Most solar plan check rejections on the DC side come down to one thing: the conductor sizing math was incomplete. A wrong wire gauge, a missing derating step, or an Isc value — the module’s short-circuit current rating from the spec sheet — pulled from the wrong column, and the job is back in the queue.

NEC 690.8 conductor sizing requirements govern every DC circuit on a PV system. The section covers how to calculate maximum circuit current, how to set minimum conductor ampacity, and what correction factors apply before you land on a wire gauge. Solar conductor sizing under 690.8 compounds across multiple steps, and the version most installers learned in the field skips at least one of them.

This guide covers the full sequence — PV source circuit conductor sizing, PV output circuit conductor sizing, the 125 percent rule, temperature derating, conduit fill, wire type selection, and solar wire sizing for permit documentation. We cover NEC Article 690 context separately if you need the broader picture first.

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PV source circuits are continuous loads by definition; they operate for three or more hours whenever the sun is shining. They also operate in an elevated-temperature rooftop environment that standard residential branch circuit wire sizing does not account for. NEC 690.8 is the code’s mechanism for addressing both of those realities.

Plan check reviewers check the conductor sizing calculation on every solar job because undersized conductors on the DC side are a documented fire risk. A conductor that looks adequate at nameplate current may fall short once the continuous load factor and rooftop temperature derating are applied. That gap between an intuitive gauge selection and a code-correct one is exactly what reviewers are looking for.

The practical consequence of an error is a resubmittal at best and a failed field inspection at worst. If the installed wire gauge does not match the approved plan set, the system cannot be energized until the discrepancy is resolved. Solar permit requirements broadly cover what AHJs expect, and 690.8 is one of the most inspected elements in the electrical package.

NEC 690.8 governs two DC circuit types. Knowing which conductors fall under this section and which do not is the starting point for sizing them correctly.

• PV source circuit: Conductors running from modules to a combiner box, or directly from modules to the inverter in a single-string or direct-connect topology.

• PV output circuit: Conductors from the combiner box to the inverter, where a combiner is used. The output circuit carries the combined current of all parallel source circuits.

• Inverter output circuit (AC side): Conductors from the inverter to the service equipment. This circuit is not governed by NEC 690.8.

One point that trips up installers: the DC conductors landing on the inverter are source circuits if they come directly from strings, or output circuits if they come from a combiner. There is no separate inverter input circuit type in Article 690. Those conductors are already classified as source or output circuits depending on the system topology.

A note on microinverter systems: For microinverter projects, most installer-facing conductor sizing work is on the AC side. The module leads and factory-installed wiring are typically part of listed equipment. Branch-circuit trunk conductor sizing follows AC continuous load rules, not the 690.8 DC sizing sequence.

NEC 690.8(A)(1) defines the maximum PV source circuit current as the sum of the parallel-connected module short-circuit current (Isc) ratings multiplied by 125 percent. This first factor is called the enhancement factor. It accounts for real-world solar irradiance conditions that can exceed the 1,000 W/m² standard test condition, including snow reflection, high-altitude exposure, and cold-weather efficiency gains.

This is not the continuous load factor. The continuous load factor comes in Step 2. The two 125 percent multipliers serve different purposes, and conflating them leads to both double-counting errors and under-sizing errors.

Single string source circuit: 11.8 × 1.25 = 14.75A maximum current.

Two parallel strings at a combiner output: 2 × 14.75 = 29.5A maximum output circuit current.

Cold-weather voltage rating: a parallel check that belongs at this stage

Source circuit conductors must also carry an insulation voltage rating that covers the maximum system voltage after cold-temperature correction to Voc. This is a NEC 690.7 requirement, not a 690.8 requirement, but it belongs in the design stage alongside the current calculation.

A conductor sized correctly for ampacity can still fail plan check if its voltage rating does not cover the temperature-corrected string Voc. This matters especially when sizing solar string wire for high-voltage string inverter systems.

Alternate large-system path (690.8(A)(1)(2)): For systems 100 kW or larger, a PE-stamped simulation using an industry-standard method is permitted. The calculated value cannot be less than 70 percent of the standard Isc × 1.25 result. The Sandia National Laboratories PV Array Performance Model is the method referenced in the NEC informational note. This path does not apply to residential jobs. Solar engineering services can provide PE-stamped calculation support for commercial systems that use this alternate path.

