Conductor and Wire Sizing for PV: An NEC 690 Primer

Why plan check keeps flagging your conductors

Most conductor corrections are not math errors. They come from doing the math in the wrong order, or stopping one step early. A designer picks a wire that carries the operating current, moves on, and the reviewer bounces it because the ampacity was never derated for a hot rooftop run. The wire was fine on paper and undersized on the roof.

The fix is to treat sizing as a defined sequence rather than a single lookup. NEC Article 690 governs photovoltaic systems and leans on Article 310 for how conductor ampacity actually works, and the two articles have to be read together. The National Electrical Code is published by the NFPA as NFPA 70, and local jurisdictions adopt it with their own amendments, which is why the same design can clear one AHJ and get flagged by the next. This primer walks the logic, not the tables. Keep your adopted code edition open next to it for the actual values.

The two current questions 690.8 asks

NEC 690.8 covers circuit sizing and current for PV systems, and it really asks two things in order. What is the maximum current this circuit can produce? And what conductor can carry that current continuously without overheating? People collapse these into one question and that is where trouble starts.

The first question sets a calculated maximum current. On a PV source circuit that maximum is built from the module's rated short-circuit current, because sunlight can push a string past its nameplate operating point. On an inverter output circuit the maximum comes from the inverter's continuous rated output. The second question then applies a continuous-duty factor, because PV circuits run at or near their maximum for hours at a stretch and the code treats that as a continuous load. The result of both steps is the ampere value your conductor has to beat. The DOE notes that PV output tracks irradiance, which is exactly why sizing starts from short-circuit and continuous behavior rather than a brief peak.

Sizing source circuits from short-circuit current

Start at the array. A PV source circuit is the wiring from the modules up to the first combining point, and its maximum current is not the operating current you read off a spec sheet. It is derived from the module's short-circuit current, Isc. The code applies a 125% factor to that Isc to account for irradiance that runs above standard test conditions on bright cold days, so the working number is Isc times 1.25 for the string.

Here is the part that catches people. That 125% is the current-producing side of the calculation. When you then size the conductor for continuous duty, a second 125% factor enters, and the two can stack into an effective multiplier well above a single 1.25. NFPA describes this layered approach in Article 690, and the intent is margin against sustained high output, not a single safety pad. Size the source-circuit conductor so its ampacity, after the derating in the next section, still clears that stacked current. Skipping straight from Isc to a wire gauge is the single most common reason a source circuit gets kicked back.

Output and inverter circuits and the continuous factor

Output circuits behave differently. A PV output circuit or an inverter output circuit is sized from the continuous current the equipment actually delivers, and the 125% continuous-load factor is applied to that value. For an inverter, the maximum continuous output current shows up on the datasheet, and you multiply it by 1.25 before you go looking for a conductor.

The reason for the continuous factor is thermal. A conductor rated for a given ampacity assumes it can shed heat, and a circuit that runs for three hours or more at its maximum does not get the cooling breaks that an intermittent load does. The 125% keeps the conductor operating below the point where sustained heat degrades the insulation. This is why an inverter's own overcurrent rating and the conductor feeding it are linked. You are protecting insulation from a load that, on a sunny afternoon, basically never lets up.

Ampacity derating for temperature and conduit fill

Now the second question. A conductor's listed ampacity is a starting figure tied to a reference ambient temperature and a small number of conductors in free air or a raceway. Real PV runs violate both assumptions, and Article 310 tells you how to correct for that. Two corrections dominate on solar jobs.

The first is ambient temperature. A conduit strapped to a dark roof in Phoenix in July sits far above the reference ambient, and the correction factor for that heat is a number below 1.0 that you multiply the base ampacity by. The hotter the environment, the smaller the factor, and rooftop conduit gets an additional bump for the air layer trapped near the roof surface. The second is conduit fill. Once more than three current-carrying conductors share a raceway, they heat each other, and Article 310 applies an adjustment factor that also drops below 1.0 as the count climbs. Bundle a lot of source-circuit conductors into one homerun and the usable ampacity can fall well under the printed value.

