Common mistakes when wiring solar PV

Undersized conductors on the DC string

The fastest way to fail an inspection is sizing PV source and output conductors like a standard branch circuit. NEC 690.8(A)(1) requires the maximum circuit current to be calculated as 125% of the module short-circuit current (Isc), and then 690.8(B) adds another 125% for continuous duty before any temperature or conduit fill corrections. That is 156% of Isc before you even open the ampacity tables.

Run the math in that order every time. Skip the stacking and you end up with #10 THWN-2 where #8 belongs, especially on modern 60-cell modules pushing 13 to 14 amps Isc.

• Isc x 1.25 (690.8(A)(1)) = adjusted current
• Adjusted current x 1.25 (690.8(B)) = minimum conductor ampacity
• Apply 310.15(B) temperature correction for rooftop conduit, typically 40C to 60C adder
• Apply conduit fill derate from 310.15(C)(1) if more than 3 current-carrying conductors

Grounding and bonding shortcuts

Module frames are not grounded by being bolted to a rail. NEC 690.43 requires an equipment grounding conductor sized per 690.45, and the bonding path through the racking has to be listed for that purpose. WEEBs, bonding jumpers, and manufacturer grounding lugs are not interchangeable, check the racking listing.

On the DC side, the EGC runs with the circuit conductors. Do not share a ground bar with the AC service unless the inverter listing and system topology allow it. Transformerless inverters change the rules, and 690.41 now permits ungrounded arrays with functional grounding, but the EGC path for fault current is still mandatory.

If the racking manufacturer specifies a torque spec for the grounding hardware, use a calibrated torque wrench. A loose WEEB will read fine at commissioning and fail two winters later when galvanic corrosion eats the contact.

Rapid shutdown misunderstood

NEC 690.12 has evolved in every code cycle since 2014. The 2020 and 2023 editions require module-level shutdown, meaning conductors inside the array boundary must drop to 80V or less within 30 seconds. A string-level contactor at the combiner does not satisfy this, you need MLPE (optimizers or microinverters) with a listed initiator.

The initiator itself is commonly missed. It must be readily accessible at the service disconnect, labeled per 690.56(C), and the label needs the specific diagram required by the adopted code year. Inspectors are checking label graphics now, not just presence.

Overcurrent protection on the AC side

The 120% rule in NEC 705.12(B)(3)(2) is where back-fed breaker installs go wrong. The sum of the main breaker and the PV breaker cannot exceed 120% of the busbar rating, and the PV breaker must land at the opposite end of the bus from the main. Center-fed panels do not qualify.

When the 120% rule does not work, you have options, but they must be documented on the plans.

1. Supply-side tap per 705.11, ahead of the main service disconnect
2. Line-side connection with a fused disconnect sized to the PV output
3. Main breaker derate, replacing the 200A main with a 175A to free up bus capacity
4. Load-side subpanel with its own calculated bus rating

Each path has a labeling requirement under 705.10 and 690.56. Skipping the permanent plaque at the service is a guaranteed correction notice.

Conduit, wire management, and the rooftop environment

Rooftop conduit temperatures are the single most underestimated variable. NEC Table 310.15(B)(2)(a), now referenced through 310.15(B) in the 2023 NEC, adds up to 33C to the ambient for conduit within 13mm of the roof surface. That pushes a 90C conductor into derate territory fast.

USE-2 and PV Wire are not interchangeable with THWN-2 inside conduit in all cases. PV Wire is required for exposed single-conductor runs between modules, but once you transition into EMT or LFMC, you switch to THWN-2 or RHW-2. Document the transition point at the first junction box.

Mount the rooftop combiner or transition box on standoffs. An inch of air gap under the enclosure keeps the internal temperature 10 to 15 degrees lower and extends the life of every terminal inside it.

Labeling, signage, and the paperwork that fails jobs

NEC 690.13(B), 690.54, 690.56, and 705.10 each require specific labels with specific wording. Generic "PV SYSTEM" stickers do not meet the requirements. The maximum system voltage, short-circuit current, operating current, and operating voltage all belong on the DC disconnect label per 690.53.

Engraved phenolic or UV-stable reflective labels are the standard. Inkjet labels fade inside one summer and trigger a callback on the next annual inspection. Keep a labeling kit on the truck with the current code-cycle language preprinted, and verify which NEC edition your AHJ has adopted before you fabricate.

• DC disconnect: Vmax, Imax, Voc, Isc (690.53)
• AC point of connection: "PV system disconnect" or equivalent (690.13)
• Service equipment: permanent plaque showing PV is present (705.10)
• Rapid shutdown: red label with the adopted-cycle diagram (690.56(C))