You're right to be concerned about the "nut shim" solution. It's a band-aid fix and highly unlikely to meet code or manufacturer's installation requirements. Here's a breakdown of the issues and a potential path forward

Your concerns are valid:
Expansion/Contraction

PVC expands and contracts significantly with temperature changes. In the Northeast, the temperature swings can be quite drastic. Without accounting for this, the conduit will exert force on the strut, clamps, and potentially the weatherhead and cable tray, leading to:

Stress on Conduit

Potentially cracking or breaking the PVC, especially at connections.

Damage to Supports

The force can bend or break the unistrut, loosen fasteners, and weaken the overall structure.

Compromised Weatherhead Seal

Movement can damage or loosen the weatherhead connection, allowing moisture to enter.

Unsafe Condition

Conduit and wiring systems are designed to withstand certain mechanical stresses. Unaccounted-for expansion/contraction can lead to failure and create a hazardous situation.

"Fixed Point" Argument

The engineer's argument that an expansion fitting isn't needed because the conduit doesn't go to a "fixed point" is flawed. The unistrut clamps are, for all intents and purposes, a fixed point. They restrict longitudinal movement of the conduit at that location. It does not matter that the other end goes to a cable tray, the point of the expansion fitting is to allow the conduit to move in the strut clamp.

Inadequate Solution

The "nut shim" method is simply not a designed solution. It's ad-hoc, unreliable, and unlikely to be recognized by any standards or codes. It also creates a situation where the conduit can slide laterally within the clamp, which is not desirable. It would also not be maintainable. When someone comes back to add a new circuit, it is unlikely that they will remove the "nut Shim".

Manufacturer's Instructions

You're correct that manufacturers' instructions typically specify tightening the clamp, not loosening it with shims. Deviating from these instructions can void warranties and raise serious liability concerns.

Code Compliance

NEC 352.44 requires expansion fittings to compensate for thermal expansion and contraction where the length change is expected to be 1/4 inch or greater. The temperature in the northeast easily meets this requirment.

Why the engineer might be pushing back



Cost

Expansion fittings add cost to the project, both in terms of materials and labor.

Aesthetics

Some engineers might consider expansion fittings visually unappealing.

Lack of Understanding

Some engineers may not fully appreciate the forces involved with PVC expansion and contraction, or they may overestimate the flexibility of the system.

Your Next Steps


Document Your Concerns (In Writing)

This is critical. Send a formal email (or letter) to the engineer, outlining your concerns, referencing the relevant sections of the NEC (352.44), and explaining why the "nut shim" solution is inadequate. State that it does not meet manufacturer instructions. Explain that if the conduit fails and hurts someone that you documented that there may be a problume with their design.
2.

Calculate Expansion

To strengthen your argument, calculate the expected expansion/contraction of the PVC conduit based on the length of the run and the temperature extremes in your region. Many online calculators can help with this. This provides concrete evidence of the need for expansion fittings.3.

Propose Acceptable Solutions (Again)

Reiterate your recommended solutions:

Expansion Fittings

This is the most reliable and code-compliant solution. Specify the correct type and size of expansion fitting.

Flexible Conduit Clamps (e.g., B-Line B2417-4ZN or Equivalent)

These clamps allow for movement while still providing secure support.4.

Escalate if Necessary

If the engineer continues to resist, escalate the issue to your supervisor, the project manager, or the authority having jurisdiction (AHJ). It's essential to ensure the electrical system is safe and code-compliant.5.

Request a Site Meeting

If possible, request a site meeting with the engineer, contractor, and the AHJ to discuss the issue in person. This can help facilitate a better understanding and a more collaborative solution.

Key Argument to Emphasize

*

Safety and Code Compliance

The primary goal is to ensure a safe and code-compliant installation. The NEC and industry best practices require accounting for thermal expansion and contraction in PVC conduit.

In summary, your concerns are well-founded. Document everything, calculate the expansion, propose acceptable solutions, and be prepared to escalate if necessary. The safety and long-term reliability of the electrical system are at stake. Flag for review