Once maximum circuit current is established in Step 1, NEC 690.8(B) sets the minimum conductor ampacity. Two sizing paths are available. Picking one and applying it correctly is essential; using elements of both simultaneously produces an over-sized conductor or an apparent pass that does not hold up under conditions-of-use review.

Conductor ampacity must be not less than 125 percent of the maximum current from Step 1. No additional correction factors for conditions of use are required under this path. The combined multiplier applied to module Isc is 1.25 × 1.25 = 1.5625, rounded to 1.56 in practice.

Conductor ampacity after all correction and adjustment factors for temperature and conduit fill must be not less than the Step 1 maximum current. This path allows a smaller conductor if conditions support it, but requires the full derating sequence. It is the path most residential jobs end up on because rooftop temperature conditions typically drive the calculation.

Applying 1.25 × Isc × 1.25 to get minimum ampacity and then also derating for rooftop temperature on top of both factors double-counts the continuous load correction. If you are on the 690.8(B)(2) path, the temperature and conduit fill derating replaces the second 1.25 multiplier. It is not added on top of it.

Worked example continued:

Step 1 maximum current = 14.75A.

Under 690.8(B)(1): minimum conductor ampacity = 14.75 × 1.25 = 18.44A.

From NEC Table 310.16, a 14 AWG copper conductor rated at 90°C has a base ampacity of 25A. 25A is above 18.44A, so 14 AWG passes this specific check before temperature derating. The next two sections show why that answer almost always changes once rooftop conditions are applied.

Temperature correction is where residential solar conductor sizing most commonly fails plan check. It is the step most frequently omitted from plan sets submitted by installers who learned the 1.56 rule without learning what comes after it.

NEC 310.16 lists conductor ampacities at a baseline ambient temperature of 30°C (86°F). Conductors operating at higher ambient temperatures require a correction factor from NEC Table 310.15(B)(1). The correction factor reduces the usable ampacity of a conductor in proportion to how far the operating environment exceeds 30°C.

The rooftop temperature adder:

NEC 310.15 includes a specific provision for raceways or cables exposed to direct sunlight on or above rooftops where the raceway sits at or near the roof surface. Where the distance from the roof surface to the bottom of the raceway is less than 7/8 inch, a temperature increment must be added to the outdoor design ambient before applying correction factors. The applicable increment and the exact subsection designation have shifted across recent NEC editions, so confirm against the edition your AHJ has adopted before finalizing calculations.

In practice, using a common rooftop adder of 33°C applied to a design ambient of 38°C produces an effective ambient of 71°C for conductor sizing purposes. At 71°C, the correction factor for a 90°C-rated conductor from NEC Table 310.15(B)(1) is approximately 0.58.

14 AWG at 90°C base ampacity of 25A × 0.58 = 14.5A adjusted. Below 18.44A. 14 AWG fails.

12 AWG at 30A × 0.58 = 17.4A adjusted. Also below 18.44A. 12 AWG fails.

10 AWG at 40A × 0.58 = 23.2A adjusted. Above 18.44A. 10 AWG passes.

When more than three current-carrying conductors share a single raceway, NEC 310.15(C) requires an adjustment factor that reduces each conductor’s usable ampacity. On multi-string residential systems, this derating applies to the conduit run carrying multiple source circuits from the array to the inverter or combiner.

Counting current-carrying conductors correctly:

Both the positive and negative conductors of each source circuit are current-carrying. Equipment grounding conductors are not counted. A four-string system with four positive and four negative conductors in one conduit has eight current-carrying conductors.

Combined derating: where gauge requirements jump

For the same four-string system with eight current-carrying conductors, the conduit fill factor is 0.70. Combined with the 71°C temperature correction factor of 0.58, the combined derating is 0.70 × 0.58 = 0.41.

A 10 AWG conductor at 40A × 0.41 = 16.4A adjusted ampacity. That no longer meets the 18.44A requirement from Step 2. 8 AWG at 55A × 0.41 = 22.55A. Passes.

