You are correct in your understanding of NM-B cable and its limitations. Let's break down the situation and the code considerations

Current Code (NEC) on NM-B in Conduit:
Dry Locations

NM-B is permitted to be installed in conduit in dry locations.

Wet Locations

NM-B is not permitted to be installed in conduit in wet locations. The inside of a conduit in a wet location is considered a wet location itself.

Has anything changed?

The fundamental rules regarding NM-B in wet locations have not changed significantly in recent NEC editions. The NEC continues to restrict NM-B cable from being installed in wet or damp locations or where exposed to weather.

Specific Scenario: Exterior Wall, Enclosed Porch, Metal Box

Let's analyze your specific application:1. Coming Through Exterior Wall

The point where the cable penetrates the exterior wall is likely considered a damp or potentially wet location, especially if the wall is exposed to rain or moisture.2.

Running Down Wall in Enclosed Porch

An enclosed porch, while offering some protection, can still be considered a damp or potentially wet location depending on local climate conditions and how well the porch is sealed from the elements. Condensation, leaks, and humidity can all contribute to this classification. If the porch is considered a dry location, then NM-B could be run in conduit down the wall to the switch. However, the entry point through the exterior wall remains a concern.3.

Metal Box

A metal box itself isn't necessarily the determining factor. The issue is the environment inside the conduit leading to the box.

Verdict

Considering these points, there is high potential your installation is not code-compliant due to the possible damp or wet conditions the NM-B will be exposed to.

Solution: Using a Junction Box and XHHW-2

Your suggestion of using a junction box and transitioning to XHHW-2 is a valid and common solution to address this issue. Here's how it works and why it's compliant:1. Install a Junction Box in the Attic (Dry Location)

Place an approved junction box in the dry attic space near the exterior wall penetration.2.

Connect NM-B to XHHW-2

Run the NM-B cable to the junction box. Use appropriately sized and UL-listed wire connectors (e.g., wire nuts or similar) to connect the NM-B conductors to individual XHHW-2 wires.3.

Run XHHW-2 in Conduit

Install conduit from the junction box, through the wall, and down the wall of the porch to the metal switch box. Pull the XHHW-2 wires through the conduit.4.

Terminate XHHW-2 in Switch Box

Connect the XHHW-2 wires to the switch in the metal box.

Why this is Compliant



NM-B Stays in Dry Location

The NM-B cable is entirely within the dry attic space.

XHHW-2 for Wet/Damp

XHHW-2 is rated for wet locations and can be installed inside conduit in those environments.

Important Considerations



Grounding

Ensure proper grounding throughout the entire circuit, including the metal box, conduit, and equipment grounding conductor (EGC).

Conduit Fill

Follow the NEC's conduit fill requirements to avoid overcrowding the conduit. Use the appropriate size conduit for the number of XHHW-2 wires you are using.

Box Fill

Ensure the junction box and the switch box are large enough to accommodate all the wires, connectors, and devices according to the NEC's box fill calculations.

Connectors

Use connectors approved for the specific types of wire and conduit you are using.*

Professional Advice

When in doubt, consult with a licensed electrician to ensure your installation meets all applicable codes and safety standards.

In summary, your proposed solution of using a junction box and XHHW-2 is the correct way to handle this situation and ensure compliance with the NEC. Flag for review