powertoolspros.net · Extension-lead voltage drop and reel loading calculation record · PLANNING ESTIMATE

Extension-lead voltage drop

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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

△ Live calculation unavailable

JavaScript is off. Use the formula, fixed worked example and blank worksheet. Edited fields do not update the example.

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

Health and Safety Executive, Electricity – Systems in buildings — accessed 2026-07-24

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

Site-lead data guide · 110 V and 230 V

Choosing CSA, length and current inputs for a site lead

Scope: portable extension leads and cable reels used with UK 110 V centre-tapped-to-earth (CTE) or 230 V site supplies. This calculator does not size fixed wiring, select a conductor, establish current-carrying capacity or reproduce a BS 7671 design table.

Scroll the table sideways to view every column.

Input	What to record	Boundary before calculation	Source context
Conductor CSA	The marked or documented conductor cross-sectional area in mm², plus the cable type and number of loaded conductors.	Obtain an mV/A/m coefficient that applies to that actual cable and state whether it is already a complete-circuit value. The calculator does not derive a coefficient from CSA alone.	The IET worked method shows that CSA, coefficient, current and length are linked, but its numerical table concerns a fixed-wiring EV circuit and is not copied as a portable-lead table.
One-way length	The lead path in metres from its supply point to the tool or load.	Enter the one-way length once when using the stated complete-circuit coefficient convention. Do not silently double it.	The IET formula uses mV/A/m × current × length ÷ 1000; the coefficient convention must be carried with the number.
Operating current	A representative load current for the actual tool and operating condition, plus any material startup-current information.	Compare current separately with the marked rating for the actual wound or unwound reel state. Startup behaviour and protective-device operation are outside the steady-current arithmetic.	Electrical Safety First says reel rating depends on the cable fitted and the reel must be unwound to achieve its marked current rating.
Supply and lead state	110 V CTE or 230 V at the lead input; connector/rating; wound or unwound; thermal cut-out; damage check; whether another lead is in series.	Use one lead/reel, fully unwind it and stop for damage, heat or repeated protective-device operation. A voltage-drop result does not override the marked rating.	HSE construction electrical-systems guidance supplies the 110 V CTE site context; Electrical Safety First overloading guidance supports the separate load-rating check.

Same lead, different entered supply voltage

With an entered complete-circuit coefficient of 18 mV/A/m, 10 A and 25 m one-way length, the drop is 4.50 V. That is 4.09% of an entered 110 V supply and 1.96% of an entered 230 V supply. The volts lost are unchanged because the coefficient, current and length are unchanged; the percentage changes with supply voltage. Neither percentage is a fixed-wiring verdict or a current-capacity check.

For the complete site-supply sequence, read Planning 110 V power for site tools. Transformer loading remains a separate calculation and has not been changed as part of this update.