NEC 690.7(A) · Solar PV

How to calculate max PV system voltage for the cold

The manufacturer temperature-coefficient method, the Table 690.7(A) method, and the engineered method — with a worked string-sizing example and a free link to the official table.

Need the actual table? Look up Table 690.7(A) in the free NFPA viewer →

Which 690.7(A) method applies?

Method	When to use it	What you need	Notes
Manufacturer temperature coefficient	Whenever the module listing/datasheet gives a Voc coefficient — the usual case	Module Voc at STC, Voc temp coefficient, design low temperature	Most accurate; typically yields a longer allowable string than the table
Table 690.7(A) correction factor	Crystalline/multicrystalline modules when you are not using the coefficient method	String Voc and design low temperature	Conservative; not for thin-film modules
Engineered (industry standard method)	Systems 100 kW and larger	Documented, stamped design by a licensed PE	Allows site-specific modeling instead of worst-case

Our own summary of the 2023 NEC, not the code text. Look up 690.7 and Table 690.7(A) in the free NFPA viewer (nfpa.org/freeaccess) and verify against the edition adopted by your AHJ. Not engineering advice.

Coefficient method, step by step

Step	What you do	Watch out for
1	Get the design low temperature for the site	The Informational Note points to the ASHRAE extreme annual mean minimum design dry-bulb — not the record low and not the average January low
2	From the datasheet, get Voc at STC (25 °C) and the Voc temperature coefficient	Coefficient may be in %/°C or V/°C — keep the units straight; it is negative
3	ΔT = design low − 25 °C (negative in cold climates)	Sign errors are the #1 mistake — cold should raise voltage
4	Cold Voc = Voc × [1 + (coefficient % ÷ 100) × ΔT]	For a V/°C coefficient: Cold Voc = Voc + coefficient × ΔT
5	String voltage = cold Voc × modules in series	Compare to the lowest of: inverter max input, module max system voltage, and the 690.7 limit for the building type
6	Round the module count down	One module over the limit can void the inverter warranty and fail inspection

Worked example — coefficient method, 1000 V inverter

Input / step	Value	Source
Module Voc at STC	49.5 V	Module datasheet (example)
Voc temperature coefficient	−0.28 %/°C	Module datasheet (example)
Design low temperature	−20 °C	ASHRAE extreme annual mean minimum (example site)
ΔT	−20 − 25 = −45 °C	Step 3
Cold-corrected Voc	49.5 × [1 + (−0.0028 × −45)] = 49.5 × 1.126 ≈ 55.7 V	Step 4
Max modules per string	1000 ÷ 55.7 ≈ 17.9 → 17 modules	Round down
String max voltage	17 × 55.7 ≈ 948 V	Under 1000 V

Example module and site values — use your actual datasheet and site data. The Table 690.7(A) method uses a code-published factor instead of step 4; look it up in the free NFPA viewer.

Common field mistakes

Mistake	What to do instead
Using the Isc or Pmax coefficient	Use the Voc temperature coefficient
Getting the sign backwards so voltage drops in the cold	A negative coefficient times a negative ΔT raises Voc
Using the record low or a random weather-site low	Use the ASHRAE extreme annual mean minimum for the nearest station
Checking only the inverter limit	Also check the module’s max system voltage and the 690.7 building-type limit
Applying the table factor to thin-film modules	The table is for crystalline and multicrystalline silicon — use the manufacturer data
Adding a module during layout changes without re-running the math	Any string change means a new cold-Voc check

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Sizing strings for the coldest sunny morning

Look up 690.7 and Table 690.7(A) in the free NFPA viewer. This page explains the methods; it does not reproduce the table.

Module Voc is rated at 25 °C. On a cold, clear morning the cells sit near ambient temperature while the sun is already strong, and voltage climbs above nameplate. Section 690.7(A) makes you size strings for that worst case so nothing in the DC circuit — inverter input, module insulation, disconnects, conductors — sees more than its rating.

Three ways to get there. The manufacturer coefficient method uses the module’s own Voc temperature coefficient and is the one you will use most. The table method multiplies string Voc by a code-published factor for the design low temperature — simple, conservative, and limited to crystalline and multicrystalline silicon. Systems of 100 kW and up may use a stamped engineered design instead.

Worked example. A module with a Voc of 49.5 V and a coefficient of −0.28 %/°C, at a site with a −20 °C design low: ΔT is −45 °C, so Voc rises 12.6% to about 55.7 V. Against a 1000 V inverter, 1000 ÷ 55.7 = 17.9, so the string tops out at 17 modules, about 948 V. Always round down.

Maximum voltage is only half of the string check — the other half is current and voltage drop. Run the numbers with the voltage drop calculator, size PV source conductors with the NEC 310.16 ampacity chart, and apply rooftop and temperature derates with the ampacity correction factors guide. Field PM keeps string layouts, datasheets, and inspection photos with the job.

FAQ

How do I calculate maximum PV system voltage?+

Under NEC 690.7(A), correct each module’s open-circuit voltage to the site’s design low temperature and add them up for the string. With the coefficient method: cold Voc = Voc × [1 + (coefficient %/100) × (low temp − 25 °C)]. Example: a 49.5 V module at −0.28 %/°C and −20 °C gives about 55.7 V; 17 in series is about 948 V.

Should I use the temperature coefficient or Table 690.7(A)?+

When the manufacturer provides a Voc temperature coefficient, use it — it is the more accurate method and usually allows a longer string. Table 690.7(A) is an alternative for crystalline and multicrystalline silicon modules and is generally more conservative. Systems of 100 kW or more may use a stamped engineered design instead. Look up the table in the free NFPA viewer at nfpa.org/freeaccess.

What low temperature do I use for PV string sizing?+

The Informational Note to 690.7(A) points to the ASHRAE Handbook extreme annual mean minimum design dry-bulb temperature for the nearest weather station. Some AHJs publish the value they expect, so check local requirements.

Why does PV voltage go up in the cold?+

A solar cell’s open-circuit voltage rises as cell temperature falls. On a clear, cold morning the array can see full sun while the cells are still near ambient temperature, which produces the highest voltage of the year. That is the condition 690.7 sizes for.

What is the maximum PV system voltage allowed?+

Section 690.7 sets maximum PV system voltage limits based on the building type, with a lower limit for one- and two-family dwellings than for other buildings, and the string must also stay within the inverter and module ratings. Check 690.7 in your adopted edition for the exact limits.

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