PV Source Circuit Conductor Sizing per NEC 690.8

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

This is NOT one running chain of multipliers. 690.8(A) sets the circuit's maximum current, and then 690.8(B) makes you run TWO independent checks against that current and size for whichever one demands more conductor. Running them as a single chain is the most common way this calculation goes wrong, because it applies the continuous-duty 125 percent and the derates to the same number.

Step 0, common to both checks:

• Start with the module short-circuit current (Isc) from the module nameplate
• Multiply by 1.25 per NEC 690.8(A)(1) (the irradiance-edge-of-cloud factor). Call the result the maximum circuit current

Check 1, per 690.8(B)(1):

• Multiply the maximum circuit current by another 1.25 for continuous duty (the standard 125 percent for any conductor carrying current more than 3 hours)
• Compare that against the conductor's ampacity from NEC Table 310.16 with NO correction and NO adjustment applied

Check 2, per 690.8(B)(2):

• Take the maximum circuit current as-is, with no second 1.25
• Compare that against the conductor's ampacity AFTER temperature correction per NEC Table 310.15(B)(1)(1) and AFTER conduit-fill adjustment per NEC Table 310.15(C)(1) if more than 3 current-carrying conductors share a raceway

Size for whichever check the conductor fails, or the one it passes with less margin. Select conductor type and size 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 + 33 degrees C rooftop adder (NEC 310.15(B)(2)) for conduit less than 7/8 inch above the surface = 80 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 the 80 degree C operating temperature derived above: 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

Check both conditions of 690.8(B) and take the larger requirement; they are alternatives, never multiplied together. 690.8(B)(1): 12.75 x 1.25 = 15.94 A, compared against the conductor's 90 C ampacity BEFORE any correction or adjustment, which for 10 AWG is 40 A. Passes. 690.8(B)(2): 12.75 A compared against the corrected ampacity, 40 x 0.41 x 0.80 = 13.12 A. Passes, with little margin. 10 AWG is compliant here.

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:

• Less than 7/8 inch (22 mm) above rooftop: 33 degrees C adder (60 F)
• 7/8 inch or more above rooftop: no adder under NEC 2020/2023 310.15(B)(2)

Note these are temperature DIFFERENCES, so they convert at 1.8 degrees F per degree C, not by the absolute-temperature formula. The older four-tier table (33/22/17/14 C, that is 60/40/30/25 F) was the NEC 2014 and earlier rule and still applies in jurisdictions on those editions; check which edition yours has adopted.

• 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 7/8 inch, which under the current rule drops the adder to zero outright (the 3.5 inch figure techs still quote is a leftover tier from the retired four-tier table, and it buys you nothing extra now); 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. For the 690.8(B)(1) check the two factors combine to 1.56 times Isc, but that check runs against the conductor's ampacity BEFORE correction and adjustment factors. The 690.8(B)(2) check uses 1.25 times Isc against the CORRECTED ampacity. Take the larger of the two; never apply 1.56 and the derates to the same number.
• 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)