300.4E Revision - 2023

Merry Christmas


Senior Member
There's a proposed revision to 300.4E to clarify that is using malleable iron boxes with RMC, that they don't have to comply with the rule regarding spacing underneath metal roof decking. It looks like someone submitted a PI that would have allowed it if a "heavy duty" box was used, and the committee didn't like that language as it isn't clear what a heavy duty box is So instead they went with the term malleable iron. This would mean that sheet metal boxes and the cheaper die cast conduit bodies would not be allowed, but the other heavier types would be. I like this allowance but I think "malleable iron" may not be a sufficiently clear definition. If I am for some reason using aluminum or stainless, I can't use a thick wall stainless or aluminum body? I wonder if anyone has suggestions on a better choice of language. Would it be proper to say that the heavier duty ones use "drop forged" manufacturing process, while the die cast ones don't? Does UL (in its now obsolete whitebook) have something that can be used to clearly differentiate the types of boxes and conduit bodies that could be used instead of just the term "malleable iron?"

(E) Cables, Raceways, or Boxes Installed in or Under Metal-Corrugated Roof Decking.
A cable, raceway, or box, installed in exposed or concealed locations under metal-corrugated sheet roof decking, shall be installed and supported so there is not less than 38 mm (11⁄2 in.) measured from the lowest surface of the roof decking to the top of the cable, raceway, or box. A cable, raceway, or box shall not be installed in concealed locations in metal-corrugated, sheet decking–type roof.
Informational Note: Roof decking material is often repaired or replaced after the initial raceway or cabling and roofing installation and might be penetrated by screws or other mechanical devices designed to provide “hold down” strength of the waterproof membrane or roof insulating material.
Exception No. 1: Rigid metal conduit and intermediate metal conduit, with malleable iron fittings or boxes, shall not be required to comply with 300.4(E).
Exception No. 2: The 38 mm (11⁄2in.) spacing is not required where metal-corrugated sheet roof decking is covered with a minimum thickness 50 mm (2 in.) concrete slab, measured from the top of the corrugated roofing.


Code Historian
Electrical Design
Looking at my old 2016 whitebook
Junction and pull boxes are (BGUZ)
They use a term "cast-metal" that might be better.
Conduit fittings are (DWTT) nothing jumps out at me