BS 7671 Voltage Drop Calculator

Calculate voltage drop for single‑phase and three‑phase circuits in accordance with BS 7671 using tabulated mV/A/m values. Check compliance with the 3% and 5% design limits for lighting and other loads.

Always verify final designs against the latest edition of BS 7671 and manufacturer data. This tool is for design assistance and education, not a substitute for professional judgement.

Voltage Drop Calculator (BS 7671)

A
m

Length of the run from origin to load (not loop length).

Results

Enter the circuit details above and click “Calculate voltage drop”.

How the BS 7671 voltage drop calculation works

BS 7671 does not ask you to calculate voltage drop from first principles (resistance, reactance, power factor). Instead, it provides tabulated voltage drop values in mV/A/m for different cable types, installation methods and conductor materials.

General BS 7671 voltage drop formula

For both single‑phase and three‑phase circuits, using the tabulated value:

\( \Delta V = \dfrac{mV/A/m \times I_b \times L}{1000} \)

  • \(\Delta V\) = voltage drop in volts (V)
  • mV/A/m = tabulated value from BS 7671 (millivolts per ampere per metre)
  • \(I_b\) = design current in amperes (A)
  • \(L\) = circuit length in metres (m)

The percentage voltage drop is then:

\( \% \Delta V = \dfrac{\Delta V}{U_n} \times 100 \)

  • \(U_n\) = nominal system voltage (e.g. 230 V or 400 V)

Recommended BS 7671 voltage drop limits

BS 7671 recommends that the total voltage drop from the origin of the installation to the point of utilisation should not normally exceed:

  • 3% of nominal voltage for lighting circuits
  • 5% of nominal voltage for other circuits (socket outlets, fixed appliances, etc.)

These are design recommendations, not automatic disconnection limits, but they are widely used as design targets. If the DNO or upstream system already has a known voltage drop, you should ensure the combined drop still meets these limits.

Worked example

Consider a 230 V single‑phase ring final circuit:

  • Design current \(I_b = 26\) A
  • Circuit length \(L = 35\) m (origin to furthest point)
  • Cable: 2.5 mm² Cu twin & earth, tabulated value 18 mV/A/m
  • Load type: “other” (5% limit)

Step 1 – voltage drop in volts:

\( \Delta V = \dfrac{18 \times 26 \times 35}{1000} = \dfrac{16380}{1000} = 16.38 \text{ V} \)

Step 2 – percentage voltage drop:

\( \% \Delta V = \dfrac{16.38}{230} \times 100 \approx 7.1\% \)

This exceeds the recommended 5% limit, so you would typically increase cable size or shorten the run. The calculator highlights this automatically and lets you quickly test alternative cable sizes.

Single‑phase vs three‑phase in BS 7671 tables

BS 7671 provides separate mV/A/m values for single‑phase and three‑phase circuits. The calculator:

  • lets you choose the system type (single or three‑phase)
  • offers typical values for common cable sizes and materials
  • allows a custom mV/A/m so you can copy directly from the tables

Neutral and harmonic effects

In some installations (especially with non‑linear loads), the neutral conductor may carry significant current, increasing effective voltage drop. BS 7671 tables already include typical assumptions, but in edge cases you may want to allow a small margin.

The “Approximate additional drop due to neutral / harmonics” option applies a modest uplift to the calculated value as a reminder to check the design carefully. For critical designs, always refer to manufacturer data and detailed calculations.

FAQ

Do I use Ib or In for voltage drop?

BS 7671 defines voltage drop in terms of the design current \(I_b\), i.e. the expected load current under normal operation. Many designers use the protective device rating \(I_n\) as a conservative approximation if \(I_b\) is close to \(I_n\).

Should I use route length or loop length?

The tabulated mV/A/m values in BS 7671 are based on the one‑way route length of the circuit, not the total loop length. Enter the distance from the origin to the furthest point of utilisation.

Can I mix BS 7671 tables with manufacturer data?

Yes. BS 7671 allows the use of manufacturer’s data where it is more accurate or specific to the cable type. In that case, simply enter the manufacturer’s mV/A/m figure into the Custom mV/A/m field.

Is this calculator valid for all editions of BS 7671?

The method (using mV/A/m × Ib × L) is consistent across recent editions, but tabulated values and notes can change. Always confirm that the mV/A/m value you use comes from the current edition of BS 7671 or up‑to‑date manufacturer data.