Solar PV String & MPPT Sizing Calculator

Calculate temperature-corrected max Voc, min/max modules per string, inverter MPPT window, and 156% DC wire sizing per NEC §690.7 & §690.8.

NEC edition

Inputs

Module Open-Circuit Voltage (Voc @ STC)

Module Max Power Voltage (Vmp @ STC)

Module Short-Circuit Current (Isc @ STC)

Voc Temperature Coefficient (% / °C)

%/°C

Lowest Design Ambient Temperature (°C)

°C

Inverter Max DC Input Voltage (Limit)

Inverter Min MPPT Operating Voltage

Inverter Max MPPT Operating Voltage

Result

Calculated result

6 – 14 Panels / String (41.7V Cold Voc)

⚠️ ✓ Safe Stringing: A series string of 14 panels will peak at 583.8V during winter, safely below the 600V limit.

Code references

NEC §690.7(A) Maximum Photovoltaic Voltage — Temperature Calculation
NEC §690.7(C) Photovoltaic Source and Output Circuits on Dwellings — 600V Cap
NEC §690.8(A) Circuit Sizing and Current — 125% Maximum Current
NEC §690.8(B) Conductor Ampacity and Overcurrent Device Sizing — 125% Continuous

Module Voc @ STC (25°C) 37.50 Volts (Coeff: -0.28%/°C) Module Datasheet

Cold-Weather Max Voc 41.70 Volts @ -15°C (111.2% of STC) NEC §690.7(A)

Max Modules per String 14 Panels (583.8V max ≤ 600V limit) Inverter Max V ÷ Cold Voc

Min Modules for MPPT Startup 6 Panels (168.9V @ 60°C cell temp ≥ 150V) Min MPPT ÷ Hot Vmp

Recommended String Window 6 to 14 Modules in Series MPPT Operating Window

PV Conductor Ampacity (156%) 21.56 Amps (13.8A Isc × 1.5625) NEC §690.8(A) & (B)

Recommended DC Fuse / Breaker 25A DC Rating NEC §690.9

⚡ Test your knowledge

NEC Article 220 was renumbered in the 2026 code cycle. What is the new article number?

A. Art. 100
B. Art. 120
C. Art. 200
D. Art. 230

💡 NEC 2026 renumbered Article 220 (Branch-Circuit, Feeder, and Service Calculations) to Article 120.

Energy Management Systems were moved from Article 750 to which article in NEC 2026?

A. Art. 100
B. Art. 125
C. Art. 130
D. Art. 150

💡 NEC 2026 moved Energy Management Systems from Article 750 to Article 130.

Which NEC standard size overcurrent device comes after 90A?

A. 95A
B. 100A
C. 110A
D. 105A

💡 Per NEC §240.6(A), the standard sizes are …90, 100, 110, 125… There is no 95A standard size.

How it works & the NEC rules behind it

Solar PV modules produce higher voltage in cold temperatures. Under NEC §690.7, the maximum photovoltaic system voltage must be calculated using the lowest expected ambient temperature to ensure system voltage never exceeds equipment ratings (e.g., 600V for residential dwellings per §690.7(C), or 1000V/1500V for commercial installations).

Key PV String Sizing Rules:

1. Cold-Weather Maximum Voc (§690.7(A)): Calculated using manufacturer temperature coefficients:
Max Voc = Voc(STC) × [1 + (Temp Coeff (%/°C) × (Lowest Temp °C − 25°C))].
Alternatively, Table 690.7(A) correction factors can be used.
2. Maximum Panels per String: Floor(Inverter Max DC Voltage ÷ Max Module Voc). Ensures the string never exceeds the inverter maximum voltage limit or 600V residential code cap.
3. Minimum Panels per String (MPPT Tracking): Sized at high summer temperatures (e.g. +40°C to +65°C cell temp): Ceil(Inverter Min MPPT Voltage ÷ High-Temp Vmp).
4. DC Conductor & OCPD Sizing (§690.8): PV source circuits are subject to two 125% factors (125% for maximum current irradiance boost × 125% continuous duty = 156.25% of Isc):
Min Conductor Ampacity = Module Isc × 1.5625.

Formulas & equations

Cold-Weather Max Voc = Voc_STC × [1 + (α_Voc × (T_min − 25°C))] (§690.7(A)) Max Modules / String = Floor(Max Inverter Voltage ÷ Cold-Weather Max Voc) Min Modules / String = Ceil(Inverter Min MPPT Voltage ÷ Hot-Weather Vmp) DC Conductor Ampacity = Module Isc × 1.25 × 1.25 = Isc × 1.56 (§690.8)

NEC reference table

Lowest Expected Ambient Temp	NEC Table 690.7(A) Multiplier	Typical 400W Module Voc (STC 37.2V)	Max Cold Voc
20°C to 24°C (68°F to 76°F)	1.02	37.2 V	37.9 V
10°C to 14°C (50°F to 58°F)	1.06	37.2 V	39.4 V
0°C to 4°C (32°F to 40°F)	1.10	37.2 V	40.9 V
−10°C to −6°C (14°F to 22°F)	1.14	37.2 V	42.4 V
−20°C to −16°C (−4°F to 4°F)	1.18	37.2 V	43.9 V
−30°C to −26°C (−22°F to −14°F)	1.23	37.2 V	45.8 V

Worked example, step by step

Example: Sizing 400W Solar Panels on a 600V Inverter (Winter Min −15°C)

Module: 400W (Voc = 37.2V, Vmp = 31.0V, Isc = 13.8A, Temp Coeff = −0.28%/°C). Inverter: 600V Max DC, MPPT 150V–480V.

1. Cold Temp Delta: −15°C − 25°C = −40°C.
2. Max Cold Voc: 37.2V × [1 + (−0.0028 × −40)] = 37.2V × 1.112 = 41.37 Volts.
3. Max Modules / String: 600V ÷ 41.37V = 14.5 → 14 Modules Maximum (14 × 41.37V = 579.2V < 600V).
4. Min Modules (Hot 55°C): Hot Vmp = 31.0V × 0.90 = 27.9V. Min modules = 150V ÷ 27.9V = 5.37 → 6 Modules Minimum.
5. Conductor Ampacity (156%): 13.8A × 1.5625 = 21.56 Amps → #10 AWG Copper PV Wire (30A+).

Frequently asked questions

Under NEC §690.7(C), photovoltaic source and output circuits installed in or on one- and two-family dwellings are strictly limited to a maximum voltage of 600 Volts between conductors to mitigate shock and arcing risks for first responders and occupants.

Under NEC §690.8(A)(1), the maximum current is 125% of module Isc to account for sunlight reflections (edge-of-cloud enhancement). Then §690.8(B) requires an additional 125% factor because solar power is continuous (operating 3+ hours), resulting in 1.25 × 1.25 = 1.5625 × Isc.

Inputs

Take this toolkit with you on the job

Studying for your electrical exam?

How it works & the NEC rules behind it

Key PV String Sizing Rules:

Formulas & equations

NEC reference table

Worked example, step by step

Frequently asked questions

Why is the residential solar PV voltage limit capped at 600 Volts? −

Why does PV conductor sizing require two 125% factors (156.25%)? +

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