NEC Solar PV
Article 690 governs solar PV installations under the NEC, with its own conductor sizing logic layered on top of the general ampacity tables.
NEC Article 690 is the dedicated section governing solar photovoltaic systems, and it layers PV-specific requirements on top of the general conductor ampacity rules covered elsewhere in the code — rather than replacing them.
Article 690 distinguishes between the PV source circuit (conductors between modules within an array, carrying string-level current) and the PV output circuit (conductors between the array and the inverter or another distribution point, potentially carrying combined current from multiple strings). Each has its own sizing considerations, and combining currents at a combiner point is exactly where undersizing mistakes happen if the combined current isn't recalculated from scratch.
| Step | What happens |
|---|---|
| 1 | Determine module Isc from the datasheet |
| 2 | Apply the applicable safety multiplier (see the 1.25×Isc article — NEC's continuous-load provisions can result in an effective ~1.56× factor once stacked) |
| 3 | Select conductor size meeting that minimum ampacity, correctly temperature- and grouping-derated for a rooftop or conduit installation |
| 4 | Verify voltage drop separately against the DC-specific tighter limit |
| 5 | At any combiner point, recalculate from the combined current of all paralleled strings — not the per-string current |
PV source circuit conductors routed on or near a rooftop under panels regularly see surface temperatures well above standard ambient — sometimes 30°C+ above the surrounding air temperature on a hot, sunny day. Applying the base ampacity table without a proper rooftop temperature correction is one of the most common PV-specific undersizing mistakes, since it's easy to forget that the panels themselves are creating a hot microclimate for the cable running beneath them.
Article 690 also has its own requirements for equipment grounding and, in some configurations, functional grounding of the DC system — distinct from the general grounding electrode conductor sizing covered in NEC Table 250.66 (see the dedicated grounding article in this series).
Size your PV source and output circuits in the NEC PV/Solar DC Cable Sizing calculator.