NEC Article 690 covers the installation and safety requirements for solar photovoltaic (PV) systems. It defines the components like arrays, modules, inverters, and disconnecting means, and outlines how circuits must be sized, protected, grounded, and labelled. The article applies to both stand-alone and interactive systems, with or without energy storage. It does not apply to large-scale utility PV power plants covered under other NEC provisions, such as Article 705.

Unlike other electrical systems, Article 690 addresses conditions unique to PV systems, such as voltage rise in cold weather, continuous backfeed from sunlight, and rapid shutdown.

Here is the summary of the main points found in NEC Article 690:

Scope and Applications

• Applies to solar PV systems, including arrays, inverters, charge controllers, and energy storage.

• Covers both stand-alone systems and utility-interactive systems.

• Large-scale utility plants are excluded and governed under Article 705.

Circuit Requirements

Voltage calculations

• NEC requires PV system voltage to be based on the lowest expected ambient temperature because solar modules produce their highest voltage in cold weather, and equipment must be rated for that worst-case condition. Cold weather increases open-circuit voltage (Voc).

• NEC Table 690.7 gives correction factors for crystalline silicon modules because they generate higher voltage in the same conditions. Example: if the lowest expected temperature is –10°C, multiply module Voc by 1.14 to determine maximum circuit voltage.

• This corrected voltage is used to size cables and other equipment so that equipment ratings aren’t exceeded in winter conditions.

Current calculations

• NEC requires PV currents to be based on module short-circuit current (Isc) × 125%.

• Conductors then must be sized at 125% of that current again (because PV circuits are continuous loads).

• The result is effectively a 156% multiplier:

• Example: if module Isc = 10 A,

• Step 1: 10 A × 1.25 = 12.5 A (design current)

• Step 2: 12.5 A × 1.25 = 15.6 A (minimum conductor ampacity)

Overcurrent and Protection

• PV source, output, and inverter circuits require overcurrent protection sized at 125% of the maximum current.

• Arc-fault protection is required for PV circuits operating at 80 V or higher.

• Ground-fault protection must isolate the faulted circuit and provide an indicator.

Disconnecting Means and Rapid Shutdown

• PV systems must have a readily accessible disconnect near the point of entry.

• The system can have no more than six disconnects.

• Rapid shutdown is required on buildings: controlled conductors must drop to 30 V or less within 10 seconds for firefighter safety.

Grounding and Bonding

• PV systems can be grounded or ungrounded, but equipment grounding conductors are mandatory in all cases.

• Metallic module frames, racking, and enclosures must be bonded to maintain electrical continuity and safety.

Marking and Labelling

• PV modules must be marked with Voc, Isc, max system voltage, and max power ratings.

• Raceway covers, junction boxes, and pull boxes must be labelled: “WARNING: PHOTOVOLTAIC POWER SOURCE.”

• Buildings with PV systems must include directories showing all disconnect locations and RAPID SHUTDOWN plaques.

Wiring Methods (690.31)

• All raceway and cable wiring methods listed in the NEC can be used for PV arrays if specifically listed for PV use.

• Because PV modules often reach 70–90°C or higher in direct sun, conductors must be listed for wet locations and have a temperature rating of at least 90°C, though 105°C and 150°C products are also common.

• NEC requires a sufficient length of cable to allow safe replacement for devices with an integral enclosure.

Permitted wiring types

• NEC allows PV wire, USE-2 cable, MC (metal-clad) cable, and raceways for PV source and output circuits.

• When to use each:

• PV wire is used outdoors for module interconnections and array wiring.

• USE-2 cable is a common choice for direct burial and outdoor runs in ground-mounted systems. It is UV- and moisture-resistant, but less flexible than PV wire.

• TC-ER cable (Tray Cable – Exposed Run) can be used outdoors in PV output circuits feeding utility-interactive inverters, provided the installation is in not-readily-accessible locations and the cable is secured at intervals not exceeding 6 feet. It must include an equipment grounding conductor within the cable jacket.

• MC cable is typically used inside buildings or at roof penetrations, since the NEC requires metallic protection for PV conductors indoors.

• Raceways (conduit) are used for longer runs inside buildings or between equipment. It offers the highest mechanical protection and compliance with NEC’s requirement for metal containment indoors. Raceways are required when PV conductors enter a building or are in accessible locations where physical protection is necessary. Still, they are not required for outdoor array wiring if the PV wire or USE-2 listed is used and properly supported.

Outdoor arrays

• A single-conductor PV wire is permitted for connections between modules in the array.

• USE-2 and PV wire are permitted in exposed outdoor locations for PV source circuits.

• Cable trays may be used if PV wire is supported at intervals not exceeding 12 inches and secured at intervals not exceeding 4.5 feet.

• This wire must be sunlight-resistant, moisture-resistant, and rated for at least 90 °C.

• It may be installed in cable trays or exposed raceways, but must be supported and secured at required intervals.

Inside buildings

• Once PV source or output circuits penetrate a building, NEC requires them to be in metal raceways, MC cable, or enclosures until the first accessible disconnect.

• If flexible metal conduit (FMC <¾ in.) or small MC cable (<1 in.) is used across joists, they must be protected by guard strips.

• Embedded conductors in roofing materials must be marked for continuous exposure to sunlight and weather.

Flexible cords

• Allowed only for tracking PV arrays that move to follow the sun.

• These cords must be hard-service, extra-hard usage, and sunlight- and water-resistant.

Small-conductor cables

• 16 AWG and 18 AWG conductors may be used for module interconnections if they are sunlight- and moisture-resistant.

Labeling

• All raceways, cable trays, junction boxes, pull boxes, and enclosures that contain PV conductors must be labelled:

• “WARNING: PHOTOVOLTAIC POWER SOURCE.”

• Labels must be reflective, permanent, red background with white text, and spaced ≤10 feet apart.

Storage Batteries

• Article 480 applies first: all batteries must meet the general NEC requirements for installation, ventilation, overcurrent protection, and maintenance access.

• Additional 690 provisions:

• In dwellings, battery systems should operate at 50 V or less, unless guarded.

• Disconnecting means are required for strings over 24 cells (48 V nominal) to allow safe servicing.

• Listed current-limiting devices (like fuses) must be installed close to the batteries to protect against short-circuit faults.

• Battery enclosures and racks must prevent accidental contact and be made of non-conductive or properly guarded conductive material.

At NNC, you can shop for PV wire, USE-2, MC cable, TC-ER, connectors, and battery cables that meet NEC 690 requirements for safe, code-compliant solar installations.