NFPA presents the National Electrical Code as a consensus safety standard that establishes the baseline for electrical installations, with its legal force coming from adoption by states and local jurisdictions rather than from the NFPA itself.

NFPA, Understanding NFPA 70 (NEC)

Apply both corrections to the base ampacity, then compare the derated result against the current from 690.8. If the derated ampacity is still above the calculated current, the conductor holds. If not, go up a size and check again. Terminal temperature ratings matter here too. Even a 90C-rated conductor is often limited to its 75C ampacity column because the lugs and breakers it lands on are rated 75C, and you cannot use ampacity your terminations cannot handle.

Matching overcurrent protection to the conductor

Overcurrent protection for PV lives in NEC 690.9, and its job is to protect the conductor, not the inverter. That framing settles a lot of confusion. The overcurrent device rating is generally set at 125% of the calculated circuit current, and it has to sit at or below the conductor's ampacity so the device trips before the wire overheats.

Two things trip designers up. Fuses and breakers come in standard sizes, so you often round to the next standard rating, and the code allows that within limits as long as the device still protects the conductor. And some PV source circuits do not require overcurrent protection at all when the available fault current cannot exceed the conductor and module ratings, which is a 690.9 allowance, not a shortcut you assume. When in doubt, size the conductor first, then choose the device that protects it, never the reverse. A breaker picked before the wire is a correction waiting to happen.

Voltage drop as a design target, not a code limit

Voltage drop is where good engineering and code compliance part ways. The NEC treats conductor voltage drop as informational guidance, not a mandatory limit, so a plan reviewer will not reject a set purely because a run drops more than you would like. That does not make it free. Every volt lost in the wire is energy the array produced and never delivered.

As a rule of thumb, many designers target roughly 1 to 2 percent drop on a given conductor segment and keep the total system drop modest, though the right number depends on run length, current, and how much lifetime yield you are willing to trade for smaller wire. On long rooftop-to-inverter or inverter-to-panel runs, voltage drop often drives you to a larger conductor than ampacity alone would require, and that is a legitimate design choice rather than a code mandate. Treat it as an economic optimization sitting on top of the safety math, not a substitute for it.

The sizing sequence and a pre-submission checklist

The table below lays out the order of operations and the article that governs each step. Values shown are illustrative to explain the logic. Use your adopted NEC edition for the actual factors and ampacity figures.

Run this list before the plan set leaves your desk. Each item maps to a place reviewers look first.

• Identified every circuit by type: source, PV output, inverter output, and feeder, since the sizing basis differs by type.
• Applied the 125% short-circuit factor on source circuits and the 125% continuous factor on output circuits, and checked whether they stack.
• Derated base ampacity for the worst-case ambient temperature at the conductor location, including the rooftop adder where it applies.
• Counted current-carrying conductors in each raceway and applied the conduit-fill adjustment before comparing to the calculated current.
• Confirmed the terminal temperature rating of every device the conductor lands on and used the matching ampacity column.
• Chosen the overcurrent device after the conductor, and verified it protects that conductor under 690.9.
• Checked voltage drop on the long runs and documented the target you designed to.
• Confirmed the equipment grounding conductor and any parallel sets are sized to match the final circuit.

Sizing errors that fail plan check

The same handful of mistakes drives most conductor corrections. None of them are subtle once you know to look.

Sizing from operating current instead of Isc. On source circuits, using the module's operating current skips the 125% short-circuit basis entirely and undersizes the wire from the first line.

Forgetting the second 125%. Applying the short-circuit factor and then treating that as the final number, without the continuous-current step, leaves the conductor short on stacked jobs.

Skipping derating. Picking a conductor whose printed ampacity clears the current, then never correcting for a 60C-plus rooftop conduit or a full raceway. This is the classic bounce.

Ignoring terminal ratings. Claiming the 90C ampacity of a conductor when the breaker terminals are rated 75C. The terminations, not the wire, set the ceiling.

