You're right, that section of the NEC can be confusing. Let's break down NEC 300.4(E) and why it seems contradictory

Understanding the Intent

The overarching goal of NEC 300.4(E) is to prevent physical damage to wiring and equipment installed under metal corrugated roof decking. Metal corrugated roofing can be sharp, potentially abrasive, and subject to movement due to temperature changes. Also, screws going through the metal would violate the integrity of a cable.

Dissecting the Two Parts of the Rule1. "A cable, raceway, or box, installed in exposed or concealed locations under metal corrugated sheet roof decking shall be installed so that there is not less than 1-1/2 in. measured from the lowest surface of the roof decking to the top of the cable, raceway, or box."

Exposed or Concealed under Metal Corrugated Sheet Roof Decking

This part addresses situations where the wiring is below the roof decking, but still somehow accessible for installation or maintenance (exposed) or hidden (concealed). The 1 1/2" rule is intended to protect the cable, raceway, or box from being damaged.

The key point

Even though "concealed" is mentioned, this portion refers to installations where the wiring is under the decking, but not within the corrugations themselves. Think of it as the space between the underside of the roof and the finished ceiling, or an open area where you can still see the underside of the decking.2.
"A cable, raceway, or box shall not be installed in concealed locations in metal corrugated sheet decking." "Concealed locations in metal corrugated sheet decking:" This is the critical distinction. This part prohibits installing wiring within the corrugations (the spaces between the ribs of the metal roof). If it's within the corrugations, it is concealed. Why the prohibition? Physical Damage

The thin metal edges of the corrugations can easily damage wiring.

Accessibility

Wiring in these spaces is extremely difficult to access for inspection, maintenance, or repair.

Screws

It encourages people to secure it with screws going through the metal decking.

The Apparent Contradiction Explained

The confusion arises from the different uses of the word "concealed." The first part refers to wiring that might be concealed under the roof decking (e.g., above a ceiling), but where the underside of the roof decking is still the uppermost surface in that space. In these situations, the 1 1/2" clearance is required to prevent damage if something comes into contact with the roof decking. The second part completely prohibits wiring hidden within the corrugations of the metal roof itself.

In Summary

NEC 300.4(E) essentially says:1. If you're running cable/raceway/boxes under metal corrugated roofing (but in a way that's still accessible or semi-accessible), keep it at least 1 1/2" away from the lowest point of the roof.2. Do NOT, under any circumstances, install cable/raceway/boxes inside the corrugations of the metal roof.Why This Matters Safety

Prevents electrical hazards caused by damaged insulation and short circuits.

Code Compliance

Avoids costly rework during inspections.

Longevity

Ensures the electrical system functions reliably for its intended lifespan.

Real-World Example

Imagine you're wiring a commercial building with a metal corrugated roof. Permitted

You run conduit along the underside of the roof, suspending it so the top of the conduit is at least 1 1/2" below the lowest point of the corrugations.

Prohibited

You tuck NM-B cable into the corrugations to hide it, or to avoid having to install conduit.You're right that the wording could be clearer, but hopefully, this explanation makes the intent of NEC 300.4(E) more understandable.

Always Consult with an Expert

Electrical codes are complex. If you're unsure about any aspect of the NEC, consult with a licensed electrician or electrical inspector in your area. They can provide guidance specific to your situation and ensure code compliance. Flag for review