UK Cable Sizer — BS 7671 Appendix 4
WireWrite's UK cable sizer applies the three checks every
electrician must satisfy before specifying a conductor:
current-carrying capacity, voltage drop, and short-circuit thermal
withstand. The calculator is free, requires no login, and works
offline as a PWA after first load.
The three checks an electrician must satisfy
1. Current-carrying capacity (Iz ≥
Ib). The conductor's de-rated current rating
must equal or exceed the design current of the circuit. BS 7671
Appendix 4 supplies the base tabulated rating; reference method,
grouping factor Cg, ambient temperature factor
Ca and thermal-insulation factor Ci all
reduce that figure. The calculator applies these for you when you
pick a reference method.
2. Voltage drop. Voltage drop across the
conductor is calculated from Vdrop = (I × L
× mV/A) / 1000, where mV/A is the conductor's tabulated
voltage drop per amp per metre. BS 7671 Regulation 525.201 caps
drop at 3 % for lighting and 5 % for other circuits where the
installation is supplied directly from a public LV distribution
network, with higher limits for private LV supplies.
3. Adiabatic short-circuit check. The
conductor must survive the prospective fault current for the time
taken by the upstream protective device to clear the fault. The
adiabatic equation k²S² ≥ I²t (where k is the
conductor material constant, S the cross-sectional area in
mm², I the fault current and t the disconnection time)
checks this in the background.
Why reference method changes the answer
The same 2.5 mm² cable has a different current-carrying
capacity depending on how it's installed. Clipped direct to a
surface (Method C) runs cooler than buried in thermal insulation
(Method A) or in conduit grouped with other circuits (Method B).
The calculator's reference-method picker applies the BS 7671
Appendix 4 correction factors automatically so you don't have to
chase them manually for each circuit.
Why oversizing is safe and undersizing isn't
An oversized cable wastes copper but is safe. An undersized
cable overheats under continuous load, degrades its insulation,
and can ignite. The calculator returns the smallest legal size
for the inputs you provide — round up one nominal size if
your installation has correction factors not covered (for example,
grouped in a hot loft space).
Digital electrical certificates for UK electricians. Create BS 7671 compliant Electrical Installation Certificates (EIC), Minor Works Certificates and Electrical Installation Condition Reports (EICR) on-site. From £5/month + VAT.
BS 7671 Certificates Supported
- Electrical Installation Certificate (EIC) — new installations, additions and alterations
- Minor Electrical Installation Works Certificate (MEIWC) — minor works on existing circuits
- Electrical Installation Condition Report (EICR) — periodic inspection and testing
- Schedule of Inspections and Schedule of Circuit Details & Test Results included
- Running header, footer and Page N of M on every page — BS 7671 Appendix 6 compliant
Why UK Electricians Choose WireWrite
- 14-day free trial — no card required
- Install as a mobile app (PWA) on phone or tablet
- Digital signatures, photos and PDF email delivery
- BS 7671 calculators: cable sizing, voltage drop, maximum demand
- Accepted by NICEIC, NAPIT, ELECSA, Stroma and other competent person schemes
BS 7671 Calculators
- Cable Sizing Calculator
- Voltage Drop Calculator
- Maximum Demand Calculator
Start creating BS 7671 certificates today: Visit wirewrite.co.uk