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Extension-lead voltage drop

Job / reference: ______________________________Printed date: ____

Formula registry version 1.0.0 · researched 2026-07-24

PS-05 · Power & supply

Extension-lead voltage drop

Use a cable/manufacturer coefficient whose complete-circuit convention is known. Reel loading remains a separate entered-rating decision.

PLANNING ESTIMATE

Cable, supply and reel data

Voltage-drop inputs
Actual reel/lead conditions

Method and worked example

ΔV = (mV/A/m × A × one-way m) ÷ 1000

Drop % = ΔV ÷ entered supply V × 100

Worked example: 18 × 10 × 25 ÷ 1000 = 4.50 V; at 110 V this is 4.09% and an estimated 105.5 V at the far end.

Blank worksheet

mV/A/m source and convention:
A / one-way m / supply V:
Reel state / marked rating / thermal cut-out:
ΔV / end V / percentage:

Lead calculation result

Result recordPLANNING ESTIMATE

Worked example — default entered values

4.50 V drop
105.5 V estimated end voltage
4.09% of 110 V

Above the entered 3% advisory screen — this is not a wiring-regulations verdict.

Assumptions register

18 mV/A/m · 10 A · 25 m one-way · 110 V supply · mV/A/m complete-circuit coefficient applied to the entered one-way length (not doubled) · unwound rating 13 A · thermal cut-out yes · series leads no · damage state no · advisory 3%

The entered mV/A/m value uses the complete-circuit convention and applies to the actual cable.

Length is the one-way lead length expected by that convention; it is not doubled again.

Current is approximately steady and the entered voltage exists at the lead input.

The result covers this lead only; upstream wiring, startup current and series leads are separate.

Advisory — investigate lead length, supply conditions, startup current and any additional series leads.

△ Reel conditions are a separate gate

Use the marked rating for the actual wound/unwound state. Fully unwind reels, confirm thermal protection and do not daisy chain. Stop for damage, overheating, smell, sparks, smoke or repeated protective-device operation.

PLANNING ESTIMATE. Voltage-drop arithmetic does not establish ampacity or a use verdict. The marked wound/unwound reel rating, full unwinding, thermal protection, cable condition and manufacturer instructions govern.

Source register · registry version 1.0.0

Institution of Engineering and Technology, How voltage drop can affect EV charging point open PEN detection devices — effective 2023-07-01 — accessed 2026-07-24

Electrical Safety First, Extension leads, cable reels and block adaptors — accessed 2026-07-24

Electrical Safety First, Overloading sockets — accessed 2026-07-24

Formula researched 2026-07-24