Voltage Drop Calculator per BS 7671 for Solar PV Systems
Solar PV voltage drop under BS 7671 Section 712 applies to both DC string cables and AC output cables. The 5% limit from Table 4Ab applies to the AC side, while DC cable sizing must ensure the inverter's MPPT range is maintained. Excessive DC voltage drop reduces energy yield by shifting the operating point away from maximum power.
Quick Reference Table
| BS 7671 Voltage Drop Considerations — Solar PV — BS 7671 (18th Edition + Amendment 2:2022) | ||
|---|---|---|
| Parameter | Value / Requirement | Clause Reference |
| AC side voltage drop limit | 5% (11.5 V at 230 V single-phase) | Appendix 4, Table 4Ab |
| PV installation requirements | Section 712 specific provisions | Section 712 |
| DC cable mV/A/m values | Use manufacturer data or Table 4D1B Column 2 (two-core DC equivalent) | Table 4D1B |
| Permissible drop measurement point | From meter position to inverter AC output terminals | Regulation 525.1 |
| DC string cable sizing | Determined by inverter MPPT voltage window, not Table 4Ab | Regulation 712.512.2 |
How to Calculate Voltage Drop for Solar PV Systems
- 1
Calculate DC string voltage and current
Determine the Voc and Isc of each PV string at STC conditions. Apply temperature correction for the coldest expected ambient temperature (Voc increases in cold weather). The total string voltage must remain within the inverter MPPT range after accounting for cable voltage drop.
- 2
Size DC string cables for voltage drop
Calculate DC voltage drop using VD = 2 × Isc × R × L, where R is the conductor resistance per metre and L is the one-way cable length from the string to the inverter. Keep DC voltage drop below 1-2% to maximise energy yield — this is a design choice, not a BS 7671 requirement.
- 3
Size AC cables from inverter to consumer unit
The AC cable from the inverter to the consumer unit is subject to the standard BS 7671 voltage drop limits. Use Table 4D1B mV/A/m values with the inverter's maximum output current.
- 4
Check total AC voltage drop
For domestic PV systems, the AC cable run is typically short (under 15 m), making voltage drop minimal. For commercial roof-top systems with inverters in plant rooms, the run may be longer. Verify the drop is within the 5% limit per Table 4Ab.
- 5
Document DC and AC voltage drops separately
Record the DC and AC voltage drop calculations separately in the design documentation. Include string Voc, cable lengths, conductor sizes, and the resulting voltage at the inverter DC input and AC output terminals.
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Calculate Voltage Drop NowBS 7671 vs IEC 60364 Cable Sizing Comparison
| Parameter | BS 7671 | IEC 60364 |
|---|---|---|
| Scope | UK & derivatives | International (adopted by 60+ countries) |
| Voltage drop limit | 3% lighting / 5% other | 4% lighting / 5% other (typical) |
| Reference ambient temp | 30°C (air), 20°C (ground) | 30°C (air), 20°C (ground) |
| Installation methods | Reference Methods A-G (Appendix 4) | Reference Methods A-G (Table B.52.1) |
| Grouping factors | Table C.3 (BS specific) | Table B.52.17 (international) |
| Disconnection time (230V) | 0.4s final / 5s distribution | 0.4s final / 5s distribution |