Solar Cable Size Calculator (UK)
Free UK solar cable size calculator. Enter voltage, current, and cable run length to get the smallest copper cross-section that stays within BS 7671 voltage-drop limits.
Prepared by the Solar Calculator HQ editorial team. Review our formula, source and limitation standards.
Solar Wire Size Calculator
How to use this calculator
Enter four values:
- System voltage — typical UK domestic strings run at 250–600 V DC; 12 V / 24 V / 48 V for leisure and off-grid setups
- Current — the maximum amps the circuit will carry (read panel Imp from the data sheet, or charge controller output rating)
- One-way length — distance in metres from the array to the inverter or charge controller
- Max voltage drop % — 3% is the MCS recommendation, 2% is excellent, 1% is best-in-class
The calculator finds the smallest cable cross-section that keeps drop within your target. Smaller cable is cheaper and easier to install — bigger cable wins only when no smaller size meets your spec.
Why cable sizing matters more than UK installers admit
A cable that’s too small does three bad things on a UK rooftop:
- Wastes energy as heat in the conductor
- Reduces voltage at the inverter — MPPT trackers can clip, charge controllers can underperform
- Heats up under load — sun-baked roof void cables can sit at 60–70°C, accelerating insulation ageing and reducing the 25-year warranty’s real lifespan
Going one cross-section larger than the minimum is one of the cheapest performance upgrades on a DIY install — copper costs scale roughly linearly with cross-section, but loss reductions are exponential.
The voltage-drop math
The calculator tries every standard cross-section (1.5, 2.5, 4, 6, 10, 16, 25 mm²) and computes the voltage drop for your inputs. It picks the smallest cable that meets your maximum drop percentage.
DC voltage drop formula:
V_drop = 2 × Length(m) × Resistance(Ω/m) × Current(A)The factor of 2 accounts for the round trip — current flows out through the positive conductor and back through the negative. Resistance values come from BS EN 60228 copper tables at 25°C.
| Cross-section | Ω/km @ 25°C |
|---|---|
| 1.5 mm² | 12.10 |
| 2.5 mm² | 7.41 |
| 4 mm² | 4.61 |
| 6 mm² | 3.08 |
| 10 mm² | 1.83 |
| 16 mm² | 1.15 |
| 25 mm² | 0.727 |
Each step up roughly drops resistance by 35–40%, which is why moving from 4 mm² to 6 mm² is usually enough to fix marginal drop on UK domestic strings.
Typical UK PV cable sizes
| Run length | 5 A | 10 A | 20 A | 30 A |
|---|---|---|---|---|
| 5 m | 1.5 mm² | 2.5 mm² | 4 mm² | 6 mm² |
| 10 m | 2.5 mm² | 4 mm² | 6 mm² | 10 mm² |
| 20 m | 4 mm² | 6 mm² | 10 mm² | 16 mm² |
| 30 m | 6 mm² | 10 mm² | 16 mm² | 25 mm² |
| 50 m | 10 mm² | 16 mm² | 25 mm² | 35 mm² |
Assumes 3% max drop on a 48 V DC system. Higher string voltage cuts cable size dramatically — a 600 V string carries the same kW as a 48 V string at one-twelfth of the current, and voltage drop scales with current squared.
Voltage drop versus ampacity — pick the larger size
Two separate constraints govern UK cable choice:
- Ampacity (current-carrying capacity): the cable must safely carry the load without overheating. Set by BS 7671 Table 4D2 or equivalent, with derating for grouping, ambient temperature and conduit fill.
- Voltage drop: the cable must keep the load above its minimum operating voltage. Set by your design choice — typically the MCS 3% DC recommendation.
For long runs from a barn or garage roof, voltage drop usually wins. For short, high-current battery-bank runs, ampacity wins.
Always use the larger of the two requirements. This calculator handles voltage drop only — confirm ampacity with BS 7671 tables or have a competent person verify your design.
BS 7671 and MCS code references
- BS 7671:2018 Amendment 2 (the 18th Edition Wiring Regulations) — Appendix 4 voltage-drop limits, Table 4D2 cable ampacity
- BS EN 50618 — DC PV-rated cable (H1Z2Z2-K) specification
- MCS Standard MIS 3002 — installation requirements for grid-connected PV
- IET Code of Practice for Grid-Connected Solar PV Systems — design guidance and connection requirements
The Energy Saving Trust and Solar Energy UK both publish design guides aligned with these standards. DNOs increasingly request a written voltage-drop calculation as part of G98/G99 connection paperwork.
What it costs to get cable wrong
A 4 kWp UK domestic system installed under MCS in 2026 typically costs £6,500–£8,500 turnkey including 0% VAT under the current ZER scheme (Energy Saving Trust survey, MCS Installation Database, MyBuilder and Checkatrade quotes). Annual generation is around 3,400–3,800 kWh. A persistent 4% voltage drop above the 3% target costs roughly 40 kWh/year — about £14/year at the 2026 Ofgem price cap of 27 p/kWh. Across a 25-year warranty that’s around £350.
Compare that to the cost of upsizing 30 m of 4 mm² to 6 mm² — about £45–£70 in materials at UK trade prices. Cable upgrades almost always pay back several times over.
Related solar calculators
- Solar voltage drop calculator — full BS 7671 drop calculation
- Solar panel tilt calculator — UK roof pitch optimisation
- Solar panel orientation calculator — south versus east-west yield
- Solar charge time calculator — battery charging from PV
For grid-connected installations, only an MCS-certified installer can register your system for the Smart Export Guarantee (SEG) and submit DNO G98/G99 paperwork. Always demand a written cable-sizing calculation as part of the system design pack before commissioning.