1. The voltage-drop formula
Every metre of cable has resistance, and the current through it turns part of the solar power into heat. The drop follows one relationship: Delta U = 2 x L x I x rho(T) / S, where L is the one-way route length, I is the operating current, S is the cross-section in square millimetres, and rho is conductor resistivity, which grows with temperature.
2. Why temperature belongs in the calculation
A rooftop conductor at 70 C conducts about 20 % worse than the 20 C textbook value. A proper calculator asks about temperature while many silently assume a cold wire, which under-sizes the cable for summer peaks.
3. Worked example
Take a string of 12 modules at 510 V and 13.8 A with a 25 m route to the combiner or inverter. On 2.5 mm2 copper at 85 C the drop is about 1.19 %, above a 1 % target. On 4 mm2 it falls to 0.74 %, roughly 52 W at full power. Over a year that difference is about 20 kWh.
4. The two checks that must both pass
The loss check picks a cross-section under the voltage-drop target; the safety check confirms the same cable carries 1.25x Isc on the hottest day without exceeding its ampacity. A cable that passes losses but fails ampacity is unsafe.
Technical diagram shown at a readable responsive scale.
5. Ampacity and installation method
IEC 60364-5-52 defines correction factors for bundling, conduit and ambient temperature. A wire bundled in a conduit on a hot roof safely carries 30 to 40 % less current than in free air, so the installation method is part of the answer, not a footnote.
6. When thicker is not better
Once the drop is well under 1 %, extra copper buys almost nothing. The money is usually better spent raising the string voltage, which cuts losses quadratically for free. Cable sizing is a balance, not a max-out.
This is general technical guidance. Confirm final ratings, installation conditions and applicable local requirements against approved project documentation and a qualified engineering review.
Frequently asked questions
What size cable do I need for PV DC?
It depends on route length, string voltage and current; a 4 or 6 mm2 solar cable covers most residential strings, but use the formula with your exact panel and run length.
Is 1 % voltage drop required on the DC side?
It is a widely recommended design target, not a legal limit. Each percent of drop is energy paid for and never received; 2 to 3 % is safe and sometimes reasonable on a tight budget.
How do I calculate PV voltage drop?
Use Delta U = 2 x L x I x rho(T) / S with the one-way length, operating current, temperature-adjusted resistivity and cross-section; the factor 2 accounts for the return path.
Does a thicker cable always pay off?
No. Below a 1 % drop, extra copper adds little; raising string voltage reduces losses more efficiently.
Bring the electrical inputs to the first review.
Share the approved module data, site temperature range, cable route, string configuration and destination-market requirements. NEUTRON can review the equipment envelope against the project inputs.
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Technical note: verify final ratings, standards, documents and configuration against approved project documentation and the applicable local requirements.