A cold snap hits and your inverter logs show voltage spikes above its rated maximum. The warranty claim gets denied. This scenario plays out when installers skip proper NEC 690.7 voltage calculations.
NEC 690.7 establishes maximum voltage limits for solar PV systems: 600V for residential one and two family dwellings, 1000V for commercial and multifamily buildings, and up to 1500V for ground mounted utility scale systems complying with 690.31(G). To calculate maximum system voltage, multiply the sum of series connected module open circuit voltages (Voc) by a temperature correction factor from Table 690.7(A) based on the lowest expected ambient temperature, or use manufacturer provided temperature coefficients. This calculation determines conductor sizing, equipment ratings, working space requirements, and string sizing limits for code compliant solar installations.
Quick Reference: NEC 690.7 Voltage Limits
- Residential (1-2 family): 600V max
- Commercial/Multifamily: 1000V max
- Ground-mount utility: 1500V max (per 690.31(G))
- Formula: Max Voltage = ΣVoc × Temperature Correction Factor
Understanding The Scope Of NEC 690.7
NEC 690.7 defines maximum voltage as the highest voltage between any two conductors of a circuit or any conductor and ground. Installers use this value to select conductors, cables, and equipment, determine working space requirements, and verify circuit voltage ratings. Related installation requirements appear throughout Article 690.
Maximum PV system DC circuit voltage must comply with specific limitations based on building type and installation location.
Voltage Limits By Building Type
NEC 690.7 sets clear voltage restrictions based on building type:
One and Two Family Dwellings: DC circuits cannot exceed 600 volts. This lower threshold adds safety for residential solar installations where homeowners may be present.
Commercial, Industrial, and Multifamily Buildings: DC circuits cannot exceed 1000 volts within or originating from building attached arrays. Commercial solar projects must verify all equipment meets this threshold.
Systems Exceeding 1000 Volts: Circuits above 1000 volts must comply with 690.31(G), which covers higher voltage systems between 1000 and 1500 Vdc.
Methods For Calculating Maximum PV System Voltage
The NEC provides three methods for calculating maximum DC circuit voltage. Each method corrects the sum of series connected module Voc values for the lowest expected ambient temperature. The Denver example that follows demonstrates a complete calculation. Detailed voltage calculations follow similar principles across all methods.
Method 1: Manufacturer Instructions
Add the open circuit voltage (Voc) of all series connected modules, then apply the manufacturer’s temperature coefficient to correct for the coldest expected temperature. When manufacturers provide Voc temperature coefficients, installers must use these values per NEC 110.3(B).
Method 2: Table 690.7(A) Correction Factors
For crystalline and multicrystalline silicon modules, multiply the rated Voc by the correction factor from Table 690.7(A). The table covers ambient temperatures below 25°C (77°F):
For temperatures below -40°C (-40°F) or non-crystalline modules, follow manufacturer instructions instead.
Method 3: Engineered Design For Large Systems
Systems 100 kW or greater may use a documented, stamped design from a licensed professional electrical engineer using an industry standard calculation method.
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Practical Application Example
Consider a commercial PV system design in Denver with the following specifications:
Module Specifications:
- Module Voc: 45.5V
- Module Vmp: 37.2V
- Temperature Coefficient of Voc: −0.33%/°C
- Modules per string: To be determined
Inverter Specifications:
- Maximum DC Input Voltage: 1000V
- MPPT Voltage Range: 200V to 950V
Site Temperature Data (from ASHRAE):
- Lowest Expected Temperature: −23°C (−9°F)
- Highest Expected Temperature: 35°C (95°F)
Step 1: Calculate Maximum Module Voltage At Coldest Temperature
Voc (at −23°C) = Voc × (1 + Temperature Coefficient × (−23°C − 25°C))
Voc (at −23°C) = 45.5V × (1 + (−0.0033) × (−48))
Voc (at −23°C) = 45.5V × (1 + 0.1584)
Voc (at −23°C) = 45.5V × 1.1584 = 52.7V
Step 2: Calculate Maximum String Size
Maximum Modules = Inverter Max Voltage ÷ Module Voc (cold)
Maximum Modules = 1000V ÷ 52.7V = 18.98
Round down to 18 modules maximum per string.
Step 3: Verify String Voltage At Cold Temperature
String Voltage (cold) = 18 modules × 52.7V = 948.6V
This value falls below the 1000V inverter limit and within the 950V MPPT range upper limit.