This is where multi-string residential jobs produce wire gauge requirements that genuinely surprise installers who sized correctly for a single-string example. The jump from 10 AWG to 8 AWG happens not because of a calculation error but because the combined derating on bundled conductors in a hot rooftop environment is substantial.

Using the 90°C column from NEC Table 310.16 when applying correction and adjustment factors is correct. But that is not the end of conductor selection. NEC 110.14(C) governs the temperature at which conductors can actually terminate at equipment.

Most solar equipment, including inverters, combiners, and disconnects, has terminals rated at 75°C. Where equipment is rated at 75°C, the conductor’s ampacity must not exceed the corresponding value in the 75°C column of Table 310.16, even if the conductor insulation is rated higher.

Use the 90°C column when applying correction and adjustment factors in the derating math. Then confirm that the selected conductor gauge, referenced against the 75°C column, still meets the ampacity requirement. Where the 90°C derating math points to a conductor that passes but the 75°C ampacity value for that gauge is lower than required, step up to the next gauge.

This is a common gap in plan sets submitted without a full calculation block. A reviewer who checks 110.14(C) will flag a conductor that passes the derating math under 90°C rules but falls short at the 75°C termination limit. The NFPA NEC reference tables cover Table 310.16 ampacity columns at both 75°C and 90°C for quick cross-reference.

The PV output circuit runs from the combiner box to the inverter. Under NEC 690.8(A)(2), maximum output circuit current is the sum of all parallel source circuit maximum currents, each already calculated at Isc × 1.25. Then NEC 690.8(B) applies, along with the same temperature correction and conduit fill derating rules covered in the sections above.

Output circuits typically carry higher total current and run longer distances than any individual source circuit, so they frequently require a larger wire gauge. That is expected and correct.

Output circuit conductor sizing connects directly to overcurrent protection requirements under NEC 690.9. OCPDs on the PV output circuit are not required under 690.9(A)(1) when conductors have ampacity at least equal to the maximum current calculated under NEC 690.8(B).

The AC conductors from the inverter to the point of interconnection at the service panel are not governed by NEC 690.8. They are sized under standard continuous load rules: 125 percent of the

inverter’s rated continuous AC output current. The NEC 705.12 interconnection method and the 120% rule calculation both depend on the AC conductor sizing being correctly documented on the one-line diagram.

For microinverter systems, trunk cable and branch circuit wiring follows NEC 210 and 215 continuous load rules from the point of each microinverter’s AC output. The 690.8 DC sizing sequence applies only within factory-listed equipment for those systems.

Conductor sizing determines the gauge. NEC 690.31 determines what type of conductor is permitted and how it must be installed. The wire type directly affects the ampacity values available, the voltage rating options, and whether a given installation location is code-compliant.

• PV wire: Double-insulated, UV and moisture resistant, 90°C rating, listed for outdoor exposed use. Standard for module-to-module and module-to-combiner runs. Most modules ship with PV wire pre-attached at the junction box.

• USE-2: Single-jacketed, UV and moisture resistant, 90°C rating. Electrically similar to PV wire and permitted under NEC 690.31(C) for the same applications.

• THWN-2 or THHN: Not permitted for exposed DC source circuit runs. Standard building wire lacks the UV resistance required for outdoor exposed installation. THWN-2 is permitted inside conduit from the combiner or junction box to the inverter or disconnect.

Do not install THHN or THWN in exposed outdoor locations without conduit protection. That is a code violation and a plan check rejection even when the wire gauge is otherwise correct.

• One-family and two-family dwellings: 600V DC maximum. Conductor voltage rating must cover the cold-temperature-corrected Voc.

• Building-mounted commercial or multi-family: 1,000V DC maximum.

• Building exterior only under 2023 NEC 690.31(G): up to 1,500V DC under specific conditions, including 2,000V-rated conductors such as PV wire or USE-2, restricted height and length requirements, and prohibition from entering buildings with habitable rooms. Confirm the AHJ’s adopted edition before applying this path.

