Cable Voltage Drop

Voltage drop + Vd % against a 3% / 5% limit

Cable size (CSA)

Design current Ib

Reading 20 A

Cable run length

Reading 25 m

System

Circuit type / Vd limit

The limits for an installation fed from the public supply.

Results

Voltage drop

Vd %

3.91%

Limit used5% (11.5 V) — BS 7671 Table 4Ab, public supply

ResultWithin the selected threshold. Indicative result.

mV/A/m used18 (2.5 mm² flat T&E, Table 4D5, single-phase)

Cable length25 m

Not checked here

Current-carrying capacity (Iz) and its correction factors (Ca, Cg, Ci), earth fault loop impedance (Zs) and the protective device. Check these separately against BS 7671 Appendix 4 and Part 4.

What to buy — 2 items

2.5mm² Twin & Earth cable (6242Y)

Inc. 10% slack

28 m

Flat cable clip for 2.5mm² T&E

At 250 mm — the tightest horizontal spacing in the On-Site Guide clip table; confirm against the current edition

100 pc

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Indicative design aids — the main figures and where each comes from are listed under "Figures based on" below.

An indicative voltage-drop check only. It does not size the cable for current-carrying capacity, check earth fault loop impedance or select the protective device, and does not demonstrate compliance with BS 7671.

Figures based on

mV/A/m (70 °C thermoplastic, copper): BS 7671:2018+A4:2026 Appendix 4 — Table 4D5 for flat twin and earth (to 16 mm², single-phase) and the multicore tables for 25–35 mm² and three-phase. Cited, not reproduced: the values were transcribed from reproductions of the 2018 tables, so whether A4:2026 changed any is still to be confirmed against the printed book
Limits: 3% lighting, 5% other uses: BS 7671:2018+A4:2026 Appendix 4, Table 4Ab, for an installation supplied from a public low-voltage network. To be confirmed against the printed book; a private supply has different limits
Nominal voltages 230 V / 400 V: BS 7671:2018+A4:2026 (UK nominal Uo 230 V single-phase, 400 V between lines)
Cable +10% for slack: No source — workshop estimate: a fixed estimating assumption
Flat cable clips at 250 mm: IET On-Site Guide Table D1 (clip spacing for cables), the tightest horizontal spacing it gives; the edition matching BS 7671:2018+A4:2026 is still to be confirmed

Indicative design aids. A standard is named with its edition; each is still to be checked against the published text. "Typical" means no standard sets it: check the bag, tin, data sheet or supplier named. "No source" means a workshop estimate: adjust to suit, or, where the box says it is fixed, a fixed estimating assumption.