Step 4: Calculate Minimum String Size For Hot Conditions
Vmp (at 35°C with 30°C temperature rise) = 37.2V × (1 + (−0.0033) × (35 + 30 − 25))
Vmp (at 65°C effective) = 37.2V × (1 + (−0.0033) × 40)
Vmp (at 65°C effective) = 37.2V × (1 − 0.132) = 32.3V
Minimum Modules = Inverter Min MPPT Voltage ÷ Module Vmp (hot)
Minimum Modules = 200V ÷ 32.3V = 6.19
Round up to 7 modules minimum per string.
Design Conclusion
This system can safely operate with string sizes between 7 and 18 modules. A designer might select 15 modules per string to optimize for the inverter’s best MPPT efficiency range while maintaining adequate safety margins on both voltage limits. Professional review can help with complex projects.
DC To DC Converter Circuit Voltage
NEC 690.7(B) covers voltage calculations for DC to DC converter circuits:
Single Converter: Use the rated output DC voltage of the converter (optimizer).
Multiple Converters: Account for combined output of series connected converters per manufacturer specifications.
Why Temperature Correction Matters
PV module voltage has an inverse relationship with temperature. When temperatures drop, module voltage increases. Cold morning conditions with low irradiance can produce the highest voltages in a PV system. Proper temperature correction ensures that:
- Conductors and equipment have adequate voltage ratings
- The system remains within safe operating limits under all weather conditions
- String sizing prevents overvoltage conditions that could damage inverters or other equipment
The ASHRAE Handbook Fundamentals provides Extreme Annual Mean Minimum Design Dry Bulb Temperature data that serves as a reliable source for determining the lowest expected ambient temperature at a given installation site. Proper solar wiring selection depends on these voltage calculations.
Marking Requirements Under 690.7(D)
Installers must place a permanent, visible label showing the maximum DC voltage at one of these locations:
- DC PV system disconnecting means
- PV system electronic power conversion equipment
- Distribution equipment associated with the PV system
This label helps service personnel and first responders quickly identify system voltage levels. Related labeling requirements apply to rapid shutdown labels and rapid shutdown system identification.
Working Space Considerations
Maximum DC voltage determines working space requirements. Systems at 1000V or less follow Article 110 Part II, while systems over 1000V must comply with Part III. Proper voltage calculation ensures adequate clearances for maintenance. Review the special equipment chapter for complete requirements.
2023 NEC Updates To 690.7
The 2023 NEC made key changes to Section 690.7. Recent code changes affect voltage calculation requirements.
Reformatted Structure: Requirements now appear as a numbered list for clarity.
Reference to 690.31(G): New subsection covers systems exceeding 1000 Vdc, with installation requirements for systems up to 1500 Vdc.
Terminology Update: “PV output circuit” became “PV string circuit” in 690.7(A). Many solar terms moved to Article 100 definitions in the 2023 edition.
Article 495 Replacement: Article 490 became Article 495 (Equipment Over 1000 Vac, 1500 Vdc, Nominal). Since PV systems max out at 1500 Vdc, Article 495 does not apply. The 2023 solar code includes additional terminology and structural changes.
String Sizing And Inverter Voltage Compatibility
NEC 690.7 calculations directly impact string sizing. Designers must keep total string voltage within the inverter’s operating window under all temperature conditions. Proper design accounts for both maximum cold weather voltage and minimum hot weather voltage.
Maximum String Size Calculation
The maximum number of modules per string is determined by dividing the inverter’s maximum input voltage by the temperature corrected module Voc:
Maximum Modules Per String = Inverter Maximum Voltage ÷ Module Voc (at lowest temperature)
For example, if an inverter has a 600V maximum input and each module produces 51V at the coldest expected temperature, the maximum string size would be 11 modules (600 ÷ 51 = 11.76, rounded down).
Minimum String Size Calculation
Proper design also requires minimum string size calculations to ensure inverter operation during hot conditions:
Module Vmp (at highest temperature) = Vmp × (1 + Temperature Coefficient × (High Temperature + Temperature Rise − 25°C))
Temperature rise depends on installation method: roof mounted systems with less than 6 inch standoff add 35°C, rack mounts with greater than 6 inch standoff add 30°C.
