FREE ENGINEERING TOOL

Solar String Sizing Calculator

Calculate NEC 690 maximum system voltage, conductor ampacity, and 120% busbar rule compliance instantly. Used by P.E. engineers and solar installers nationwide.

📥 Enter Module & System Data

Module Voc (Open Circuit Voltage)

Module Isc (Short Circuit Current, A)

Modules Per String

Number of Strings in Parallel

Lowest Design Temperature (°C)

Temperature Coefficient of Voc (%/°C)

Main Panel Bus Rating (A)

Main Breaker Size (A)

📊 Engineering Results

Max System Voltage (NEC 690.7)—

Total Array Isc—

NEC 690.8 Continuous Current (×1.5625)—

Minimum Conductor Ampacity—

120% Busbar Rule — Max Solar Breaker—

120% Busbar Rule—

DC System Voltage Class—

Recommended Inverter Input Voltage—

Engineering Formulas Used

V_max = Voc × Modules_per_string × [1 + (Tc × (T_min − 25°C) / 100)]
(NEC 690.7 — Maximum System Voltage)

Where Tc = temperature coefficient of Voc (%/°C), T_min = lowest expected ambient temperature.

I_continuous = Isc × Strings × 1.5625
(NEC 690.8(A) — Continuous Duty Conductor Sizing)

NEC 690.8 requires DC conductors to be sized at 125% of the maximum current, and because this is a continuous load, an additional 125% factor applies: 1.25 × 1.25 = 1.5625.

Max_Solar_Breaker = (Bus_Rating × 1.20) − Main_Breaker
(NEC 705.12(B)(2)(3) — 120% Busbar Rule)

The solar backfeed breaker may not exceed: (Panel bus rating × 120%) minus the main breaker size. If exceeded, a Main Panel Upgrade (MPU) is required.

Need a P.E. to Stamp Your String Sizing Calculations?

Our licensed Electrical P.E.s review all system calculations, prepare NEC 690-compliant single-line diagrams, and stamp plan sets for AHJ submission in 24-48 hours.

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