Cable Size UK: BS 7671 sizing
Cable & volt drop calculator
£9.99 · Designed for iPad. Not verified for macOS.
Cable Size UK - The Calculator That Shows Its Working
Free calculators give you a number. Cable Size UK gives you the chain of
reasoning: design current, protective device check, correction factors,
the minimum tabulated capacity your cable must meet, and volt drop as a
value and as a percentage of nominal. The intermediate values are what
an inspector asks for, so a result can be transcribed onto a design or a
certificate and defended during an inspection.
HOW IT WORKS
Enter the circuit details: the load and supply, the protective device
rating, the correction factor values that apply to the installation, and
the cable's volt drop figure and run length. The app works through the
calculation in the order the method describes and shows every step, so
you can check the working rather than trust a black box.
WHAT IT CALCULATES
- Design current (Ib) for single and three phase loads, entered as power
or as stated current, with power factor, efficiency and diversity
- Protective device check: the rating you choose is verified against the
design current (Ib not exceeding In)
- Required tabulated capacity (It): applies the correction factors you
enter for ambient temperature, grouping, thermal insulation and
rewireable fuses, and shows the combined factor
- Volt drop from the cable's millivolt per amp per metre figure, in
volts and as a percentage of nominal, checked against the 3 percent
lighting and 5 percent other-uses limits
- The full working for every result, step by labelled step
WORKS WITH YOUR OWN TABLES
This app reproduces no tabulated data. You read the factor values and
cable figures from your own copy of the tables, exactly as you would on
paper; the app applies the method, keeps the arithmetic honest and
shows the working. Where a case falls outside what it covers, it says
so rather than guessing.
WHO IT IS FOR
UK electricians, electrical designers and competent-person scheme
members, and anyone learning cable sizing who wants to see the method,
not just the answer.
A DESIGN AID, NOT A DESIGNER
Cable Size UK is a calculation aid. It does not replace the judgement of
a competent person, and every result should be verified by one before an
installation is energised. Calculations follow the methods of
BS 7671:2018 Amendment 4:2026; the app is not certified, approved or
endorsed by BSI or the IET.
PRIVACY
No sign-in, no ads, no tracking. Works offline. Your calculations stay
on your device.
Ratings & Reviews
- This app has not received enough ratings or reviews to display an overview.
Adds to the sizing chain:
- Share the working: export any calculation as text, ready to paste
into notes or a job record
- Saved circuits: name a circuit, reopen it later; inputs are saved
and results recalculate
- Maximum cable length (Lvd): how far a run can go before the volt
drop limit, for radial, distributed and ring final circuits
- Device type and installation pickers: Cf and Cc are applied for you,
as the regulations state them
- Buried circuits: soil resistivity and depth factors
- Circuits without overload protection (Regulation 433.3.1 cases)
- Private LV supply volt drop limits
The developer, Plan CD, indicated that the app’s privacy practices may include handling of data as described below. For more information, see the developer’s privacy policy .
Data Not Collected
The developer does not collect any data from this app.
Accessibility
The developer has not yet indicated which accessibility features this app supports. Learn More
Information
- Provider
- Rik Watson
- Size
- 1.6 MB
- Category
- Utilities
- Compatibility
Requires iOS 26.0 or later.
- iPhone
Requires iOS 26.0 or later. - iPad
Requires iPadOS 26.0 or later. - Mac
Requires macOS 26.0 or later and a Mac with Apple M1 chip or later. - Apple Vision
Requires visionOS 26.0 or later.
- iPhone
- Languages
- English
- Age Rating
4+
- 4+
- Copyright
- © 2026, Rik Watson, Omega Point Limited.
