NEC 690.31 PV wiring methods and conductors

Once you have modules and an inverter chosen, the conductors that connect them have to satisfy NEC 690.31, the section that governs PV wiring methods. It decides what cable you can run where, when it must be inside conduit, and how the raceway gets marked. These are quiet requirements that trip up a surprising number of first-time permit sets.

Exposed cable: PV Wire and USE-2

The conductors from the modules to the first junction or combiner are often exposed to weather and sunlight, so they need a listed cable rated for that duty:

• PV Wire (Photovoltaic Wire): a listed single-conductor cable with heavy insulation, sunlight-resistant and rated for the wet, hot, inside-the-array environment. It can be used as free-air single conductors within the array.
• USE-2: a common alternative for grounded PV systems, also sunlight- and moisture-rated.

Ordinary THWN-2 building wire cannot be run exposed in free air the way PV Wire can — it belongs inside a raceway. This is the single most common conductor mix-up on residential plans.

When conduit is required

690.31(D) is emphatic about DC PV circuits inside a building: they must be contained in a metal raceway (like EMT) or a metal enclosure. The reasoning is firefighter safety — a responder cutting into a wall needs the PV DC contained and identifiable, not run as loose cable. Key placement rules:

• DC conductors transitioning from roof to interior should enter a metal raceway at the point they penetrate the building.
• The raceway and any junction boxes must be marked "PHOTOVOLTAIC POWER SOURCE" or "SOLAR PV DC CIRCUIT" at intervals and at turns/penetrations.

Readily accessible connectors and the 8-foot rule

690.31 also carries provisions about connectors and cable management. Connectors must be of a listed type, guarded or located so they are not a shock or fire hazard, and cabling must be secured and supported so it does not chafe against the roof or racking. Exposed cable that could be mistaken for something else must be managed and, where required, guarded from readily accessible contact.

Conductor sizing still applies

690.31 is about the method; the size still comes from ampacity and derating. Run your conductors through the temperature and conduit-fill corrections and check the result against the wire ampacity tables in 310.16. Rooftop conduit runs get hot, so the rooftop temperature adder can push you to a larger conductor than the raw current suggests.

What a reviewer looks for

The reviewer checks that exposed runs use a sunlight-rated cable, that indoor DC is in metal raceway, and that the PV warning labels appear where required. A plan that shows THWN-2 running exposed across a roof, or unmarked DC conduit inside the garage, will come back with corrections.

OneLine Studio is a design aid, not a permitting authority. Confirm the enforced NEC edition and any local wiring-method amendments with your AHJ, and have a licensed electrician or PE review, sign and stamp the set before you file or energize. For the broader set of drawings that surrounds these details, see what is in a solar permit set.

Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.