BS 7671 Cable Sizing Calculator
Professional BS 7671 Cable Sizing Calculator for UK electrical design. Compute required tabulated capacity (Iz), voltage drop, and adiabatic short-circuit checks using IET factors.
Design data
Apply correction factors and table data taken directly from BS 7671:2018+A2:2022.
Correction factors
How to Use This Calculator
Collect the design current, installation corrections, and tabulated parameters from BS 7671:2018+A2:2022. Choose the appropriate calculator mode—Sizing, Verify cable, or Full check—based on how many verification steps you want to perform.
After clicking Calculate, the tool will update Iz_min, verify your protective device rating, estimate voltage drop, and compute the adiabatic requirement when you provide short-circuit data. Always confirm the results against licensed tables and manufacturer guidance.
Step-by-step
- Gather design current Ib, correction factors (Ca, Cg, Ci, Cf), and protective device rating In.
- Use the calculator to compute Iz_min = Ib / (Ca × Cg × Ci × Cf) and ensure the chosen cable meets that capacity.
- Add tabulated data (It), cable CSA, and the mV/A/m coefficient to verify thermal limits and voltage drop.
- Enter circuit length, power factor, and voltage drop limit to check %ΔV against the nominal voltage.
- For adiabatic checks, provide fault current I_sc, disconnection time t, and the k factor from BS 7671.
Authoritative methodology
Standards reference: IET Wiring Regulations BS 7671:2018+A2:2022 (18th Edition, Amendment 2). This calculator applies the official Iz, voltage drop, and adiabatic formulas; users must source tabulated values (It, mV/A/m, k) from their licensed copy of BS 7671 or manufacturer data.
Glossary of variables
- Ib — Design load current (A).
- In — Protective device rating (A).
- Iz — Cable current-carrying capacity under conditions (A).
- Iz_min — Minimum required tabulated capacity before applying factors (A).
- Ca — Ambient temperature correction (≤ 1.0).
- Cg — Grouping correction (≤ 1.0).
- Ci — Thermal insulation correction (≤ 1.0).
- Cf — Additional special conditions correction.
- m — Voltage drop coefficient (mV/A/m) from Appendix 4.
- L — Circuit conductor length (m).
- I_sc — Prospective fault current (kA).
- k — Adiabatic material/insulation factor (A·s0.5/mm²).
Worked example
For a single-phase 230 V lighting circuit with Ib = 32 A and a grouped installation (Ca = 1.00, Cg = 0.80, Ci = 1.00, Cf = 1.00), the total correction factor F equals 0.80 and Iz_min = 40 A. Choose a cable with It ≥ 40 A. If the cable length is 30 m with m = 18 mV/A/m, ΔV equals 17.28 V (≈7.5 % of 230 V), so the drop exceeds a 5 % limit and you would need a larger conductor or shorter run. For I_sc = 6 kA, t = 0.4 s, and k = 143 (Cu/XLPE), S_min ≈ 26.3 mm², so the chosen CSA must meet or exceed that value.
Frequently Asked Questions
Does this calculator include the BS 7671 tables?
No. Those tables are copyrighted by the IET. Enter table values from your licensed copy.
Which correction factors should I use?
Use the factors that match your installation method and conditions from BS 7671 (ambient, grouping, thermal insulation, etc.). Include any additional derating as Cf.
How do I select the correct mV/A/m value?
Appendix 4 provides mV/A/m values by conductor construction and system arrangement. Pick the row that matches the cable type and whether it is single- or three-phase.
What is the relationship between Ib, In, and Iz?
Ensure Ib ≤ In ≤ Iz. The protective device rating must be at least the design current and no larger than the cable’s capacity.
Do I need to consider installation method and temperature rise?
Yes. Installation conditions, ambient temperature, and any thermal insulation reduce cable capacity and are captured through Ca, Cg, Ci, and Cf.
Is the adiabatic check for the line conductor or CPC?
The adiabatic equation verifies the conductor that must withstand the fault current; designers often use it for the CPC but may apply it to line conductors for worst-case scenarios.
Does power factor affect voltage drop?
The mV/A/m coefficient already accounts for R and X. If your load has a significant reactive component, select the appropriate table entry or use manufacturer data that reflects that power factor.