You're on the right track with leaving extra flex and adjusting the boxes as the metal goes up. Here's a breakdown of the best approach, considering conduit/flex, ribbed metal, and alignment

Key Considerations:
Code Compliance

Always adhere to local electrical codes and the National Electrical Code (NEC). Specifically, pay attention to box support, conductor fill, and conduit/flex support.

Watertightness

Ribbed metal walls are often part of a weather-resistant system. Penetrations must be sealed properly to prevent leaks. This is crucial.

Durability

Consider the long-term durability of your installation. Think about vibration, movement of the metal panels due to temperature changes, and potential for physical damage.

Panel Thickness

Know the thickness of the ribbed metal. This affects the type of connectors and box extensions you might need.

Steps and Best Practices


Planning & Layout



Coordinate

Talk to the metal wall installers before you start. Understand their layout and attachment methods. Knowing where the ribs will be placed precisely is crucial.

Mark Locations

Clearly mark the desired locations of the boxes on the framing before the metal goes up. Use a laser level or plumb bob to ensure vertical alignment if necessary.

Offset Considerations

Deliberately plan to offset the boxes slightly from the exact center of where you think the ribs will land. This gives you wiggle room. A small offset is better than being directly on a rib.2.

Box Mounting & Conduit/Flex



Support Structure

The boxes must be securely mounted to a structural element (stud, purlin, etc.), not just to the ribbed metal. The metal is cladding, not a structural support. Use appropriate mounting brackets or straps. Consider using adjustable box hangers if you need more flexibility in placement before the metal goes up.

Conduit/Flex Choice



Flexible Metal Conduit (FMC or "Flex")

The most common and practical choice. It's flexible enough to allow for minor adjustments.

Liquidtight Flexible Metal Conduit (LFMC)

Recommended if there's any chance of moisture exposure inside the wall. This is often worth the extra cost for added protection.

Rigid Metal Conduit (RMC) or Intermediate Metal Conduit (IMC)

Rarely used for final connections to boxes behind ribbed metal due to the difficulty of making adjustments. If you use RMC or IMC for the main runs, transition to flex near the boxes.

Conduit/Flex Runs



"Saddle" Bends

For short runs of conduit/flex that must cross a rib, use "saddle" bends (a series of small bends) to contour the conduit around the rib. However, try to avoid crossing ribs if possible.

Slack is Key

Leave plenty of extra flex (at least 12-18 inches, depending on the rib height and spacing) at the box. This gives you the freedom to move the box up, down, left, or right to avoid the rib. Temporarily secure the extra flex so it doesn't get in the way.

Connectors



Straight Connectors

Use straight connectors to the boxes. Angle connectors can create unnecessary stress on the flex and the box.

Proper Fit

Ensure the connectors are properly sized for the conduit/flex you are using. Tighten them securely. For LFMC, use liquidtight connectors.3.

Installation with Metal Panels



Communication

Stay in constant communication with the metal installers. Let them know you might need to adjust box positions slightly.

Pilot Holes

If the metal needs to be drilled for the box, drill a pilot hole smaller than the knockout size in the box. You can then enlarge it to the correct size as needed.

Adjustment

As the metal panels are being installed:

Identify Conflicts

Immediately check if the box location interferes with a rib.

Adjust Box Position

Loosen the box mounting screws and carefully shift the box slightly to avoid the rib. Use the extra flex you left to your advantage.

Secure Box

Once the box is clear of the rib, re-tighten the mounting screws securely.4.

Sealing and Weatherproofing



Backer Plates/Gaskets

Consider using a backer plate made of metal or durable plastic between the box and the metal wall. This provides a flat surface and helps to seal the penetration. Use a closed-cell foam gasket between the backer plate and the metal to ensure a weathertight seal.

Sealant

Use a high-quality, paintable, exterior-grade sealant (e.g., polyurethane or silicone) around the box where it meets the metal wall. Be generous with the sealant, but apply it neatly. Follow the sealant manufacturer's instructions.

Watertight Boxes/Covers

Consider using weatherproof boxes and covers, even if the inside of the wall is technically "dry." It's cheap insurance against leaks.5.

Final Inspection



Check Support

Double-check that all boxes are securely supported by the structure.

Check Sealing

Verify that all penetrations are properly sealed against moisture.

Check Conduit/Flex

Ensure that the conduit/flex is properly supported and that there are no sharp bends or kinks.

Check Fill

Make sure the wire fill in the boxes and conduit/flex does not exceed code limits.

Important Notes



Avoid Cutting Ribs

Never cut or notch the ribs of the metal panels to accommodate a box. This compromises the structural integrity and watertightness of the wall.

Oversized Boxes

Consider using slightly larger boxes (e.g., deep 4" square boxes) to provide more room for wiring and connections, especially if you're using conduit.

Document Everything: Take photos of the installation process, especially before the metal panels are fully installed. This can be helpful for future maintenance or troubleshooting.By following these guidelines and communicating effectively with the metal installers, you can achieve a safe, code-compliant, and durable electrical installation behind ribbed metal walls. Always prioritize safety and consult with a qualified electrician if you have any doubts. Flag for review