Long leads punish compressors
Voltage drop that looks tolerable for a resistive heater can still cause hard starts or poor performance on compressor-based equipment.
Electrical calculator · BS 7671 / voltage drop / temporary HVAC
BS 7671 Voltage Drop Envelope for Temporary HVAC Loads
Estimate voltage drop over long leads for heaters, air movers, dehumidifiers and portable AC, with compressor start warnings.
A planning aid, not a certification cable design package.
Field notes
Field notes for HVAC voltage drop envelope
Practical checks to run before this calculator result turns into a site decision.
Use realistic length and load
Measure the route rather than guessing. Include the load that actually runs together, not just the newest machine being added.
Treat amber as a layout prompt
Move equipment closer, use a better supply point, split circuits or change cable size before the site becomes unreliable.
FAQ
HVAC voltage drop envelope FAQ
Short answers written for UK temporary electrical and HVAC planning work.
What is the HVAC voltage drop envelope used for?
Estimate voltage drop over long leads for heaters, air movers, dehumidifiers and portable AC, with compressor start warnings. It is mainly for UK electrical review work, 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 current BS 7671 values, manufacturer device data, measured results, earthing arrangement, correction factors and site installation conditions. 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.
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