Choosing the breaker first. Sizing overcurrent protection before the conductor, then discovering the device no longer protects the wire. The DOE has backed automated permitting tools like SolarAPP+ precisely because consistent, correct calculations move projects through review faster, and conductor math is one of the checks those tools enforce.

Copying a value across editions. Reusing a factor from an older NEC cycle after the jurisdiction adopted a newer one. Always confirm the edition your local AHJ has adopted before you trust a number.

Designing conductors in PVCAD

Doing this sequence by hand across dozens of circuits is where errors creep in, especially when a module swap ripples through every downstream conductor and device. That is the case for putting the calculation inside the drawing. PVCAD, an AutoCAD plugin from PVComplete, handles wire and conductor sizing and generates NEC-compliant construction documents for projects up to about 5 MW, so the ampacity, derating, and overcurrent logic stay tied to the same model that produces your plan set. When the array changes, the conductor schedule updates with it instead of drifting out of sync with the single line. For teams that keep getting the same plan-check corrections, moving the math into the design tool removes the manual re-entry step that most of those corrections trace back to. The DOE Solar Energy Technologies Office frames faster, more reliable design and permitting as a direct lever on installed cost, and consistent conductor sizing is a small but real part of that.

Frequently asked questions

How do you size PV wire?

Size PV wire in two stages. First calculate the circuit's maximum current under NEC 690.8, using 125% of short-circuit current on source circuits and 125% of continuous current on output circuits. Then pick a conductor whose ampacity, after you derate it for ambient temperature and conduit fill under Article 310, still exceeds that current, and match the overcurrent device to the result.

What is the 125% rule in solar?

There are actually two 125% factors, and PV work uses both. One inflates a source circuit's current to 125% of the module short-circuit current to cover irradiance above test conditions, and the other is the continuous-load factor that sizes conductors and overcurrent devices at 125% of the continuous operating current. NFPA builds both into Article 690, and on many source circuits they stack.

What is the difference between a PV source circuit and an output circuit?

A source circuit is the wiring from the modules to the first combining or aggregation point, and it is sized from short-circuit current. An output circuit carries the combined current from that point onward, and it is sized from continuous operating current. Because the sizing basis differs, mislabeling a circuit is a fast way to get the current calculation wrong. See how output tracks conditions in the DOE overview of PV performance.

Does the NEC set a maximum voltage drop for solar?

No. The NEC treats conductor voltage drop as informational guidance rather than a mandatory limit, so plan check will not reject a design on voltage drop alone. It still matters economically, because drop is lost energy, and many designers target roughly 1 to 2 percent per segment as a rule of thumb. The DOE homeowner guide touches on how wiring losses affect delivered output.

Why does conduit fill reduce a conductor's ampacity?

Conductors in a shared raceway heat each other, so once more than three current-carrying conductors run in the same conduit, Article 310 applies an adjustment factor below 1.0 to the base ampacity. Combined with a high rooftop ambient temperature, the usable ampacity can drop well under the printed value, which is why derating has to happen before you compare against the 690.8 calculated current.

Can PVCAD size conductors to NEC?

Yes. PVCAD handles wire and conductor sizing and produces NEC-compliant construction documents for projects up to about 5 MW, keeping the ampacity and overcurrent logic tied to the same model that generates your plan set. When the array changes, the conductor schedule updates with it. Faster, more consistent design is a goal the DOE Solar Energy Technologies Office connects directly to lower installed cost.

Sources

1. NFPA - Understanding NFPA 70 (NEC)
2. NFPA 70 (National Electrical Code) - product page
3. DOE - Solar Energy Technologies Office
4. DOE - Solar Performance and Efficiency
5. DOE - Streamlining Solar Permitting with SolarAPP+
6. DOE - Homeowner's Guide to Going Solar
7. PVComplete - PVCAD (wire sizing and NEC construction documents)
8. PVComplete - Design Services