Cable Sizing Calculator per BS 7671 for Solar PV Systems
Cable sizing for BS 7671 solar PV systems follows Section 712 requirements, selecting DC string and main cables to carry 1.25 times Isc per Regulation 712.433.1, applying roof-space temperature derating from Table 4B1 for ambient temperatures up to 60 °C, and verifying voltage drop to minimise DC power losses across the array.
Quick Reference Table
| BS 7671 Solar PV Cable Sizing References — BS 7671 (18th Edition + Amendment 2:2022) | ||
|---|---|---|
| Parameter | Value / Requirement | Clause Reference |
| PV cable sizing current | 1.25 × Isc for string and array cables | Regulation 712.433.1 |
| Roof-space temperature correction | Ambient up to 60 °C in roof voids | Table 4B1 |
| DC cable current ratings | Single-core PV cable clipped in free air | Table 4D1A, Method C |
| Voltage drop on DC side | Minimise losses — industry target < 3% | Appendix 4 |
| RCD requirements for PV AC | Type B RCD for inverters with DC fault current | Regulation 712.411.3.2.1 |
How to Calculate Cable Sizing for Solar PV Systems
- 1
Calculate the PV string current
Determine the short-circuit current (Isc) of the PV string from module datasheets and multiply by 1.25 per Regulation 712.433.1 to get the design current for DC cables.
- 2
Assess roof-space ambient temperature
Measure or estimate the maximum temperature in the roof void or behind the PV array. Temperatures can reach 55-65 °C in summer, requiring significant derating from Table 4B1.
- 3
Select DC cable size
Choose solar-rated (H1Z2Z2-K) cable with a current rating that exceeds the derated design current. Typical residential PV strings use 4 mm² or 6 mm² DC cable.
- 4
Check DC voltage drop
Calculate the total DC cable voltage drop from modules to inverter. Keep total drop below 3% to avoid significant energy yield losses over the system lifetime.
- 5
Size the AC cable from inverter to consumer unit
Size the AC output cable per standard BS 7671 methods using the inverter's maximum AC output current as the design current, applying normal derating factors.
Try the Cable Sizing Calculator
Run compliant BS 7671 calculations for solar pv systems — free, instant results with full clause references.
Calculate Cable Sizing 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 |