Dry Wall- Effectively Insulated or Grounded Wall?

[eemdude]

I have a 480V MCC that I am installing in electric room whose walls are composed of two layers of gypsum drywall. I am using Table 110.26 in NEC to consider working space in front of the MCC. In condition 2 of this Table, the code says to consider concrete, ceramic tile, and brick as grounded. I am trying to determine if a drywall installation is "effectively insulated". Can anyone point in the direction of how I can make this determination. Thanks

 

[davidbeach]

Yep, grounded. Put up a layer of 1/2 inch plywood and have an insulated surface.

 

[rbulsara]

I also can't see drywall qualifying as "effectively insulated". They often absorb moisture and can be conductive.

Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

Grounded.

 

[eemdude]

I will consider the wall as grounded. What is interesting is that I couldn't find any documentation searching the internet for several hours that even touched on this issues. I thought of some testing that I could do with a megger on the installation. Before doing the actual test, I megged at 1000Vdc a piece of drywall that I had in my garage (hot and humid). The drywall tested over 2 giga-ohms with both probes firmly embedded in the sheetrock. It megged over 2 gigs with the probes being only inches apart! There was a couple of reasons that I didn't do any further testing. One is that persons may invalidate the testing on the grounds that as you say conditions (moisture content) may change. I don't think that in my case it will, but that it still a valid argument. The other reason that I chose to call the drywall installation as grounded is that there are metal screws that attaches the dry wall to a metal stud frame. The metal wall stud frame is attached to the building's metal steel frame which is attached the building's ground grid.

 

[dpc]

If you consider it ungrounded, you can reduce clearance by what - 6 inches. No one will thank you for that. If you consider it grounded, additional clearance is required, which is generally a good thing. And you are covered if someone does something with the wall in the future that would increase its conductivity.