Inverters & String Math

DC Cable Voltage Drop & Wire Sizing Rule

Core Question: What is the maximum permissible voltage drop for DC solar home runs, and how is cable cross-section calculated?

Governing Physics Formula / Design Rule
Voltage_Drop_% = (2 * Length * Current * Resistivity) / (Area * Voltage) <= 1.5% (Max 2.0%)

Executive Technical Summary

Keep DC voltage drop from solar array to inverter under 1.5% at maximum operating current (Imp). High resistance in undersized cables dissipates valuable solar energy as heat.

Detailed First-Principles Explanation

Solar systems operate at peak current for thousands of hours over 25+ years. Copper resistivity rho = 0.0175 ohm*mm²/m. For a 30-meter 15A string at 400V DC, a 4mm² wire has a 1.97% drop (118W continuous heat loss), while upgrading to 6mm² cuts loss to 1.31%, paying for the copper in under 18 months.

Critical Field Pitfall & Safety Warning

Do not size DC solar cables based only on thermal ampacity (fire safety); always calculate 25-year cumulative kWh resistance loss economics.

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