Inverter Damage Prevention
Exceeding maximum DC input voltage can cause permanent inverter damage or fire. Many inverters log peak voltage, and manufacturers may void warranties if logs show limit violations. Conservative sizing protects equipment and maintains warranty coverage. Related safety requirements include arc fault protection and proper fuse servicing.
Sourcing Temperature Data For Voltage Calculations
Accurate temperature data is essential for voltage calculations. The NEC recommends ASHRAE data for determining lowest expected ambient temperature. This solar code guide provides additional context on temperature requirements.
ASHRAE Extreme Annual Mean Minimum Design Dry Bulb Temperature
This value represents the coldest temperature expected at a location based on historical weather data. Solar ABCs provides free ASHRAE data by zip code for US locations. Solar Design Temps offers worldwide data.
Record Low Temperature vs Design Temperature
Designers can choose between two approaches:
Record Low Temperature: The coldest temperature ever recorded at the location. This conservative approach provides the highest safety margin but may result in shorter strings than necessary.
Extreme Annual Mean Minimum: A statistically derived value that accounts for typical cold extremes without using absolute record lows. This approach often allows longer strings while maintaining code compliance.
For a location like Albuquerque, the record low temperature is −17°F while the extreme minimum dry bulb temperature is 10°F. This difference can mean the choice between 13 and 14 modules per string on a 600V system.
Advanced Design Topics
Cold Climate Considerations
Cold climate systems require careful voltage calculations. Arrays in Minnesota or Alaska experience significantly higher peak voltages than those in Arizona or Florida. Off-grid systems in remote cold locations face additional sizing constraints.
Most crystalline silicon modules have a Voc temperature coefficient between −0.25% and −0.35% per °C. For every degree below 25°C, module voltage increases by this percentage.
Example: A module with 45V Voc and −0.30%/°C coefficient at −30°C:
Voltage Increase = 45V × 0.003 × 55 = 7.43V
Maximum Voc = 45V + 7.43V = 52.43V
Cold climates often require shorter strings. A system accommodating 12 modules per string in Southern California may only support 10 modules in Northern Minnesota due to temperature induced voltage rise.
Bipolar PV Systems
NEC 690.7(C) covers bipolar source and output circuits per Article 690. For bipolar systems, maximum voltage equals the highest voltage between conductors when one conductor is solidly grounded and each two wire circuit connects to a separate subarray. Bipolar configurations can double system voltage compared to unipolar designs.
Common Calculation Mistakes And How To Avoid Them
Several errors lead to voltage miscalculations and code violations.
Using Vmp Instead Of Voc For Maximum Voltage
Always use open circuit voltage (Voc), not maximum power voltage (Vmp). Voc represents peak voltage, which occurs at open circuit during cold temperatures with low irradiance.
Ignoring Temperature Correction
Multiplying module Voc by module count without temperature correction underestimates true maximum voltage. This error causes systems to exceed inverter limits on cold days.
Using Incorrect Temperature Data
Use reliable sources like ASHRAE data, not estimates or average winter temperatures. Record lows or extreme minimum design temperatures provide appropriate safety margins.
Forgetting Cell Temperature vs Ambient Temperature
At dawn, cell temperatures match cold ambient temperatures because irradiance is low. As irradiance increases, cells heat above ambient. Coldest cell temperatures and highest voltages occur during early morning cold snaps with clear skies.
Key Takeaways For Solar Installers
Proper NEC 690.7 application requires attention to critical factors in the general requirements of Article 690:
- Select the appropriate calculation method based on module type and system size
- Use accurate local temperature data for correction factors
- Verify all equipment voltage ratings exceed calculated maximum
- Install required voltage labels at approved locations
- Ensure working space complies with Article 110 requirements
These requirements help PV installations meet code compliance while maintaining safe operating conditions.
Conclusion
NEC 690.7 provides the foundation for safe voltage management in solar installations. Clear voltage limits, multiple calculation methods, and labeling requirements protect installers and end users from electrical hazards.
Accurate voltage calculations remain critical for equipment selection, string sizing, and system design. Installers must account for cold temperature conditions that push module voltages to peak levels.
The 2023 NEC updates improved clarity and added provisions for systems between 1000 and 1500 Vdc. Staying current with code changes ensures installations meet safety standards while leveraging new higher voltage allowances. Additional resources from Solar Permit Solutions and other industry sources can support ongoing compliance.
