Data Source and Methodology
Authoritative source: The IET Wiring Regulations — BS 7671:2018+A2:2022, Appendix 4 (Voltage drop), including representative tables such as 4D2A and 4E. Official publication: The Institution of Engineering and Technology (IET), 2022. See the IET publication page: BS 7671:2018+A2:2022. For an accessible overview, see NAPIT’s guidance: How to determine voltage drop limits.
All calculations are strictly based on the formulas and tabulated data provided by this source.
The Formula Explained
Using tabulated values (BS 7671 Appendix 4):
Single-phase and three-phase with tabulated mV/A/m:
Inline: \( \Delta V = \dfrac{mV\!/\!A\!/\!m \times I_b \times L}{1000} \)
Block: \[ \Delta V = \frac{k \cdot I_b \cdot L}{1000} \] where \(k\) is the tabulated voltage-drop factor in mV/A/m appropriate to the circuit type.
Percentage voltage drop: \[ \Delta V_{\%} = \frac{\Delta V}{V_n} \times 100 \]
Maximum allowable length for a given limit \(p\%\): \[ L_{\max} = \frac{\left(\frac{p}{100} \cdot V_n\right) \cdot 1000}{k \cdot I_b} \]
General formulation with resistance and reactance (reference):
Single-phase: \( \Delta V = I \cdot (R \cos\varphi + X \sin\varphi) \cdot L \)
Three-phase: \( \Delta V = \sqrt{3} \cdot I \cdot (R \cos\varphi + X \sin\varphi) \cdot L \)
Glossary of Variables
- System type: Single- or three-phase circuit.
- Nominal voltage (Vn): The system nominal voltage (L–N for single-phase, L–L for three-phase).
- Design current (Ib): The current used for design of the circuit segment assessed.
- Run length (L): One-way run length of the circuit in metres.
- mV/A/m (k): Tabulated voltage-drop factor from BS 7671 Appendix 4 for the chosen cable family, size, and circuit type.
- ΔV (V): Voltage drop across the run.
- ΔV%: Voltage drop as a percentage of Vn.
- Limit (3% / 5%): Common design limits for lighting and other circuits, respectively.
- Lmax: Maximum permissible one-way length for the chosen parameters to remain within the limit.
- Minimum CSA: Smallest conductor cross-sectional area that meets the selected limit with the given Ib and L.
How It Works: A Step-by-Step Example
Inputs: Single-phase, Vn = 230 V, Ib = 20 A, L = 30 m, limit = 3% (lighting), family = PVC 70°C copper, size = 4 mm². From BS 7671 Appendix 4 tables, the tabulated value is \(k = 11\ \text{mV/A/m}\) for a typical two-core single-phase multicore circuit.
Calculation:
\( \Delta V = \dfrac{11 \times 20 \times 30}{1000} = 6.6\ \text{V} \)
\( \Delta V_{\%} = \dfrac{6.6}{230} \times 100 = 2.87\% \)
Result: 2.87% ≤ 3% ⇒ compliant for lighting. If the limit were 5% (other uses), this would also comply comfortably.
Frequently Asked Questions (FAQ)
Do the tabulated mV/A/m values already include the return path?
Yes. For the stated circuit type (e.g., 2-core single-phase or 3-core three-phase multicore), the BS 7671 tabulated factors account for the relevant return path.
Should I use 230 V or 400 V?
Use 230 V for single-phase (line-to-neutral) circuits and 400 V for three-phase (line-to-line) circuits unless your specification states otherwise.
What if my installation uses XLPE 90°C cables?
Select the “XLPE 90°C” family. These cables typically have slightly lower voltage drop factors than PVC 70°C. Verify values against the latest BS 7671 Appendix 4 tables.
How is minimum CSA determined here?
The tool searches the tabulated sizes for the chosen family and system, computing drop for each, and returns the smallest size that satisfies the selected limit.
Can I override the mV/A/m value?
Yes. Toggle “Use custom mV/A/m” to input a value from BS 7671 or a validated manufacturer datasheet.
Is this calculator sufficient for full compliance?
No tool can replace professional judgment. Confirm all inputs, ambient conditions, grouping, thermal constraints, cable ratings, and the latest standard and client requirements.
Where can I read more about voltage drop limits?
See NAPIT’s explainer: Voltage drop limits within electrical installations.
Quality note: Values and presets are representative for common copper multicore circuits per BS 7671 Appendix 4. Always cross-check against your exact cable construction and the latest amendment of BS 7671.