National Electric MFGTemporary power • HVAC load tools
Methodology

Electrical calculator · AC exhaust / ducting / derate

Portable AC Exhaust Duct Derating Calculator

Estimate cooling performance loss from long exhaust duct runs, bends and recirculation risk.

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For temporary AC installs where hot exhaust routing is the real constraint.

Field notes

Field notes for AC exhaust derate

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

Site check

Exhaust and condensate are part of the load plan

Portable AC fails when hot exhaust air recirculates or condensate has nowhere to go. Check ducts, pumps and drainage before sizing more units.

Site check

Compressor starts need headroom

Running current is not the whole story. If the site has generators, long leads or shared circuits, leave margin for start current and voltage sag.

Site check

Watch mixed-use rooms

Server rooms, event rooms and workshops change load during the day. Recheck assumptions after doors, people, equipment and extraction patterns are known.

FAQ

AC exhaust derate FAQ

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

What is the AC exhaust derate used for?

Estimate cooling performance loss from long exhaust duct runs, bends and recirculation risk. 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 cooling load, exhaust duct route, condensate route, compressor start current, available circuit rating and whether other site loads share the supply. 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

See super flows