Class II, Div 1, group F, pull boxes/LB's

[dennis5150]

The contractor asked if he could use NEMA 4x enclosures or Form 7 LB's as pull points (pull boxes) and sealed all of the conduit openings with seaing putty (duct seal). No arcing, sparking or heat generating devices, terminations, or splices would be inside, these are simply pull boxes or LB's, and the cables would simply pass through. It would seem that this would comply with 502.10 (4), and 502.15 (1) and FPN, but would appreciate any input. Thanks.

 

[rbalex]

The form 7s are fine and forget the putty.

In Class II, Division 1, the 4X boxes are more problematic. Technically, they are not dustight, and duct seal won't change that; they would also need something like a NEMA 3 rating. See Table 110.20 in NEC 2008 and later.

On top of that, the raceway connections to the enclosures must be fully threaded (bosses)[502.10(A)(4)]; double-locknut and locknut bushing won't do. [502.30(A)]

 

[dennis5150]

I have reviewed table 110.20, in the past and have noticed that it shows an "X " in the windblown (outdoor) dust... column for NEMA 3 and 4, 4X and circulating (indoor) dust... column for NEMA 4, and 4X. Is your preference for NEMA 3 based on the FPN at the bottom of the table? I took the omission of NEMA 4 and 4x as an inadvertant error, which is curious, since the NEMA250 and UL50 standards both define 3, 4 and 4x the same with respect to dust tightness as well as the body table 110.20. I agree that the duct seal will not change that, but the objective here is to prevent dust migration through the conduit since there is no ignition source (arcing,sparking, or heat generating devices) inside the pullbox or LB and contains only nonincendive field wiring. So if duct seal is used at the conduit openings on a NEMA 3, 4, or 4X, transmission of dust from one location to another would not occur. The Form 7LB's have threaded bosses so wrenchtight connections prevent ingress of dust, but the gasket on the cover may still possibly allow some dust, but again the duct seal would prevent any migration.
Am I missing something???

 

[rbalex]

You may wish to go back to your NEMA 250 and UL 50 and see which enclosures are permitted to be marked dusttight - other "descriptions" not withstanding; i.e., Table 110.20 FPN is correct.

I apologize for misreading where the duct seal was intended to be used; nevertheless, it is hard to see it as "permanent" per 502.15(1). It's a judment call I wouldn't want to make.