DC source and output circuits entering a building must be in metal raceways, Type MC cable complying with NEC 250.118(10), or metal enclosures from the point of building penetration to the first readily accessible disconnect. Conduit and junction boxes containing DC conductors must be permanently labeled per NEC 690.31(D). Solar labeling requirements cover label sizing, wording, and placement specifications.

AHJs and utilities check conductor sizing documentation against a consistent set of expectations. A plan set that passes a knowledgeable reviewer’s check includes all of the following, not just the final wire gauge.

☐ Module Isc from the current manufacturer spec sheet; model number must match the equipment list exactly

☐ Number of parallel strings per source circuit; total string count at the combiner for output circuit sizing

☐ Maximum circuit current calculation per 690.8(A)(1) shown on the SLD or in a separate calculation block

☐ Conductor ampacity requirement per 690.8(B) with the sizing path stated: 690.8(B)(1) or 690.8(B)(2)

☐ Effective ambient temperature identified, including the rooftop adder per the AHJ’s adopted NEC edition

☐ Temperature correction factor from NEC Table 310.15(B)(1) applied and shown

☐ Number of current-carrying conductors in conduit identified; conduit fill adjustment applied where more than three are bundled

☐ Final adjusted conductor ampacity compared to required, with wire gauge selection justified

☐ Termination temperature limit confirmed per NEC 110.14(C)

☐ Conductor type identified (PV wire, USE-2, THWN-2) and confirmed appropriate for the installation location

☐ Conductor voltage rating confirmed against temperature-corrected system Voc per NEC 690.7

☐ DC circuit labeling requirements noted per NEC 690.31(D)

☐ Isc taken from an outdated spec sheet, or Imp used instead of Isc

☐ Rooftop temperature adder omitted from the effective ambient calculation

☐ Both the 690.8(B)(1) continuous load factor and temperature derating applied simultaneously, double-counting the continuous correction

☐ Conduit fill adjustment not applied when four or more source circuits share a conduit

☐ Wire gauge selected using the 90°C ampacity column without checking the 75°C terminal compatibility requirement under NEC 110.14(C)

☐ THWN listed in the wire schedule for a run that includes exposed outdoor DC sections

☐ Conductor voltage rating not confirmed after cold-temperature Voc correction

☐ OCPD not shown or not correctly coordinated with conductor ampacity on the output circuit

NEC 690.8 is an ampacity section. Voltage drop is a separate sizing check that can push conductor selection above the ampacity minimum, particularly on longer DC runs where resistive losses accumulate.

Industry practice targets 2 percent or less DC voltage drop for source circuits. The NEC does not mandate a specific maximum, but excessive voltage drop produces real performance consequences: inverter low-voltage shutdowns, narrowed MPP tracking range, and reduced energy yield over the life of the system. On long runs, the conductor that passes the 690.8(B) ampacity check may be one or two gauges smaller than what the voltage drop target requires.

• Ground-mount arrays with DC runs exceeding 30 to 40 feet from array to inverter

• Roof-mounted arrays with the inverter at ground level

• High Vmp string configurations where even moderate resistance produces meaningful voltage loss

• Any job where the inverter’s minimum operating voltage is close to the design string Vmp at high temperatures

The final wire gauge must satisfy both the ampacity floor and the voltage drop target. Whichever requirement produces the larger conductor wins.

The SolSmart simplified permit commentary includes pre-calculated sizing tables for common residential configurations that incorporate both ampacity and voltage drop for systems up to four source circuits with Isc at or below 12.8A.

The NEC 690.8(A) enhancement factor and the 690.8(B) ampacity requirement have been consistent across recent editions. What has changed is how NEC 310.15 addresses rooftop temperature, what wiring methods NEC 690.31(D) permits inside buildings, and what the 2023 NEC 690.31(G) provisions allow for high-voltage DC exterior runs.

Before finalizing conductor sizing calculations for a job, confirm which NEC edition the AHJ has adopted. The rooftop temperature treatment has been revised across the 2017, 2020, and 2023 cycles, and using a subsection reference from the wrong edition is a plan check trigger in jurisdictions where reviewers verify code citations.

