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PV Source Circuit Conductor Sizing per NEC 690.8

Reference Compliance Updated Jun 19, 2026 ยท Solar

Why this matters

A PV source-circuit conductor that is undersized for the source-circuit current carries above-ampacity current any time the array delivers above-nameplate output. Above-ampacity current degrades the insulation thermally; the failure shows up 6 to 10 years post-install as a melted MC4 connector, a charred junction box, or a roof fire. NEC 690.8 sets the calculation method; the calculation is straightforward once the multipliers are correctly applied. Most field errors are missed multipliers, not arithmetic.

The calculation steps

NEC 690.8(A) and 690.8(B) together establish the required ampacity of a PV source-circuit conductor in five sequential steps:

  1. Start with the module short-circuit current (Isc) from the module nameplate
  2. Multiply by 1.25 per NEC 690.8(A)(1) (the irradiance-edge-of-cloud factor)
  3. Multiply by another 1.25 per NEC 690.8(B)(1) for continuous-duty (the standard 125 percent for any conductor carrying continuous current more than 3 hours)
  4. Apply temperature-correction factors per NEC Table 310.15(B)(2)(a) based on rooftop or conduit ambient
  5. Apply conduit-fill derate per NEC Table 310.15(C)(1) if more than 3 current-carrying conductors share a raceway

The result is the minimum required conductor ampacity. Select a conductor type and size with at-least that ampacity from NEC Table 310.16 or the manufacturer ampacity data.

Worked example - rooftop string

A typical residential string:

  • Module Isc: 10.2 A (Q-Cell 400 W reference)
  • 690.8(A)(1) factor: 10.2 x 1.25 = 12.75 A
  • 690.8(B)(1) factor: 12.75 x 1.25 = 15.94 A
  • Rooftop ambient assumption: 47 degrees C ambient + 30 degrees C rooftop adder (NEC 310.15(B)(2)(a)) for conduit on roof less than 13 mm above surface = 77 degrees C operating
  • Conductor selected: 10 AWG USE-2 or PV wire, 90 degree C insulation
  • 90 degree C copper ampacity at 30 degree C ambient: 40 A (Table 310.16)
  • Temperature correction at 77 degree C: 0.41 (Table 310.15(B)(1)(1))
  • Conduit fill (4 source-circuit conductors): 0.80 (Table 310.15(C)(1))
  • Adjusted ampacity: 40 x 0.41 x 0.80 = 13.12 A

13.12 A is less than the 15.94 A requirement. The 10 AWG conductor fails. Bump to 8 AWG: 90 degree C ampacity 55 A, adjusted 55 x 0.41 x 0.80 = 18.04 A. 18.04 A exceeds 15.94 A. 8 AWG passes.

This is why residential PV string conductors are 8 AWG more often than the napkin calculation suggests. The 47 degree C rooftop ambient assumption (used in California and most southern climates) and the conduit-on-roof adder dominate the math.

The rooftop ambient adder

NEC 310.15(B)(2)(a) (now 310.15(B)(2) in 2023) gives the adder for raceways and cables exposed to direct sunlight on or above rooftops:

  • 0.5 inch or less above rooftop: 33 degrees C adder (90 F)
  • 0.5 to 3.5 inches above rooftop: 22 degrees C adder (60 F)
  • 3.5 to 12 inches above rooftop: 17 degrees C adder (40 F)
  • More than 12 inches above rooftop: 14 degrees C adder (35 F)
  • Cables not in a raceway, on rooftop: no adder, but the cable's own insulation rating governs

A conduit run sitting flat on a south-facing roof in Phoenix, with a 47 degree C ambient, sees 47 + 33 = 80 degrees C operating temperature. That kills conductor ampacity.

Strategies to reduce the adder: mount the conduit on a strut so the conduit clears the roof by at least 3.5 inches; route the conduit on the north slope; use free-air cable runs (PV wire on the array side) instead of conduit until the cable reaches a wall penetration.

The 47 degree C question

NEC does not specify a national ambient temperature; the AHJ does. ASHRAE 99th-percentile design dry-bulb temperatures by city are the typical source. Common AHJ values:

  • Phoenix, Las Vegas, Palm Springs: 47 to 49 degrees C
  • Los Angeles, San Diego, Sacramento: 37 to 40 degrees C
  • Atlanta, Dallas, Houston: 35 to 37 degrees C
  • Chicago, Denver, Seattle: 30 to 33 degrees C

Use the local value; do not default to 30 degree C from Table 310.16's column header.

PV wire vs THWN-2 vs USE-2

The conductor type matters:

  • PV wire (per UL 4703): rated for 90 degree C wet, 105 to 150 degree C dry, sunlight-resistant, single-conductor. Used between modules and at the array side up to the first junction box.
  • USE-2: rated 90 degree C wet, sunlight-resistant, suitable for direct-burial. Acceptable for module interconnection in some manufacturer specifications.
  • THWN-2: rated 90 degree C wet, 90 degree C dry, in raceway only. Used inside conduit from the array junction box to the inverter or combiner.

Do not run THWN-2 unprotected on a rooftop; the insulation will UV-degrade within 18 months. PV wire or USE-2 is required for any exposed run.

Source-circuit vs output-circuit

NEC distinguishes:

  • Source circuit: conductors between modules and between strings and the first piece of equipment (combiner or inverter input). Sized per 690.8(A)(1) and 690.8(B)(1).
  • Output circuit: DC conductors from the combiner to the inverter. Sized per 690.8(A)(2) - the sum of the source-circuit currents.

A 4-string array with 10.2 A per string has source-circuit conductors sized for 10.2 A times the multipliers; the output-circuit conductor between the combiner and the inverter is sized for 40.8 A (4 x 10.2 A) times the multipliers. Different conductors, different calculations.

Common errors

  • Designer forgets the second 1.25 factor. The 690.8(A)(1) and 690.8(B)(1) factors stack; the total is 1.56 times Isc, not 1.25.
  • Designer uses module Imp (operating current) instead of Isc. Imp is the operating current at maximum power; Isc is the short-circuit current and is roughly 5 to 8 percent higher. NEC 690.8 explicitly calls for Isc.
  • Designer omits the rooftop temperature adder. The 47 degree C ambient calculation alone is not the rooftop temperature; add the conduit-on-roof correction.
  • Designer uses Table 310.16 ampacity without temperature correction. Table 310.16 columns are at 30 degree C ambient. Operating-temperature ampacity is reduced significantly above 30 degrees C.

Documentation for permit

The plan set carries:

  • Module manufacturer cut sheet with Isc highlighted
  • Conductor calculation table showing each step of the 690.8 math
  • AHJ-stated ambient design temperature reference
  • Selected conductor type, size, insulation rating

The conductor calculation is the second-most-cited permit RFI after the 120 percent rule. Show the work.

References

  • NEC 2023 Section 690.8 (PV circuit sizing and current calculations)
  • NEC 2023 Table 310.15(B)(2) (rooftop temperature adders for raceways and cables)
  • NEC 2023 Table 310.15(B)(1)(1) (ambient temperature correction factors)
  • NEC 2023 Table 310.16 (allowable ampacities for insulated conductors)
  • UL 4703 (Photovoltaic Wire standard)

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