You're right to question the inspector's call. The commentary is vague and seemingly misapplied. Let's break it down

Understanding the Inspector's Concern (Possibly)

The inspector likely sees the installation as being exposed to physical damage, leading to the 300.4 violation. Here's the likely thought process: Low Height

The box is close to the ground in a flower bed.

Potential Damage

This makes it vulnerable to gardening tools (shovels, weed whackers), foot traffic, or even just accidental impact.

PVC Conduit Concerns

The inspector might be extending the physical damage concern from the box to the PVC conduit feeding it, though that's a weaker argument.

Why You're Questioning the Call is Valid No Specific Box Height Requirement

You're absolutely correct. The NEC does not specify a minimum height for junction boxes in general.

300.4 Is Vague

300.4 talks about protecting conductors, raceways, and cables from physical damage. It doesn't explicitly mention boxes/enclosures.

"Subject to Physical Damage" is Subjective

This is the key point. It's up to the Authority Having Jurisdiction (AHJ, in this case, the inspector) to determine if the location makes the installation "subject to physical damage."

Schedule 40 PVC

While Schedule 80 PVC is generally tougher, the NEC doesn't always mandate it just because something might get hit.

Analyzing Your Points and the Code 300.5 Underground Installations

This section applies specifically to underground installations. While it mentions protection from damage for enclosures/raceways, that generally refers to underground damage (rocks, soil settling, etc.). It's less directly applicable to an above-ground box with an underground feed.

(D)(4) and Enclosure Damage Underground

Yes, 300.5(D)(4) addresses the protection of the enclosure when it is underground. Imagine a direct-buried metal box. If it's not protected, corrosion or physical damage could compromise the box and the wiring inside.

Conduit Type and Enclosure Damage (Above Ground)

The argument that the conduit type directly affects the enclosure damage risk is weak. If the box itself is vulnerable, changing the conduit doesn't really solve that problem.

How to Address This Issue1. Politely Ask for Clarification

The best approach is to speak with the inspector. Ask for a more specific explanation of why he considers the box to be subject to physical damage. You can ask the inspector what is the exact code they are referencing. A good inspector should be able to articulate the specific concern and reasoning.2.

Assess the Actual Risk

Honestly evaluate the flower bed situation. Is it likely that someone will accidentally hit the box with a shovel? Will children be playing nearby? If the risk seems low, politely point that out.3.

Possible Solutions (Negotiate)

If the inspector is firm, consider these options, in order of likely effectiveness and cost:

Guard

Install a physical barrier/guard around the box (e.g., a small fence, rocks strategically placed). This is often the easiest and most acceptable solution.

Raise the Box

If possible, raise the box slightly (if this is feasible, it might make the inspector happy).

Replace the Box

You could replace the plastic enclosure with a metal one.

Schedule 80 PVC (Maybe)

If the inspector is adamant about the conduit, you could change it to Schedule 80. However, emphasize that the box itself is the bigger concern.*

Relocate

Moving the box to a less vulnerable location is the most drastic (and likely most expensive) solution.4.

Appeal (If Necessary)

If you strongly disagree and can't reach a compromise, you have the right to appeal the inspector's decision through the proper channels (your local permitting office can tell you the procedure). But try to avoid this if possible.

In summary, the inspector's call is questionable due to the vagueness of 300.4 and the lack of a specific height requirement for boxes. Focus on understanding the specific perceived risk and finding a practical solution that addresses it. Flag for review