London ElectricalDistributors

How we check the calculations

Each calculator shows how its result is calculated, which units to use and the conditions its formula describes.

What we calculate

Our 23 tools cover relationships between electrical quantities, cable geometry, lengths, costs, and testing and protection limits. They use the values you enter and the model described on each page. The formula, your substituted inputs and the scope accompany the result.

DC Ohm’s law is restricted to a positive, constant resistance. AC power conversions assume sinusoidal steady-state supply voltages and use RMS quantities and true power factor. Three-phase conversion assumes a balanced load and uses line-to-line voltage. Energy tools use entered operating time and a single entered rate.

Testing and protection values

The testing and protection tools use a small number of values from BS 7671, the standards it refers to and the IET’s guidance. Each one follows from a published formula, and our test suite checks it against the printed figures:

  • Maximum Zs for circuit-breakers and RCBOs in TN systems: Zs ≤ U0 × Cmin ÷ Ia (Regulation 411.4.4), with Cmin = 0.95. For 0.4 s, Ia is the top of each type’s instantaneous tripping range in BS EN 60898-1; for type D at 5 s, Table 41.3 uses 10 × In. This reproduces every entry in Table 41.3.
  • Maximum measured Zs: 0.8 × U0 × Cmin ÷ Ia (Appendix 3), rounded up to two decimal places, as the IET prints it for a 10 °C ambient in its 2024 corrections to the On-Site Guide (8th edition, Table B6) and Guidance Note 3 (9th edition, Table A5). No correction for other ambient temperatures is applied.
  • k for copper protective conductors (Tables 54.2 and 54.3): the IEC 60949 formula, rounded to a whole number.
  • Conductor resistance: the BS EN 60228 maximum at 20 °C, with the 1.20 multiplier for 70 °C from the On-Site Guide (8th edition, Table I3).

Fuses are not covered, because their Ia comes from each fuse’s time-current curve, which we do not hold.

Which edition of BS 7671

From 15 October 2026 the only current edition is BS 7671:2018+A4:2026. BS 7671:2018+A2:2022 and Amendment 3 (2024) are withdrawn. In Amendment 4, Regulation 411.4.4 still requires Zs × Ia ≤ U0 × Cmin, with Cmin = 0.95 for supplies under the Electricity Safety, Quality and Continuity Regulations; the IET printed the Amendment 4 text of the regulation in Wiring Matters in July 2026.

None of the IET’s summaries of Amendment 4 lists a change to Table 41.3, Tables 54.2 and 54.3, Regulation 543.1.3 or Appendix 3. We have not checked those tables in the Amendment 4 text itself. The measured Zs values and the 1.20 multiplier come from the IET’s guidance written for BS 7671:2018+A2:2022. We have not yet compared them with the Amendment 4 editions of that guidance (On-Site Guide 9th edition, Guidance Note 3 10th edition).

Choose inputs that describe your calculation

Cable calculations use the dimensions or electrical properties you supply. Check the manufacturer’s data and the conditions it applies to: an outside cable diameter differs from its conductor diameter, and a voltage-drop coefficient must match the circuit arrangement.

Geometric area percentages describe areas. They do not establish a permitted fill level or whether cables can physically fit and be installed. Connected power is a sum of input ratings; maximum demand requires a separate assessment of how the equipment operates.

Numbers, rounding and invalid inputs

Calculations use decimal arithmetic with 100 significant digits internally. Results display up to eight significant figures, using half-up rounding. The approximately-equal symbol marks values rounded for display. Irrational values such as √3 and recurring quotients necessarily have finite numerical approximations. Display precision does not express measurement accuracy.

Blank values, non-numeric strings, negative input values, division by zero and unsupported modes produce no result. Positive inputs must fall between 0.000000001 and 1,000,000,000,000; some fields have narrower bounds. Power factor must be greater than zero and no more than one. Daily operating time cannot exceed 24 hours. Lamp counts and day counts must be whole numbers.

Zero is valid where meaningful, such as zero energy use or a zero tariff. The Ohm’s law tool requires positive inputs so that the requested quantities are determined. A negative LED saving is allowed as an output: it means the new lighting uses more energy.

How to use a result

Read the model before entering values. Use the specified units, check whether the power is electrical input or equipment output, and confirm that any phase assumptions apply. The example tariffs are illustrative values, not current market prices. The cost calculations exclude charges they do not explicitly include.

When copying a result, the tool includes the inputs, formula and assumptions. Changes to inputs clear the previous result so an old answer is not displayed beside new values.

References

Sources accessed 4 September 2026; testing and protection sources checked 28 September 2026, and against Amendment 4 sources on 15 October 2026. The IET is linked for current installation standards and guidance. Apart from the values listed above, its tables are not reproduced in our calculator engine.

If something looks wrong

Report the calculator, inputs, selected mode and expected result through our issue-report page. Include the source for the expected value where possible. Do not rely on a result you have reason to doubt.