Why size for 125 percent of Isc?

NEC 690.8 treats PV output as a continuous current and adds a 125 percent factor for ampacity, and a second 125 percent for terminal temperature derating, giving 156 percent overall in many cases. This tool applies the first 1.25 design factor so your conductor and overcurrent device are sized for sustained high irradiance.

How is voltage drop calculated?

Voltage drop equals current times the round-trip resistance, which is two times the one-way length times the conductor's resistance per metre. Resistance per metre is the material resistivity divided by the cross-sectional area. The tool reports the drop as a percentage of operating voltage.

What voltage drop is acceptable?

Industry practice keeps DC string drop under about 2 to 3 percent so the array delivers near its rated power. Higher drops waste energy as heat in the cable and can stop an MPPT controller from reaching the battery charge voltage, especially on long runs.

Why does copper beat aluminium?

Copper has lower resistivity, about 0.0172 ohm-mm per metre versus 0.0282 for aluminium, so for the same cross-section copper drops less voltage and carries more current. Aluminium is cheaper and lighter but needs a larger cross-section for the same performance.

Does this replace an ampacity table?

No. Always confirm the chosen conductor's rated ampacity against the NEC table for its insulation type and installation conditions, including temperature and conduit fill derating. This tool gives the design current and voltage drop, not the full table lookup.

Solar DC Cable Sizing Calculator (NEC 690.8)

DC string cables in a solar array must carry the array’s continuous current safely and deliver power without wasteful voltage drop. This calculator applies the NEC 690.8 design factor and computes round-trip voltage drop for copper or aluminium conductors of a chosen size.

How it works

NEC 690.8 sizes PV conductors at 125 percent of short-circuit current for continuous duty. Voltage drop is the operating current times the round-trip resistance of the run:

Design current = Isc × 1.25
R per metre = resistivity ÷ area(mm²)
Vdrop = I × (2 × length × R per metre)
Vdrop % = Vdrop ÷ operating voltage × 100

Resistivity is about 0.0172 Ω·mm²/m for copper and 0.0282 for aluminium.

Example

A 10 A string on 6 mm² copper over 15 m drops 10 × (2 × 15 × 0.0172 / 6) ≈ 0.86 V, about 2.4 percent on a 36 V string — within the usual 2 to 3 percent target.

Tips

Keep DC string voltage drop under 2 to 3 percent.
Confirm the conductor’s rated ampacity in the NEC table for its insulation.
Long runs favour a larger cross-section or a higher string voltage.