Solar PV Circuit Sizing Under NEC 690: Complete Source-Circuit Worked Example

The permit came back rejected: “PV source circuit conductors undersized per NEC 690.8.” The installer had sized the wire to the 125% continuous load rule they use for every other circuit. NEC 690.8 uses a different multiplier structure, and missing it means either a rejected permit or, worse, conductors that run hot every summer afternoon when the array is at peak output.

Here’s the complete calculation for a single PV source circuit, starting from the module nameplate and ending with a wire size that passes inspection.

NEC 690.8: Why PV Conductors Get a Double 1.25× Multiplier

Most branch circuits use one 1.25× multiplier for continuous loads (NEC 210.19(A)). PV source circuits use two separate 1.25× factors stacked on each other, for a combined 1.56× applied to the module’s short-circuit current (Isc). The two-step structure is in NEC 2023 § 690.8:

The first 1.25× accounts for extended irradiance conditions — modules routinely exceed their nameplate Isc under high-irradiance conditions such as cloud-edge diffusion (sometimes called the “edge-of-cloud” effect). The second 1.25× is the standard continuous-load conductor sizing factor. Source: NFPA 70 (NEC), 2023 edition, § 690.8.

The ampacity from this calculation is before any temperature correction or conduit fill derating. On a rooftop installation, temperature derating is rarely optional — conduit in direct sun on a dark roof routinely reaches ambient temperatures that cut ampacity by 20–25%.

The Worked Example: A 10-Module, 400W String

Given a PV source circuit with these module nameplate values:

• Rated power: 400W
• Short-circuit current (Isc): 10.25A
• Maximum power current (Imp): 9.81A
• Open-circuit voltage (Voc): 49.3V
• Maximum power voltage (Vmp): 40.8V
• 10 modules wired in series (one string, one source circuit)
• String Vmp: 40.8 × 10 = 408V DC
• 100 ft one-way conduit run on rooftop to inverter combiner
• Outdoor design ambient: 35°C

Step 1 — Maximum Circuit Current (690.8(A))

Step 2 — Minimum Conductor Ampacity Before Derating (690.8(B))

The conductor you select must have a derated ampacity at least 16.02A. For most standard branch circuits this is trivially met by 14 AWG (25A at 90°C per NEC 2023 Table 310.16). On a rooftop run, temperature derating changes the picture.

Step 3 — Rooftop Temperature Correction

NEC 2023 Table 310.15(B)(3)(c) adds a temperature adder to the outdoor ambient when conductors run in conduit within ½” of a rooftop surface: +17°C.

• Outdoor design ambient: 35°C
• Rooftop adder: +17°C
• Adjusted ambient: 52°C

From NEC 2023 Table 310.15(B)(1), the correction factor for a 90°C-rated conductor at 51–55°C ambient: 0.76.

THWN-2 is rated 90°C (dry) and is the standard conductor for PV source circuits in conduit. Using NEC 2023 Table 310.16 ampacity at 90°C column:

Step 4 — Confirm the Wire Size

14 AWG THWN-2 is the minimum-compliant conductor for this source circuit: derated ampacity of 19.0A exceeds the NEC 690.8(B) minimum of 16.02A, and the conductor is rated for the system voltage (600V for most residential PV; 1,000V or 2,000V DC-rated for string voltages above 600V).

String Voc at cold temperature must also stay within conductor and equipment voltage ratings. Low-temperature Voc correction (NEC 690.7): at −10°C with a temperature coefficient of −0.29%/°C:

10 modules: 54.3 × 10 = 543V DC — within standard 600V DC equipment ratings.

DC Voltage Drop: The Second Sizing Check

Temperature correction confirms conductor thermal adequacy. Voltage drop confirms energy efficiency. Use the operating current (Imp = 9.81A, not the 1.25× sized current) for voltage drop calculations — the multiplier is a safety factor, not the actual operating current.

K = 12.9 (copper resistivity constant), L = one-way run length in feet, CM = conductor circular mils.

The chart below shows voltage drop percentage vs. source circuit run length for both wire sizes, at the Imp operating current of 9.81A and 408V string voltage:

Data source: VD formula with K = 12.9 (copper), Imp = 9.81A, string Vmp = 408V, CM values from NEC 2023 Chapter 9, Table 8.

Where This Calculation Breaks Down

The worked example above covers a straightforward single-string, single-source-circuit installation. Several conditions change the calculation:

Parallel strings (combiner boxes): When multiple strings are paralleled at a combiner box, the output circuit carries the sum of all string currents. For four parallel strings with Isc = 10.25A each: combined Isc = 4 × 10.25 = 41.0A, max current = 41.0 × 1.25 = 51.25A, conductor minimum = 51.25 × 1.25 = 64.1A. The source circuit wires (string to combiner) stay at 16.02A minimum, but the output circuit (combiner to inverter) is now 64.1A minimum before derating.

NEC 2023 § 690.8(A) Exception: For specific equipment that limits current to a documented maximum and where the manufacturer provides a value lower than Isc × 1.25, that lower value can be used as the maximum circuit current. This exception applies to some microinverter and power optimizer systems. Check the equipment listing and manufacturer documentation — the AHJ will ask.

NEC 2026 changes: The 2026 NEC includes revisions to PV system provisions. If your AHJ has adopted 2026, verify the current article numbers and any changes to the conductor sizing multipliers before finalizing the design.

For related wire sizing work on the AC side of the inverter, see the discussion of ampacity tables and continuous-load derating for branch circuits. For long feeder runs where voltage drop dominates the sizing decision, the same calculation approach applies as shown in voltage drop for long wire runs.

The wire size calculator handles the NEC 690.8 calculation for standard source circuits, including rooftop temperature correction and voltage drop — enter the module Isc, string voltage, run length, and outdoor ambient, and it returns the minimum wire size for both the thermal and voltage drop constraints.