BS 7671 Cable Sizing Calculator

How this BS 7671 cable sizing calculator works

The goal of this tool is to give a quick, transparent indication of a suitable LV cable size to BS 7671 for typical 230/400 V installations. It follows the usual design sequence:

1. Start from the design current \( I_b \).
2. Apply correction factors to find the required tabulated current-carrying capacity \( I_t \).
3. Select a cable size with \( I_z \ge I_t \).
4. Check volt drop against the chosen limit.
5. Check earth fault loop impedance \( Z_s \) against the maximum allowed for the protective device.

1. Current-carrying capacity and correction factors

BS 7671 requires that the cable's current-carrying capacity \( I_z \) is not less than the design current corrected for installation conditions. A common design approach is:

The calculator estimates \( C_a \), \( C_g \) and \( C_i \) from your inputs using simplified curves inspired by BS 7671 tables. It then searches through a list of standard conductor sizes and their approximate tabulated capacities \( I_z \) for the selected cable type and installation method.

2. Volt drop check

For each candidate cable size, the tool calculates the volt drop using the standard formula:

The percentage volt drop is then:

3. Earth fault loop impedance (Zs) check

To give a quick indication of disconnection times, the calculator estimates the circuit earth fault loop impedance as:

The value of \( R_1 + R_2 \) is estimated from typical resistance-per-metre values for copper conductors at operating temperature. The resulting \( Z_s \) is compared with the maximum you enter from the appropriate BS 7671 table for your protective device and disconnection time.

4. Limitations and good practice

• The internal tables are simplified and do not reproduce BS 7671 tables verbatim.
• Only a small subset of cable types and installation methods is included.
• Thermal effects, harmonics, voltage unbalance and special locations are not fully modelled.
• Final designs must always be checked by a competent person against the full standard and manufacturer data.

Worked example

Suppose you have:

• Single-phase 230 V ring final circuit
• Design current \( I_b = 32 \,\text{A} \)
• Twin & earth 70 °C PVC cable, clipped direct
• Ambient temperature 30 °C, no grouping, no insulation
• Circuit length 25 m (one-way)
• Max volt drop 3 %
• Max \( Z_s = 1.37 \,\Omega \) (e.g. 32 A type B MCB, TN system)

The calculator will:

1. Estimate \( C_a \approx 1.00 \), \( C_g = 1.00 \), \( C_i = 1.00 \).
2. Compute \( I_t = 32 / (1.0 \times 1.0 \times 1.0) = 32 \,\text{A} \).
3. Check standard sizes (e.g. 2.5, 4, 6 mm²) until it finds one with \( I_z \ge 32 \,\text{A} \).
4. Calculate volt drop and \( Z_s \) for each size and select the smallest that passes all limits.

This gives you a quick, transparent starting point before you refine the design using full BS 7671 tables and manufacturer data.

BS 7671 cable sizing – frequently asked questions

Can I change the maximum Zs values?

Yes. The calculator does not hard-code Zs limits from BS 7671. Instead, you enter the maximum Zs taken from the appropriate table for your protective device, system type and disconnection time. This keeps the tool flexible and avoids reproducing copyrighted tables.

Why do my results differ from other BS 7671 calculators?

Different tools may use different assumptions for operating temperature, conductor resistivity, grouping factors, or may interpolate directly from manufacturer data. This calculator uses simplified, rounded values for clarity. Small differences are normal; if you see large discrepancies, re-check all inputs and verify with the standard.

Can I use this for three-phase motors and long feeders?

Yes, the calculator supports three-phase 400 V circuits and long lengths, but you must pay particular attention to volt drop and starting currents for motors. For critical circuits, always cross-check with detailed manufacturer data and, where necessary, perform more detailed thermal and voltage stability studies.