Calcady™

String Cold Weather Max Voltage (Voc)

Calculate cold-temperature module Voc and maximum series string voltage from the module Voc temperature coefficient and the design low temperature, using the NEC 690.7(A) manufacturer-coefficient method.
Trades & Construction
Standard: NEC 690.7(A) temperature-coefficient method

Array & Environmental Data

Why cold matters: Module open-circuit voltage rises as cell temperature falls. A string sized from STC or summer voltages can exceed the inverter's maximum DC input voltage on a cold, bright morning, which may trip the inverter offline or damage it and can void the warranty. Use the Voc coefficient from the module datasheet (not the Pmax coefficient) and the design low temperature for the site.
Calculated result for Total String Max Voltage:

Total String Max Voltage

599.5 VDC
Estimated maximum string Voc at the design low temperature.
Calculated result for Cold Adjusted Panel Voc:

Cold Adjusted Panel Voc

49.96 Volts
Per-module Voc at the design low temperature.
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Worked Example: 12-Module String at -10 degC

Calculator default inputs, NEC 690.7(A) coefficient method

Design Scenario

Twelve modules with a data-sheet Voc of 45.5 V and a Voc coefficient of -0.28% per degC are installed at a site whose design low temperature is -10 degC.

Design Parameters
Module Voc (STC)45.5 V
Voc coefficient-0.28 %/degC
Modules in series12
Design low temperature-10 degC
Mathematical Solution
1Temperature difference
-10 - 25 = -35 degC
2Voltage multiplier
1 + (-0.0028 x -35) = 1.098
3Cold module Voc
45.5 x 1.098 = 49.96 V
4Maximum string voltage
49.96 x 12 = 599.5 V
5Same string at -32 degC
45.5 x 1.1596 = 52.76 V per module; x 12 = 633.1 V
Engineering Conclusion: At -10 degC the string reaches about 599.5 V, just under a 600 V limit. If the site design low were -32 degC the same string would reach about 633.1 V, so one fewer module (11 x 52.76 = 580.4 V) would be needed for a 600 V limit. Inverter and component ratings must also be checked.

What is Cold-Weather PV String Voltage?

A PV string is open-circuited at dawn before the inverter starts, when cells are close to the coldest ambient temperature of the day. Under those conditions module voltage is at its highest. NEC 690.7 requires the maximum PV system voltage to be calculated with a temperature correction so that inverters, disconnects and conductors are rated for it.

Mathematical Foundation

Cold-corrected module open-circuit voltage
Voc,cold=Voc,STC×[1+β100×(Tlow−25)]V_{oc,cold} = V_{oc,STC} \times \left[ 1 + \frac{\beta}{100} \times (T_{low} - 25) \right]
Voc,STCV_{oc,STC}= Module open-circuit voltage at Standard Test Conditions, from the data sheet (V)
β\beta= Manufacturer Voc temperature coefficient, entered as a negative value (%/degC)
TlowT_{low}= Design low temperature (degC); the NEC informational note points to the ASHRAE extreme annual mean minimum design dry-bulb temperature
Maximum string voltage
Vstring=Voc,cold×NV_{string} = V_{oc,cold} \times N
NN= Number of modules connected in series

Laws & Principles

  • NEC 2023 690.7(A) permits the manufacturer temperature coefficient method shown here, or the generic correction factors of Table 690.7(A) for crystalline and multicrystalline silicon modules.
  • NEC 690.7 limits maximum PV system voltage to 600 V for one- and two-family dwellings and 1,000 V on or in other buildings.
  • The informational note in 690.7(A) refers to the ASHRAE extreme annual mean minimum design dry-bulb temperature as the design low temperature.
  • Local amendments and the authority having jurisdiction may specify different design temperatures or limits.

Step-by-Step Example Walkthrough

" Ten modules with Voc 49.5 V and a Voc coefficient of -0.30% per degC are planned for a site with a design low of -20 degC. "

  1. 1. Temperature difference: -20 - 25 = -45 degC.
  2. 2. Multiplier: 1 + (-0.0030 x -45) = 1.135.
  3. 3. Cold module Voc: 49.5 x 1.135 = 56.18 V.
  4. 4. String voltage: 56.18 x 10 = 561.8 V.
Final Result: About 561.8 V, below a 600 V dwelling limit. Check that the inverter maximum DC input voltage is also above this value.

Quick Answer

Multiply module Voc by 1 + (coefficient / 100) x (design low - 25), then by the number of modules in series. Twelve 45.5 V modules at -0.28% per degC and -10 degC give 49.96 V each and 599.5 V per string.

