A new requirement in the 1990 NEC is being ignored by many installers and engineers. In some Code Books this change was not accompanied by the line in the margin and may be the cause of this. NEC 450-9 in the 1990 NEC was changed to require that transformers with ventilation openings requiring clearance from walls be labeled as such. NEC Section 110-3(b) requires that labeled and listed equipment be installed in accordance with the labeling and listing instructions. So, if a transformer is labeled to require that it be installed 4 or 6 inches from the wall then that is part of the listing and must be complied with. This requirement must be observed prior to doing any slab work or else penetrations may not allow for the required clearance. There are tansformers that are tested for installing against walls, and of course, will not be labeled to maintain clearances. The 1993 Code requires that the clearance information be applied under the marking requirements.