PV Cable Sizing and Voltage Drop: 4mm2 vs 6mm2 vs 10mm2

Why cable size matters

PV DC cable connects modules, strings, combiner boxes and the inverter. Undersized cable overheats, wastes energy and becomes a fire risk; oversized cable adds unnecessary cost and handling weight. Sizing is a balance between current capacity, voltage drop and mechanical practicality.

Current rating first

Ampacity is the starting point. A single H1Z2Z2-K cable laid on a surface at 40C ambient (IEC 60364-7-712) carries roughly 26A at 2.5mm2, 32A at 4mm2, 40A at 6mm2, 54A at 10mm2, 70A at 16mm2 and 92A at 25mm2.

• Derate by 0.82 for buried installation.
• Derate by 0.75 for bundled runs of four or more cables.
• Add the 1.25 x Isc safety factor on the string current before comparing to the rating.

Voltage drop is usually the deciding factor

Long runs are limited by voltage drop, not ampacity. The single-conductor-pair formula is dV = (2 x L x I x rho) / A. Keeping string voltage drop within 1% of system voltage is the IEC 62548 recommendation.

Example: a 1000V system, 13A string, 80m one-way run. On 4mm2, dV = (2 x 80 x 13 x 0.0215) / 4 = 11.2V, which is 1.12% and outside the limit. On 6mm2 the drop is 7.5V, 0.75%, so 6mm2 becomes the correct choice for that run.

Length and system voltage interplay

The same cable run looks different at different system voltages because the 1% budget scales with voltage. A 1500V system tolerates a much larger absolute drop than a 600V residential system, so longer strings and longer runs are practical at the higher class. Always size against the actual system voltage, not a generic rule.

H1Z2Z2-K versus PV1-F

H1Z2Z2-K (EN 50618 / IEC 62930) is the current generation solar cable with XLPE insulation and a halogen-free sheath, rated for 90C conductor temperature. PV1-F is the superseded HD 605 type with a PVC sheath and 70C rating. New projects should specify H1Z2Z2-K.

String cable versus combiner output cable

String cable typically runs at 2.5-6mm2 depending on current and length. Combiner box output runs aggregate many strings and carry higher current, so 10mm2, 16mm2 and 25mm2 are common. Check the continuous current of the aggregated feed and the terminal capacity of the equipment at both ends before selecting.

Mechanical and environmental checks

• UV resistance for exposed outdoor runs.
• Suitable cable glands and entry seals into combiner boxes and enclosures.
• Bend radius and pulling tension limits respected in tray and conduit.
• Compatibility of insulation with the operating temperature at the site.

RFQ checklist for cable

When requesting cable quotations, state the standard (EN 50618 or IEC 62930), cross-section, conductor material, colour, quantity in metres, and the derating assumptions used. The supplier should confirm ampacity and the exact resistivity used in any voltage drop calculation.