Electrical Design & Code Compliance

The NEC is updated on a three-year cycle (2017, 2020, 2023, etc.). Your local AHJ adopts a specific edition — not always the latest one. The edition your jurisdiction enforces determines which requirements apply to your installation. Always verify which NEC edition is in effect locally before finalizing a design.

Here are the primary NEC articles relevant to residential solar:

Additional articles may apply depending on your installation: Article 706 for energy storage systems, Article 480 for batteries, Article 240 for overcurrent protection, and various articles in Chapters 3 and 4 for wiring methods and equipment. We reference all applicable code sections in the plan sets we produce.

Correct wire sizing is fundamental to a safe and code-compliant installation. Undersized wire overheats; oversized wire wastes money and is harder to work with. The NEC provides a methodical process for determining the minimum wire size for each circuit segment.

DC circuits (panels to inverter)

For DC PV source and output circuits, wire sizing starts with the maximum circuit current. Under NEC 690.8, the maximum current for conductor sizing is calculated based on the short-circuit current of the modules, adjusted for conditions of use. The process involves several steps:

• Start with the module I_sc (short-circuit current) from the datasheet. For modules in parallel, multiply by the number of parallel strings.
• Apply the continuous duty factor. PV circuits are treated as continuous loads, so the conductor must be sized for at least 125% of the maximum current (NEC 690.8(B)).
• Apply temperature correction. Wire on a rooftop in direct sun can reach extreme temperatures. The conductor’s ampacity must be derated based on the expected ambient temperature using NEC Table 310.15(B)(1) correction factors. On rooftops, the ambient temperature used for derating includes an adder above the local design temperature to account for the roof surface heat (NEC 310.15(B)(2) for conduit on rooftops).
• Apply conduit fill adjustment. If more than three current-carrying conductors share a conduit, the ampacity is further derated per NEC Table 310.15(C)(1).
• Check voltage drop. While not a hard NEC requirement, keeping voltage drop under 2% on DC circuits (3% total for the combined DC and AC system) is industry best practice and recommended in NEC informational notes. Long conduit runs — especially for ground-mount systems — may require upsizing wire beyond the minimum ampacity requirement.

AC circuits (inverter to panel)

The AC output circuit from the inverter to your electrical panel is sized based on the inverter’s maximum output current, treated as a continuous load (125% factor). Temperature correction and conduit fill adjustments apply here as well, though AC runs are typically shorter and at lower temperatures than rooftop DC conduit. The wire must also be sized for the overcurrent protection device (breaker) rating.

Overcurrent protection devices (OCPD) — circuit breakers and fuses — protect conductors and equipment from damage due to currents exceeding their ratings. In a solar installation, OCPD is required on both the DC and AC sides of the system, with different rules for each.

DC overcurrent protection

Whether you need fuses or breakers on the DC side depends on your system configuration. A single string connected to a single-MPPT inverter may not require a separate DC OCPD because there’s no external source of fault current that could damage the string conductors. However, when two or more strings are connected in parallel, each string needs overcurrent protection to prevent backfeed from the other strings during a fault. NEC 690.9 governs DC OCPD requirements. The fuse or breaker rating is based on the module series fuse rating from the datasheet.

AC overcurrent protection and the interconnection method

On the AC side, the inverter’s output connects to your main electrical panel through a dedicated circuit breaker. The breaker size is determined by the inverter’s maximum output current (with the 125% continuous duty factor). But the breaker also has to comply with the interconnection rules in NEC 705, which limit the total current sources feeding a busbar.

Grounding and bonding are among the most misunderstood aspects of solar installations — and among the most common reasons for inspection failures. They serve different purposes, and the NEC requires both.

Common grounding details

• Racking bonding. Most racking systems use integrated bonding hardware (WEEB clips, lay-in lugs, or bonding washers) at rail splices and mount points. These must be installed correctly — skipping or improperly seating a bonding clip breaks the grounding path and will fail inspection.
• Panel frame grounding. Panel frames must be bonded to the racking system. Some racking clamp designs provide this bond through the clamp itself (listed for grounding); others require a separate bonding jumper or lug on each panel frame.
• EGC sizing. The equipment grounding conductor in the DC circuits is typically sized per NEC 250.122 based on the rating of the OCPD, or per NEC 690.45 for specific PV system configurations. This is specified in the plan set.
• Metallic conduit as a grounding path. Listed metallic conduit (EMT, rigid) can serve as an equipment grounding conductor if properly installed with listed fittings. However, many jurisdictions and inspectors prefer (or require) a separate copper EGC pulled through the conduit as well. We specify the requirement based on your AHJ’s preferences.

Rapid shutdown is a safety requirement designed to protect firefighters and first responders. When the rapid shutdown system is activated, conductors on or within the array boundary must be reduced to safe voltage levels quickly, so emergency crews can work on or near the roof without risk of electrocution from energized DC conductors.

NEC 690.12 defines the requirements. The specifics depend on which edition of the NEC your jurisdiction has adopted:

How different inverter types comply:

• Microinverters inherently comply — when the system shuts down, there are no high-voltage DC conductors on the roof because conversion to AC happens at each module.
• DC power optimizers comply because they reduce output voltage to a safe level (typically 1 V per optimizer) when communication with the inverter is lost.
• String inverters require additional module-level rapid shutdown transmitter/receiver devices installed at each panel. These add cost and complexity but allow you to use a string inverter while meeting the rapid shutdown requirement.

The rapid shutdown initiator is typically integrated into the inverter or the AC disconnect. When the initiator is triggered (either manually or by loss of grid power), it sends a signal to the module-level devices to shut down. The specific initiation method and equipment must be listed and labeled per the NEC.

Labeling is the most overlooked part of a solar installation and one of the easiest ways to fail an inspection. The NEC and your AHJ require specific labels at specific locations, and inspectors check them carefully. Missing or incorrect labels will result in a failed inspection even if the installation itself is perfect.

Key labeling requirements under NEC 690 include:

• Main service panel. A label indicating the presence of a PV system, its rated AC output, and the location of the AC and DC disconnects. This alerts electricians and first responders that the building has a secondary power source.
• AC disconnect. Labeled with the rated AC output current and voltage of the PV system.
• DC disconnect (if present). Labeled with the maximum DC voltage and current.
• Rapid shutdown initiator. Labeled with instructions for initiating and verifying rapid shutdown. The label format and content are specified in NEC 690.56(C).
• Conduit and junction boxes. DC conduit must be labeled with “WARNING: PHOTOVOLTAIC POWER SOURCE” at regular intervals. Junction boxes containing DC conductors must be labeled similarly.
• Ground-mounted system markers. If conduit runs underground between the array and the building, the route should be marked where it enters or exits the ground.

Labels must be durable (UV-resistant, weatherproof), legible, and use the correct color coding (typically red or orange background with white text for warnings). We include a complete labeling schedule in our plan sets so your electrician knows exactly what to install and where.

The electrical design is the most engineering-intensive part of a solar project, and it’s where mistakes are most consequential. Here’s what we deliver as part of the plan set:

Your electrician uses this plan set as their installation guide. They don’t need to design the system or look up code requirements — it’s all specified. Their job is to execute the plan accurately. If questions come up during installation, we’re available to clarify.