Safety and scope: PV source circuits are energized whenever modules are illuminated. This planning estimate does not replace the module and inverter manufacturer instructions, the adopted code, or review by the authority having jurisdiction; installation should be done by qualified persons.

Voltage Multiplier by Design Low Temperature and Coefficient

Multiplier = 1 + (coefficient / 100) x (T - 25). Multiply module Voc by the value to get cold Voc.

Coefficient (%/degC) 0 degC -10 degC -20 degC -30 degC -40 degC
-0.251.06251.08751.11251.13751.1625
-0.281.07001.09801.12601.15401.1820
-0.301.07501.10501.13501.16501.1950
-0.351.08751.12251.15751.19251.2275

Frequently Asked Questions

Why does PV string voltage rise in cold weather?

Silicon cell open-circuit voltage increases as cell temperature falls. Data sheets express this as a negative Voc temperature coefficient, for example -0.28% per degC, meaning Voc rises 0.28% for every degree below 25 degC.

Which low temperature should I use?

The NEC 690.7 informational note refers to the ASHRAE Handbook extreme annual mean minimum design dry-bulb temperature for the site. It is a statistical design value, not the all-time record low. Some jurisdictions specify their own design temperature, so check with the authority having jurisdiction.

What maximum voltage limits apply?

Under NEC 690.7 the maximum PV system voltage is limited to 600 V for one- and two-family dwellings and 1,000 V for PV systems on or in other buildings, with higher voltages permitted in some ground-mounted applications. The inverter and every other DC component must also be rated for the calculated string voltage.

Can I use the Pmax temperature coefficient instead?

No. The Pmax (power) coefficient is usually larger in magnitude than the Voc coefficient and describes power loss with heat, not open-circuit voltage. Using it would overstate the cold-weather voltage. Use the Voc coefficient from the module data sheet.

How is this different from the PV string sizing calculators?

This page calculates the cold-weather voltage for a string length you choose. The PV String Sizing calculators work the other way, starting from an inverter voltage limit to find the maximum number of modules in series.

Related Calculations

Calculation Provenance & Validation Record

Method

Applies the manufacturer Voc temperature coefficient to correct module open-circuit voltage from 25 degC to the design low temperature, then multiplies by the number of series modules.

Formula
V_{string} = N imes V_{oc,STC} imes left[1 + rac{eta}{100}(T_{low} - 25) ight]
Assumptions
  • Uses the manufacturer Voc temperature coefficient method of NEC 2023 690.7(A); cell temperature is taken equal to the design low ambient temperature, which is the usual open-circuit assumption at dawn.
  • Coefficient must be the Voc coefficient (not the Pmax coefficient) and must be negative.
  • Design low temperature must be below 25 degC; the calculator rejects warmer values.
  • Irradiance enhancement, bifacial gain and optimizer or microinverter behavior are not modeled; follow the equipment manufacturer instructions for those systems.
References
  • NFPA 70, National Electrical Code (NEC 2023) 690.7(A) - maximum PV system DC voltage and temperature correction
  • NEC 2023 Table 690.7(A) - generic voltage correction factors for crystalline and multicrystalline silicon modules
  • ASHRAE Handbook - Fundamentals, climatic design data (extreme annual mean minimum design dry-bulb temperature)
Last substantive review:
Automated test status: 3 golden test vectors passing (SCV-01, SCV-02, SCV-03)
Method, assumptions & governing standards
Standard Sourced

Calculation Methodology

Applies the manufacturer Voc temperature coefficient to correct module open-circuit voltage from 25 degC to the design low temperature, then multiplies by the number of series modules.

Governing Standard 2023

Standard:NFPA 70 (NEC) 690.7(A)

Statutory building, electrical, and mechanical codes vary by jurisdiction. Confirm local municipality amendments before installation.

Key Assumptions & Constraints

  • Uses the manufacturer Voc temperature coefficient method of NEC 2023 690.7(A); cell temperature is taken equal to the design low ambient temperature, which is the usual open-circuit assumption at dawn.
  • Coefficient must be the Voc coefficient (not the Pmax coefficient) and must be negative.
  • Design low temperature must be below 25 degC; the calculator rejects warmer values.
  • Irradiance enhancement, bifacial gain and optimizer or microinverter behavior are not modeled; follow the equipment manufacturer instructions for those systems.
Regression Tests: 3 golden vectors
Last Verified:
Primary References: 3 documented
Field Trade Notice: For trade planning and engineering estimates. Final installations must conform to project blueprints, authority having jurisdiction (AHJ) code approvals, and site-specific inspections.