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Electrical calculator · ducting / fan derate / static pressure

Ducting Static Pressure & Fan Derating Calculator

Estimate how duct length, bends, inlet restrictions and outlet restrictions reduce nominal fan airflow.

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For portable extraction, ventilation, drying airflow and AC exhaust duct planning.

Field notes

Field notes for Ducting fan derate

Practical checks to run before this calculator result turns into a site decision.

Site check

Air path matters more than fan count

Fans need a sensible intake, discharge route and make-up air path. More fans can still underperform if ducting or room geometry blocks the flow.

Site check

Ducting changes the real output

Long flexible duct, crushed bends and reducers can knock capacity down quickly. Use the result as a site sanity check, not a catalogue promise.

Site check

Avoid creating a new site hazard

Large fans and air movers add trailing leads, noise, dust movement and sometimes negative pressure. Plan power routes and access before switching on.

FAQ

Ducting fan derate FAQ

Short answers written for UK temporary electrical and HVAC planning work.

What is the Ducting fan derate used for?

Estimate how duct length, bends, inlet restrictions and outlet restrictions reduce nominal fan airflow. It is mainly for temporary HVAC, drying, cooling and site-power planning, especially where a quick pre-check is needed before selecting equipment or changing a temporary setup.

Can this replace BS 7671 design, inspection or testing?

No. It is a competent-person planning aid only. Final decisions still need current BS 7671 requirements, manufacturer data, inspection, testing, risk assessment and the actual site conditions.

What should I verify before acting on the result?

Check room volume, duct length, static pressure, intake and exhaust position, make-up air and the supply available for continuous fan operation. If any assumption is uncertain, use the result as a prompt to investigate rather than as permission to energise.

What does an amber or red result usually mean?

It normally means the margin is weak, an assumption is missing, or the load should be split, staged, moved closer to the supply, reduced or reviewed by a competent electrician before use.

Next tools in this workflow

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