Where in the NEC is it clearly stated how many extension rings can be added to an existing box. I've looked for this in article 314 but cannot find it clearly defined. I know that past practice is not to use more than one extension box. I've recently encountered some newer installations that have two and three extension rings added to existing 4x4x1.5" 1900 boxes for new circuits. Did the inspector miss it or am I out of the loop?