Solar String Sizing Calculator — NEC 690 Compliant

Calculator Mode

Solar Panel Specifications From panel datasheet

Quick Load:

Voc (Open Circuit) i

Vmp (Max Power Voltage) i

Isc (Short Circuit Current) i

Pmax (Panel Power) i

αVoc (Voc Temp. Coeff.) i

%/°C

αVmp (Vmp Temp. Coeff.) i

%/°C

NEC Note: αVoc and αVmp are two different coefficients — do not use the same value for both. Both are on your panel datasheet under "Temperature Characteristics."

Site Conditions

Temperature Unit:

Minimum Design Temp. i

°C

Use record low, not average low

Maximum Ambient Temp. i

°C

Mounting Type i

Inverter Specifications

Quick Load:

Max DC Input Voltage i

MPPT Minimum Voltage i

MPPT Maximum Voltage i

Max DC Input Current i

Number of MPPT Inputs i

MPPTs

Verify Your Proposed Design

Proposed Panels/String

panels

Proposed Parallel Strings

strings

NEC 690 Compliance Checks

4 Required Compliance Checks

— Max String Voltage ≤ Inverter Max DC —

— Min String Voltage ≥ MPPT Minimum —

— String Vmp ≤ MPPT Maximum —

— Parallel String Current ≤ Inverter Max DC —

NEC 690.9(A) Alert: 3 or more strings in parallel require individual string fuses. Confirm your combiner box or inverter input has appropriate DC-rated overcurrent protection per NEC 690.9(A).

Min Panels/String

—

Vmp at hottest day

Max Panels/String

—

Voc at coldest day

Max Parallel Strings

—

Per MPPT input

String Power

—

at optimal length

Recommended String Length

— panels per string

Targeting 90% of MPPT max for optimal energy harvest

Temperature-Corrected Voltages

Parameter	Method 1	Method 2	Status
Voc at T_min	—	—	—
Max String V (at — panels)	—	—	—
Vmp at T_cell_max	—	N/A	—
Min String V (at — panels)	—	N/A	—
String Isc × 1.25	—		—

Method 1 NEC 690.7(A)(1) — Coefficient formula (preferred) Method 2 NEC Table 690.7(A) — Table lookup (fallback)

Run a calculation to see steps.

Disclaimer: This calculator performs calculations per NEC Article 690 (2023 Edition). Always verify results with the applicable local code edition and your AHJ. Final design responsibility rests with the licensed professional of record. This tool does not replace professional engineering judgment.

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