Okay, this is a great question and a common challenge with embedded vertical conduit runs. You've correctly identified the crux of the issue: supporting conductors within a long, inaccessible vertical run. Let's break down your options and address your specific questions

Understanding the Problem & Code:
300.19 is King

You're absolutely right,

300.19 is the governing rule for vertical conductors. Failure to comply can lead to serious issues down the line (literally – conductors can pull out of terminations due to their own weight!). Accessibility Matters

Your assessment of the (1), (2), and (3) methods being impractical is spot-on given the embedded nature of the conduit.

"Other Approved Means"

This is where you need to get creative and, most importantly, get approval from the Authority Having Jurisdiction (AHJ). This phrase means exactly what it says: You need to present your solution, explain how it provides adequate support, and obtain their buy-in before installation.

Addressing Your Questions


Manufactured Solutions/Products



Wire Mesh Grips (Conduit Support)

While not specifically designed for this embedded scenario, some wire mesh grips might be adaptable. These grips are typically used to support cables externally. The challenge is to find one that can fit inside your conduit, securely grip the conductors without damaging them, and be installed reliably during the pour.

Cable Support Systems (For vertical cable runs)

Companies that manufacture cable support systems for tall buildings (think high-rise apartments or large data centers) may have experience or components that can be adapted to your situation. These companies often deal with supporting heavy data or power cables in vertical shafts.

Specialty Conductor Supports

You might have to engage a manufacturer to develop a custom solution. This can be expensive, but it could be the only way to get a truly reliable and compliant system for such a long run.2.

Previous Solutions and Experiences



Splice Boxes (Embedded Access)

While you've mentioned this isn't ideal, it's still worth revisiting the feasibility. Strategically placed, accessible splice boxes (with appropriate covers rated for the environment) can provide access points for conductor support. If you can minimize the number of boxes and make them relatively unobtrusive, it might be the most straightforward solution. Think about how often those boxes will need to be accessed once the system is in service, and design your solution accordingly.

Pour in Place Supports

Some specialty Electrical suppliers make supports that can be attached to the conduit before the pour and then the conductors are supported as they are pulled into the conduit.

Pre-Installed Conductor Support

Some installers will install a series of plastic or metal loops into the conduit, spaced according to the table in 300.19. You would have to engineer the installation of these loops to ensure that the wires can be easily pulled into the conduit.

High-Friction Conductors

While not an ideal solution, some manufacturers offer conductors with a special coating that increases friction against the conduit wall. This can reduce the overall load on the support mechanism, but you must get engineering data from the manufacturer to prove that it meets the requirements of 300.19. And still, you need to have a reliable method for supporting the conductors at the top of the run.
3.

Messenger Supported Cable



Theoretical Soundness

The concept is valid in principle. Using a messenger wire provides a robust supporting member.

Article 396 (Messenger Supported Wiring)

You're right to check Article 396, but it primarily addresses exposed messenger cable systems. However, the principles of load calculation and support are still relevant.

Challenges



Lashing

Typical lashing is designed for horizontal applications. Gravity will work against you in a vertical run. You'd need a very secure lashing method (possibly custom) or a system of clamps designed for vertical messenger cables.

Cable Weight and Sag

Even with a messenger, the conductors themselves will still have some sag between the supports. This needs to be factored into your calculations.

Space Constraint

The messenger wire, the conductors, and the securing method all take up space within the conduit. You need to ensure you're not exceeding the conduit fill limits.

Quadraplex/Triplex Cable

This is an interesting idea. These cables are designed for overhead distribution, so they inherently have a built-in messenger.

Approval is Key

The AHJ will need to be convinced that this is an appropriate use of this type of cable.

Termination and Splice

You'll need to carefully consider how to terminate and splice these cables, especially if you have multiple branch circuits within a single quadraplex cable.

Sheath

Make sure the cable sheath is suitable for a wet location, since the conduit will likely have condensation.

Recommendations and Next Steps


Engage the AHJ Early

This is the most important step. Schedule a meeting with the AHJ to discuss your problem and proposed solutions. Get their feedback and requirements before you start any detailed design.2.

Consult with a Qualified Engineer

Hire a structural or electrical engineer to perform load calculations and design a suitable support system. They can help you determine the required strength of the messenger wire, the spacing of the supports, and the appropriate lashing or clamping method.3.

Consider a Hybrid Approach

You might need to combine multiple techniques. For example, you could use a messenger wire with strategically placed wire mesh grips to provide additional support.4.

Document Everything

Keep meticulous records of all your calculations, designs, and communications with the AHJ. This will be essential for inspection and future maintenance.5.

Pull Testing and Mockups: If possible, create a mockup of your proposed support system and conduct pull testing to verify its effectiveness.This is a challenging but not insurmountable problem. With careful planning, engineering, and, most importantly, collaboration with the AHJ, you can develop a compliant and reliable solution for supporting your vertical conductors. Flag for review