Undersized DC cables cause voltage drop, power loss and fire hazards. Oversized cables waste money on copper. The goal of cable sizing is to find the minimum cross-section that safely carries the design current with acceptable voltage drop and temperature rise throughout the cable's 25-year life. This guide covers the engineering steps for sizing DC cables from PV strings to the inverter, following IEC 60364 and EN 50618 practice.
Step 1: Determine the design current
For a PV string cable, the maximum operating current is the module's short-circuit current (Isc) at STC. The cable must carry this without exceeding its rated current under the worst operating conditions.
The design current for DC string cables is:
Idesign = 1.25 × Isc(STC)
The 1.25 factor accounts for irradiance conditions above 1,000 W/m² (which occur briefly during cloud edge enhancement events) and provides a safety margin per IEC 60364-7-712.
For a typical 400 Wp module with Isc = 10.0 A: Idesign = 12.5 A per string.
For a combiner output cable serving N strings in parallel: Idesign = 1.25 × Isc × N.
Step 2: Apply temperature derating
Cable current ratings are specified at a reference ambient temperature (typically 30°C for buried cables, 40°C for conduit/free air). At higher temperatures, the cable must be derated:
| Ambient temperature | Derating factor (Cu cable, XLPE insulation) |
|---|---|
| 30°C (reference) | 1.00 |
| 40°C | 0.87 |
| 50°C | 0.71 |
| 60°C | 0.50 |
| 70°C | 0.00 (cable not suitable above 70°C for standard XLPE) |
In hot climates where roof surfaces reach 60–70°C, cables routed along the underside of panels must use cables with higher temperature ratings (120°C or 150°C rated PV cables — EN 50618 certified) rather than standard 90°C XLPE. Always specify the installation temperature, not the ambient air temperature, when sizing cables in hot climates.
Step 3: Apply installation method derating
Cables in conduit or bundled together run hotter than single cables in free air. The installation method derating factor:
| Installation method | Derating factor |
|---|---|
| Single cable, free air | 1.00 |
| 2 cables touching, free air | 0.87 |
| 3 cables in flat formation, free air | 0.79 |
| 3 cables in conduit (not more than 2 circuits) | 0.70 |
| Cables in surface-mounted trunking | 0.75 |
For the minimum required cable rating: Irated(min) = Idesign ÷ (temp derating × installation derating)
Example: 12.5 A design current, 50°C ambient, in conduit:
Irated(min) = 12.5 ÷ (0.71 × 0.70) = 12.5 ÷ 0.497 = 25.2 A minimum cable rating
From cable tables, a 4 mm² copper PV cable (EN 50618) has a free-air rating of approximately 50 A, which after derating gives 50 × 0.497 = 24.8 A — marginally insufficient. Specify 6 mm² (rated ~65 A, derated to 32.3 A) — adequate with margin.
Step 4: Check voltage drop
Voltage drop in DC string cables reduces the inverter's MPPT operating range and causes resistive power losses. The maximum acceptable voltage drop for string cables is typically:
- 1% of Vmpp at STC — for high-quality installations
- 2% of Vmpp at STC — maximum acceptable (IEC 60364 guidance)
Voltage drop formula: ΔV = 2 × L × I × ρ / A
Where L = one-way cable length (m), I = design current (A), ρ = resistivity (0.0175 Ω·mm²/m for copper), A = cable cross-section (mm²). The factor of 2 accounts for both positive and negative conductors.
For a 100 m string run (one-way), 12.5 A, 4 mm² cable:
ΔV = 2 × 100 × 12.5 × 0.0175 / 4 = 10.9 V
If Vmpp per string = 30 modules × 32 V = 960 V, then 10.9/960 = 1.14% — acceptable. For a 200 m run, voltage drop doubles to 2.3% — exceeds the 2% limit. Upsize to 6 mm² or reduce string length.
Step 5: Select the cable
PV DC cables must be certified to EN 50618 (or equivalent IEC 62930) for outdoor photovoltaic applications. Key certifications:
- UV resistance: PV cables must withstand 25+ years of UV exposure without cracking
- Temperature rating: Standard EN 50618 cables rated 90°C continuous. Specify 120°C or 150°C for hot climate rooftop installations
- System voltage: Must be rated for the system voltage — 1,000 V or 1,500 V
- Halogen-free: Required by most standards for installations where toxic fume emission in fire is a concern
Common cross-sections used in solar: 4 mm² (residential, short C&I strings), 6 mm² (most C&I string cables), 10 mm² (combiner outputs, longer runs), 16 mm² or 25 mm² (main DC trunk cables from combiners to inverter).