NEC requirements for wiring solar PV

Scope and Code Foundation

Solar PV installs fall under NEC Article 690, with supporting rules pulled from 705 (interconnected power production), 250 (grounding), 310 (conductors), and 110 (general requirements). Know which edition your AHJ has adopted before you pull wire. The 2023 NEC tightened rapid shutdown, dc arc-fault, and equipment grounding language, and a 2020 jurisdiction will read differently on the same roof.

Every PV job starts with three questions: is it dc-coupled or ac-coupled, is it grid-interactive or stand-alone, and where does it land on the service? The answers drive conductor sizing, overcurrent placement, and disconnect count. Get those wrong at the planning stage and the rework is expensive.

Conductor Sizing and Circuit Current

PV source and output circuit currents are calculated from Isc, not operating current. Per NEC 690.8(A)(1), multiply module Isc by 125 percent to get maximum circuit current. Then apply the 690.8(B) factor: another 125 percent for continuous duty, which stacks to 156 percent of Isc before you even pick a conductor.

Ambient temperature on a hot roof is brutal. Rooftop conduit runs within 7/8 inch of the surface add 33 deg C to outdoor design temp per 310.15(B)(2), and conductors above 30 deg C derate fast. Size for the worst-case cell, not the nameplate. USE-2 and PV Wire are the standard module interconnects; they are sunlight resistant and rated for wet locations and 90 deg C or 150 deg C.

• Module interconnects: USE-2 or PV Wire, single conductor, per 690.31(C).
• Inside buildings, transition to a wiring method from Chapter 3 (EMT, MC, etc.) per 690.31(D).
• Minimum 10 AWG is common for string circuits; verify ampacity after derating.
• All conductors 4 AWG and smaller must be color coded or marked at terminations.

Disconnects, Overcurrent, and Rapid Shutdown

NEC 690.13 requires a readily accessible PV system disconnect, grouped and labeled. For grid-tied inverters, 690.15 covers the equipment disconnect, which often lives at the inverter itself. On the ac side, 705.20 and 705.25 govern the interconnection disconnect. If the point of connection is load-side, the 120 percent rule in 705.12(B)(3) decides whether the busbar can take the backfeed.

Rapid shutdown under 690.12 is non-negotiable on buildings. Inside the array boundary, conductors must drop to 80 V or less within 30 seconds of initiation. Outside the array, 30 V within 30 seconds. Module-level shutdown devices (MLPE) are the common solution; verify the listing says the exact inverter model you are installing, or the system is not code-compliant.

Label the rapid shutdown initiator at the service disconnect in reflective red, and make sure the first responder can reach it without a ladder. AHJs are failing jobs for initiators hidden behind meter combos.

Grounding and Bonding

Article 690 Part V and Article 250 work together. Every exposed non-current-carrying metal part of the PV system, racking, modules, combiner enclosures, must be bonded. WEEBs (washer electrical equipment bonds) and listed lay-in lugs are the standard tools. The equipment grounding conductor (EGC) is sized per 690.45 and 250.122, based on the OCPD ahead of it, not the conductor ampacity.

Grounding electrode conductors for dc systems follow 690.47. Most modern inverters are transformerless and ungrounded (functionally grounded), so the traditional dc GEC is replaced by the inverter's internal arrangement plus an ac-side EGC sized per 250.66. Read the inverter manual: it dictates whether you land a dc GEC or not.

• Bond every module frame to rail using listed hardware, no improvised star washers.
• Irreversibly mark the EGC if it is not green, per 250.119.
• Use stainless or tin-plated copper in direct rooftop contact, never bare aluminum on a steel rail.

Working Space, Labels, and Signage

Working clearances per 110.26 apply to every disconnect, inverter, and combiner you install. 30 inches wide, 36 inches deep, 6.5 feet high, measured from the enclosure face. On a roof that means planning array setbacks so a service tech has a place to stand. If it does not meet 110.26, it does not pass.

Signage is where jobs fail inspection most often. 690.13(B), 690.54, 690.56, and 705.10 each require specific placards. They must be reflective, engraved or UV-stable, and permanently affixed. Handwritten Sharpie on a P-Touch label does not fly in any jurisdiction worth working in.

Pre-print your label kit before the truck leaves the shop. Missing a "PHOTOVOLTAIC AC DISCONNECT" placard on final inspection turns a one-hour trip into a two-day delay.

Interconnection and Commissioning

Supply-side taps under 705.11 bypass the 120 percent busbar limit but require an OCPD within 10 feet and careful coordination with the utility. Load-side connections under 705.12 are more common on residential retrofits; verify the main bus rating, main breaker size, and backfed breaker position (opposite end of the bus from the main) before you commit.

Before energizing, megger the dc circuits, verify polarity at every string, confirm rapid shutdown operation, and document open-circuit voltage against the expected Voc at site temperature. Commission under 690.4(C) means a licensed person signs off, and most utilities now require a witnessed inspection before PTO.

1. Verify AHJ inspection sign-off and utility net meter agreement.
2. Measure Voc and Isc per string, log against design values.
3. Confirm RSD shutdown at 30 seconds, verify with non-contact voltage.
4. Torque every lug to spec, mark with witness paint.
5. Hand over the monitoring login and the single-line drawing.