300.4(E)

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eds

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May be in a bind on a install. Outdoor pavilion with tongue and grove finished ceiling, (2) 2x4 placed flat and stacked on each other, 3/4 wood decking on top of that, then roof tin. Lights and receptacles to be installed in tongue and grove ceiling, flush with ceiling finish. I can achieve the 1 1/2 clearance, but the concealment is what's tripping me up. Am I misinterpreting something, it first talks about exposed and concealed locations, then seem to disallow a concealed location altogether. Rigid seems to be a option, but again not concealed, and the exception doesn't specifically list a malleable iron box.

2020 NEC
 
I would have 2 1/4” from the bottom of the wood roof decking to the back side of the 4 square box. Metal roof is secured to the wood roof decking. Boxes and conduit will be concealed between the tongue and groove ceiling and the wood roof decking.

Thinking out-loud now, is standing seem and metal corrugated the same? I don’t believe they are.
 
I think this code references commercial roofs. (although it doesn't specify that) I used to run EMT in the channel between the top of the steel truss and the corrugated tin decking. The jobs always passed inspection and no strapping of the conduit was needed. It was really a quick way to run conduit. One time, one of the long screws penetrated the EMT I couldn't get get my wire through it. A few years later (2008 NEC) Art. 300.4 (E) was added. It doesn't clearly state that you can do what you're trying to do, but IMO, if you maintain you're 1 1/2" clearance from the lowest point of the decking to the cable, I would accept it. I think this would be an AHJ call.
 

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Rick 0920 said:
I think this code references commercial roofs. (although it doesn't specify that) I used to run EMT in the channel between the top of the steel truss and the corrugated tin decking. The jobs always passed inspection and no strapping of the conduit was needed. It was really a quick way to run conduit. One time, one of the long screws penetrated the EMT I couldn't get get my wire through it. A few years later (2008 NEC) Art. 300.4 (E) was added. It doesn't clearly state that you can do what you're trying to do, but IMO, if you maintain you're 1 1/2" clearance from the lowest point of the decking to the cable, I would accept it. I think this would be an AHJ call.
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I agree what you described was the intent of the change back then, but the wording might actually fit other construction designs even if not intended.
 
kwired said:
I agree what you described was the intent of the change back then, but the wording might actually fit other construction designs even if not intended.
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I would disagree, given the high number of rooftop PV systems getting installed and the use of 4" lag bolts to attach them, I think the code also applies to residential installations.
 
acrwc10 said:
I would disagree, given the high number of rooftop PV systems getting installed and the use of 4" lag bolts to attach them, I think the code also applies to residential installations.
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The code only applies to installations using "metal corrugated roof decking" and does not apply to any other type of roof installation. There were rejected Public Inputs for the 2023 code cycle that would have made 300.4(E) apply to all types of roof decks. Those were rejected for lack of any technical substantiation that says there are issues with any other type of roof decking.

However the similar rule for luminaires found in 410.10(F) is going to apply to light fixtures installed on the bottom of any type of roof deck. In the 2020 this section also specifies "metal corrugated roof decking".
(F) Luminaires Installed in or Under Roof Decking.
Luminaires installed in exposed or concealed locations under roof decking where subject to physical damage, 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 luminaire.
Exception: The 38 mm (1 1⁄2 in.) 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.
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acrwc10 said:
I would disagree, given the high number of rooftop PV systems getting installed and the use of 4" lag bolts to attach them, I think the code also applies to residential installations.
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In a residential application, even if you have metal corrugated roofing, there is usually a wood truss design. Electricians install the NM cable on the top of the lowest part of the truss and hardly ever get near the roof with their cables. I've seen several homes in the midwest with metal roofs and this code section was never referenced. In the question from the OP however, there is very little space between the bottom of the decking and the finished ceiling. With it being an outdoor pavilion and he can maintain the required 1 1/2" clearance, I would get special permission from the AHJ prior to beginning the work.
 
don_resqcapt19 said:
The code only applies to installations using "metal corrugated roof decking" and does not apply to any other type of roof installation. There were rejected Public Inputs for the 2023 code cycle that would have made 300.4(E) apply to all types of roof decks. Those were rejected for lack of any technical substantiation that says there are issues with any other type of roof decking.

However the similar rule for luminaires found in 410.10(F) is going to apply to light fixtures installed on the bottom of any type of roof deck. In the 2020 this section also specifies "metal corrugated roof decking".
Click to expand...

interesting top of luminaire, not top of box
 
eds said:
May be in a bind on a install. Outdoor pavilion with tongue and grove finished ceiling, (2) 2x4 placed flat and stacked on each other, 3/4 wood decking on top of that, then roof tin. Lights and receptacles to be installed in tongue and grove ceiling, flush with ceiling finish. I can achieve the 1 1/2 clearance, but the concealment is what's tripping me up. Am I misinterpreting something, it first talks about exposed and concealed locations, then seem to disallow a concealed location altogether. Rigid seems to be a option, but again not concealed, and the exception doesn't specifically list a malleable iron box.

2020 NEC
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I think you are misreading that rigid isn't allowed to be concealed. The exception refers to the entirety of 300.4(E). In my reading that means RMC and IMC are exempt from all the language therein. Typical NEC language that leaves gaps in logic. But I can't think of any good reason that RMC and IMC should be exempt from the first sentence but not the second.
 
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