Getting NEC 690.8 conductor sizing right on a plan set means correctly applying the enhancement factor, the continuous load ampacity path, the rooftop temperature adder for the adopted edition, and the conduit fill derating, then showing all of it on the drawings clearly enough that a plan reviewer does not have to ask a follow-up question.

One missing factor is a plan check comment. A wrong wire gauge on the installed system is a failed inspection and a required change order before the customer can get PTO.

GreenLancer connects solar installers with licensed electrical engineers and designers for permit-ready plan sets, conductor sizing calculations, and PE-stamped electrical documentation in all 50 states. Our engineers understand what plan reviewers check and build the package to clear review on the first submission.

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NEC 690.8(A)(1) requires that maximum PV source circuit current be calculated by multiplying the sum of the parallel-connected module Isc ratings by 125 percent. This enhancement factor accounts for real-world irradiance that can exceed standard test conditions, including snow reflection and cold-temperature efficiency gains. It is separate from the continuous load factor, which is applied in Step 2.

The first 125 percent under 690.8(A)(1) is the enhancement factor, adjusting Isc upward for above-STC irradiance conditions. The second under 690.8(B)(1) is the continuous load factor, applied because PV systems run for three or more hours and are treated as continuous loads. Combined, they produce a 1.56 multiplier on module Isc. Under the 690.8(B)(2) alternative, conditions-of-use derating replaces the second factor entirely. Do not apply both simultaneously.

Using the 690.8(B)(1) path: 1.25 × 1.25 = 1.5625, rounded to 1.56. This multiplier is applied to module Isc to determine minimum conductor ampacity before conditions-of-use derating. Under the 690.8(B)(2) path, the Step 1 current sets the target, and the conductor’s ampacity after temperature and conduit fill derating must meet or exceed it.

NEC 310.15 requires adding a temperature increment to the outdoor ambient for raceways near rooftop surfaces. Where the raceway is within 7/8 inch of the roof, a commonly applied adder is 33°C. Adding that to a 38°C design ambient produces a 71°C effective ambient. At 71°C, the correction factor for a 90°C-rated conductor is approximately 0.58, which can force a conductor one or two gauge sizes larger than the base ampacity calculation suggests. Confirm the applicable adder and subsection against your AHJ’s adopted NEC edition.

NEC 690.31(C) permits PV wire and USE-2 for exposed DC source circuit runs. Both are UV and moisture resistant with a 90°C rating. Standard THWN-2 or THHN is not permitted for exposed outdoor DC runs because it lacks UV resistance. THWN-2 is permitted inside conduit on the protected portions of the circuit.

The PV output circuit runs from the combiner to the inverter. Maximum current under NEC 690.8(A)(2) is the sum of all parallel source circuit maximum currents, each at Isc × 1.25. The same 690.8(B) ampacity requirement and derating rules apply. Output circuits carry higher current and run longer distances, so a larger gauge than the source circuit is typical and expected.

NEC 110.14(C) requires that conductors not terminate above the equipment’s terminal temperature rating. Most solar equipment has 75°C-rated terminals. Use the 90°C column for derating math, then verify the selected gauge satisfies the ampacity requirement when referenced against the 75°C column. Where there is a shortfall at 75°C, step up to the next gauge.

The SLD or a separate calculation block should show the module Isc, the number of parallel strings, the maximum current calculation per 690.8(A)(1), the required ampacity per 690.8(B) with the path stated, the effective ambient including the rooftop adder, the temperature correction factor, the conduit fill adjustment if applicable, and the final wire gauge with its adjusted ampacity confirmed against the requirement.

The most frequent issues are using Isc from an outdated spec sheet or using Imp instead of Isc. Another common problem is leaving out the rooftop temperature adder from the effective ambient. Reviewers also flag plans that apply both the 690.8(B)(1) continuous factor and temperature derating at the same time. Conduit fill derating is often missed when multiple circuits share a conduit. Another frequent issue is selecting a wire gauge from the 90°C column without checking 75°C terminal compatibility under NEC 110.14(C).

No. AC output conductors are sized under standard continuous load rules at 125 percent of the inverter’s rated continuous AC output current. They fall under NEC 705.12, and the sizing must be reflected on the one-line diagram for both the interconnection calculation and the 120% rule check.