our independent review of electrical wire size for smoke exhaust fans at a sports centre under development at the time
The mechanical contractor on the job had raised concerns about cable size and power requirements due to estimated static pressures being significantly higher than figures specified by the consultant
While recalculation based on actual ducting system is standard for ventilation system pressure losses before purchasing the equipment, the difference exceeded 100% in this case, impacting main electrical components

The Costly Mistake:

The consulting firm used a generic static pressure value, leading to undersized fans and cables. This appears to be a more and more occurring approach by some consultants possibly due to engineering time saving

The Impact:

The design selected fans required a maximum running ampacity of 15Amps.

The new selected fans based on the correct system static loss required a maximum running ampacity of 32.5Amps.

The required cable size based on a distance from the electrical board to the fan of 50m changed from 2.5mm2 to 6mm2 multicore cable. True Partners Consulting Engieers wire size calculator can be used for this purprpose Wire Sizing tool!

As a result:

Fans significantly smaller than required were initially selected, requiring reselection with higher-capacity units
Cable size needed to nearly triple, impacting wiring, switchboards, VSDs, submains, etc…
Approved variation cost: $50,000!

The Lesson:

Detailed static pressure calculations are crucial for each ventilation system even at the design stage, not just throwing random figures in the schedules for the sake of document submission. This is to ensure the final static figures are within ±20% tolerance of design values
Preliminary fan selections and power requirements at design stage based on detailed static pressure calculations are required to ensure minimum impact on the Electrical discipline design requirements at the construction stage

Prevent similar issues with:

Thorough system analysis and component selection
Collaboration between disciplines (electrical & mechanical)
Utilising specialised tools like our Wire Sizing tool (compliant with AS/NZS 3008.1.1). True Partners Consulting Engineers Wire Sizing tool is a specialised, quick and easy to use tool for determining the appropriate size of wires for all electrical HVAC equipment of a project. The sizing tool calculates the optimal cable size based on the Australian Standard AS/NZS 3008.1.1 requirements. The table number utilised from Standard AS/NZS 3008.1.1:2017 is given along with the results, facilitating your cross-checking and reference of the results.

Share your experiences with static pressure calculations and wire sizing requirements below!

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TPCE excels in offering cohesive energy solutions for both new constructions and upgrades of existing facilities. Our proficiency spans MEP Design, Energy Efficiency, and Data Analysis, enabling us to craft and refine energy saving measure solutions. We transform initial ideas, concepts, or spreadsheet data into comprehensive, budget-friendly packages that are ready for implementation and adhere to all compliance Australian standards.

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