Solar Cable Sizing and Voltage Drop: DC and AC Guide
Size DC string and AC cables for rooftop solar in India: ampacity, derating, voltage drop formulas, 1.5% targets, worked 5 kW examples and standards.
Short answer: size every cable for two things: it must carry the current safely after derating for rooftop heat and grouping, and it must keep voltage drop within target, typically about 1.5% DC (at hot Vmp) and about 1.5% AC (inverter to meter). Use 4 sq mm solar DC cable for most residential strings and upsize for long runs; size AC cable for 1.25 × inverter current. Example: a 5 kW single-phase inverter 30 m from the meter needs 10 sq mm copper, not 6, to stay near 1.3% drop.
The formulas
DC and single-phase AC: Vd (volts) = 2 × L × I × ρ ÷ A
Three-phase AC (line-to-line): Vd (volts) = √3 × L × I × ρ ÷ A
Percentage drop = Vd ÷ system voltage × 100
Where: - L = one-way route length (m) - I = operating current (A): Imp for DC strings, inverter rated AC current for AC - ρ = resistivity at conductor operating temperature: copper about 0.0214–0.0225 Ω·mm²/m, aluminium about 0.036 Ω·mm²/m (warm conductor) - A = cross-section (mm²)
DC string cable
Standard: 1.5 kV DC solar cable, tinned copper, double-insulated, UV and ozone resistant, to EN 50618 / IEC 62930 / IS 17293. Never use ordinary PVC house wire on the DC side.
Current: string Isc × 1.25 (and × 1.25 again where the code requires), with derating for 60–70°C rooftop conduit temperature and grouping. Studio's derated ampacity values are about 32 A for 4 sq mm, 41 A for 6 sq mm and 57 A for 10 sq mm.
Voltage drop at hot Vmp: use the string's Vmp at about 70°C cell temperature, because that is when voltage is lowest and the percentage drop highest.
Worked DC example
String of 10 × 550 Wp (Imp 13.1 A; hot Vmp ≈ 354 V), one-way run 25 m on 4 sq mm copper:
- Vd = 2 × 25 × 13.1 × 0.0214 ÷ 4 ≈ 3.5 V
- % = 3.5 ÷ 354 ≈ 0.99% ✓
At 45 m one-way, the same string would drop about 1.8%; move to 6 sq mm (about 1.2%).
| Situation | Typical DC size |
|---|---|
| Single string, run up to about 30–35 m | 4 sq mm |
| Long run or high-current (210 mm cell) modules | 6 sq mm |
| Paralleled strings to a combiner, long trunk | 10 sq mm or more (check) |
Cable lengths: measure roof run + vertical drop + run to the inverter for both polarities, and add about 20% for routing and slack.
AC cable
Current: design current = 1.25 × inverter rated AC current. The breaker must protect the cable: breaker rating ≤ cable's derated ampacity.
Voltage drop: from inverter through the ACDB to the net meter, target about 1.5%. On weak Indian feeders, lower drop also reduces over-voltage trips, because the inverter sees grid voltage plus its own cable rise.
Worked AC example: 5 kW single-phase
- Rated current: 5,000 ÷ 230 ≈ 21.7 A; design current 1.25 × 21.7 ≈ 27.2 A
- Ampacity check: 6 sq mm copper (about 34 A derated) passes
- Voltage drop over 30 m on 6 sq mm: 2 × 30 × 21.7 × 0.0225 ÷ 6 ≈ 4.9 V = 2.1% ✗
- On 10 sq mm: ≈ 2.9 V = 1.3% ✓
So voltage drop, not ampacity, sets the size on longer runs.
Three-phase example: 10 kW
- Current per phase ≈ 10,000 ÷ (√3 × 415) ≈ 13.9 A; design 17.4 A
- 40 m on 4 sq mm copper: √3 × 40 × 13.9 × 0.0225 ÷ 4 ≈ 5.4 V = 1.3% of 415 V ✓
Aluminium for larger AC runs
At 16 sq mm and above, armoured aluminium XLPE (IS 7098) is usually cheaper and accepted by most DISCOMs. Use: - Copper-aluminium bimetallic lugs at copper terminals - Correct crimp tools and dies - Torque marks and periodic checks
Battery cables (hybrids)
48 V battery currents are high (a 5 kW inverter can draw about 110 A or more). Keep runs short (within about 3 m), use flexible copper sized so that the cable's ampacity is at least the DC breaker rating, and use proper lugs. Studio picks 25–70 sq mm by breaker size.
Standards and good practice
- DC solar cable: EN 50618 / IEC 62930 / IS 17293
- AC: IS 694 (PVC), IS 7098 (XLPE)
- Wiring practice: IS 732; PV array design: IEC 62548
- Route DC cables in UV-resistant conduit or covered tray, away from lightning down conductors
- Label every string pair (S1+, S1−) with ferrules so DCDB and SLD match
- Use the same make of MC4 on both halves of a joint; no cross-mating
Try your own numbers:
Common cable mistakes on Indian rooftops
- House wire on the DC side. Ordinary PVC wire is not UV-stable or rated for 1 kV+ DC.
- Cable lying on the slab in sun and standing water. Use conduit or covered tray, raised off the roof.
- Cross-mated MC4 connectors from different makers, which overheat over time.
- Sizing for ampacity only and ignoring voltage drop on long AC runs to the meter.
- Aluminium straight into copper terminals without bimetallic lugs.
- No ferrules or labels, which makes fault-finding slow and fails inspections.
Documenting your cable design
Write each cable's size, length, current and calculated drop into the SLD or a cable schedule. Inspectors and future service technicians can then check the design quickly, and you can defend the extra cost of a larger size to the customer.
Talk to Sunlectric
We stock 4, 6 and 10 sq mm solar DC cable, copper and aluminium armoured AC cable, MC4 connectors, lugs, glands and conduit at our Pune hub. WhatsApp +91 89567 89307 (wa.me/918956789307) with your run lengths for a cable list. Sunlectric Studio sizes DC, AC and battery cables automatically, and the Academy electrical module has practice problems. Related: string sizing, commissioning checklist.
Sources
- Cable resistivity, derated ampacity, drop limits and current factors: Sunlectric Studio bom_rules, Oct 2026 (verify against the cable maker's data at your installation conditions)
- Standards referenced: EN 50618; IEC 62930; IS 17293:2020 (PV cables); IS 694; IS 7098; IS 732; IEC 62548 (check current editions)
Questions people ask
What size DC cable is used for rooftop solar strings?
4 sq mm solar cable for most single-string residential runs; 6 or 10 sq mm when the run is long, the drop exceeds the target, or the cable carries paralleled strings. Use 1.5 kV solar cable to EN 50618, IEC 62930 or IS 17293.
What is an acceptable voltage drop for solar?
A common design target is about 1.5% on the DC side at operating voltage and about 1.5% on the AC side from inverter to the meter. Some specifications allow up to 2–3% total; follow the project or DISCOM specification.
How do I calculate voltage drop?
For DC and single-phase AC: Vd = 2 × L × I × ρ ÷ A, where L is the one-way length in metres, I the current, ρ the resistivity (about 0.0214–0.0225 Ω·mm²/m for hot copper) and A the cross-section in mm². For three-phase, use √3 instead of 2.
Can I use aluminium cable on the AC side?
Yes, for larger sizes, typically 16 sq mm and above, as armoured XLPE. Use copper-aluminium bimetallic lugs at copper terminals.