You've hit on a crucial point and are right to be cautious. Let's break down why 705.95(B) typically does not apply to the neutral conductor on the supply side (source side) of the PV disconnect, especially regarding tap conductors.Here's the reasoning: 705.95(B) Specifically Ties to the Equipment Grounding Conductor (EGC)

The core of 705.95(B) allows a reduction in neutral conductor size based on the size of the EGC. The rationale is that a ground fault condition is the limiting factor, and the neutral's ampacity can be reduced if the EGC can safely handle the fault current.

No EGC on the Source Side

As you correctly point out, there is no EGC on the source side of the PV disconnect. The EGC's purpose is to provide a low-impedance path back to the grounded conductor (neutral) and the grounding electrode system from equipment enclosures. The PV source conductors (including the neutral, if present) are isolated from the premises grounding system until they reach the inverter.

Tap Conductor Rules and Overcurrent Protection

Tap conductor rules in the NEC (e.g., Article 240.21) generally require the tap conductors to be protected against overcurrent at the point where they connect to the larger conductors. In the context of PV systems, this means the tap conductors from the PV array to the PV disconnect must be sized to handle the expected maximum current and be protected by an overcurrent device (fuse or breaker). Reduction based on 705.95(B) would violate this principle because the neutral isn't actually protected.

Fault Scenarios on the Source Side

A fault on the source side of the disconnect (e.g., a short between the positive and neutral conductors before the disconnect) would not be cleared by the EGC of the premises wiring system. The OCPD on the load side won't likely trip, the fault will be backfed into the PV array and the disconnect might not be able to interrupt that current. Faults on the DC side need to be addressed with specific DC overcurrent protection and equipment ratings.

Why 705.95(B) CAN be Used on the Load Side

On the load side (output) of the inverter, the inverter's output conductors connect to the premises wiring system. This is where 705.95(B) potentially becomes relevant. Here, the EGC runs with the other output conductors, providing a fault current path. The neutral conductor sizing is then based on the calculated neutral load or the size of the EGC, whichever is larger.

In Summary

No, downsizing the neutral conductor based on 705.95(B) is generally not permitted on the source side of the PV disconnect (including tap conductors). The neutral conductor must be sized according to the tap conductor rules and other applicable sections of the NEC to handle the expected current and be properly protected by an overcurrent device. You MUST factor in the continuous nature of the PV source, meaning that you can't apply reduction factors that require the load to be non-continuous.

Important Considerations



Authority Having Jurisdiction (AHJ)

Always consult with your local AHJ for clarification and their specific interpretation of the code. They have the final say.

Manufacturer's Instructions

Carefully review the manufacturer's instructions for the PV modules, inverters, and disconnects. They may have specific requirements for conductor sizing and overcurrent protection.*

Engineering Judgment: In complex situations, it's best to consult with a qualified electrical engineer who has experience with PV systems. They can perform the necessary calculations and ensure that the system meets all applicable code requirements.In short, stick to standard wiring practices and NEC rules for tap conductors, and don't apply 705.95(B) to the source side of the PV disconnect. You are absolutely right to be careful and seek clarification. Safety is paramount. Flag for review