Overcurrent Protection, Disconnecting Means, and Wiring Methods

Overview

NEC Article 690 governs photovoltaic systems from the array to the point where power enters the rest of the building's electrical system. Two of its most examined topics are overcurrent protection (Part III) and disconnecting means (Part IV). Getting these right is not just a code requirement — it is what keeps installers, firefighters, and homeowners safe when something goes wrong.

Overcurrent Protection for PV Circuits (NEC 690 Part III)

PV systems generate current continuously whenever the sun shines. Unlike utility power, a PV source circuit cannot simply be switched off at a transformer — the array itself is the source. This makes proper overcurrent protection especially critical.

NEC 690.9 requires overcurrent protective devices (OCPDs) — fuses or circuit breakers — to be installed in each PV source circuit and PV output circuit unless the equipment is rated to handle the available fault current without a device. An important exception: a PV source circuit conductor is not required to have an OCPD when the conductor is sized at or above the maximum current that can flow from paralleled sources. In plain terms, if only one string feeds a conductor and no parallel fault path exists, a fuse may be omitted for that conductor — but verify this condition carefully with the specific equipment and array configuration before omitting any device.

OCPDs must be listed for use in PV systems. Standard residential breakers are not automatically acceptable. The device must be rated for DC voltage at or above the maximum PV system voltage, which can exceed 600 V or even 1000 V on larger systems. Fuses used in combiners must carry a DC voltage rating equal to or greater than the open-circuit voltage of the strings they protect.

The rating of an OCPD protecting a PV source circuit conductor is calculated from the short-circuit current of the module string. NEC 690.9 sets the minimum conductor ampacity and OCPD rating at 125% of the module's rated short-circuit current (Isc). Because modules are a continuous source, this factor is applied. Always check the module datasheet for the rated Isc before specifying fuse or breaker sizes.

Disconnecting Means (NEC 690 Part IV)

NEC 690.15 requires a disconnecting means for each PV system that allows the system to be isolated from all wiring systems, including the premises wiring and the utility. Disconnects must be:

• Accessible to service personnel and, in the case of rapid-shutdown-equipped systems, to emergency responders
• Grouped where practicable — NEC 690.15 limits the maximum number of disconnecting means per system; consult the current edition of the NEC for the specific count allowed, as this has been updated across code cycles
• Marked to indicate which source they disconnect
• Rated for the DC voltage and current of the circuit they interrupt; again, standard AC-only breakers are not permitted

A rapid shutdown system (NEC 690.12) is a related but separate requirement: it reduces voltage in array conductors on the roof surface quickly so that firefighters are not exposed to high voltage while working near the array. Rapid shutdown devices are distinct from the main disconnect but must be coordinated with it.

All disconnects must open all ungrounded conductors simultaneously and must be operable without exposing the operator to energized parts.

Safety During Installation and Maintenance

Before working on any part of a PV system — combiner box, inverter, or disconnect — apply lockout/tagout (LOTO) procedures. PV source circuits cannot be de-energized by switching alone if sunlight is present; cover the array with an opaque tarp or work at night/overcast conditions where possible. Even then, treat all conductors as energized.

Required PPE for PV DC work typically includes insulated gloves rated for the system voltage, safety glasses, and arc-flash protection as determined by a hazard assessment. Never work alone on energized PV circuits.

Wiring Methods for Roof-Mounted Arrays

NEC Article 690 permits specific wiring methods for PV source and output circuits based on location and exposure.

PV Wire is a single-conductor cable listed and labeled specifically for PV use. It is sunlight-resistant, moisture-resistant, and rated for the higher temperatures seen on rooftops. PV Wire is permitted for use in PV source circuits on the roof surface and in free air.

USE-2 (Underground Service Entrance cable, 90°C wet rating) has historically been used in PV source circuits on rooftops because of its sunlight resistance and wet-location rating. The NEC permits its use in PV source circuits exposed to the sun, but it must be sunlight-resistant and listed for PV use in the location — always verify the listing on the cable's label.

THWN-2 is a thermoplastic insulated wire rated for wet locations at 90°C. It is suitable for use inside conduit but is not sunlight-resistant when exposed, so it must be protected in conduit when run on or near the roof surface. It is commonly used for PV output circuit conductors in conduit.

MC Cable (Metal-Clad) listed for PV use and sunlight-resistant may be used in some PV applications. Standard MC cable without a PV listing is not permitted in PV source circuits on rooftops. Check that the cable carries the appropriate listing mark.

For direct burial between the array structure and the building, conductors must be rated for the burial condition — PV Wire and USE-2 are commonly acceptable; THWN-2 requires conduit for burial.

In attic spaces, conductors must be protected from physical damage and must meet ampacity requirements accounting for the elevated ambient temperatures common in attics. This often means conduit or conduit fill adjustments.

Key Code Habit

Always note the edition of the NEC adopted in your jurisdiction. Code language in Article 690 has changed significantly across the 2014, 2017, 2020, and 2023 editions. When citing a section on a permit application or inspection, confirm you are citing the adopted edition.