The Code rules
The relevant Code language isn't in Art. 348, but in Sec. 710-4(a) covering acceptable aboveground wiring methods for over 600V wiring. The permitted methods include "rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, cable trays, busways, cablebus, other identified raceways, [emphasis supplied] or open runs of metal-clad cable suitable for the use and purpose."
For many years, makers of EMT have sought to have their product allowed for medium-voltage installations by authorities having jurisdiction (AHJ) as one of the "identified" raceways. In accordance with the definition in Art. 100, this would have meant that EMT was "recognizable as suitable for the specific purpose." The former UL guide card information stood in the way, until last summer when the 1995 Green Book came out, effectively leaving the decision in the hands of each individual jurisdiction.
With the publication of this latest supplement, EMT has clearly been identified as suitable for use over 600V. You can use it unless a jurisdiction goes through a formal rule making process to prohibit these applications. Of course, the installation must comply with any relevant requirements in both Art. 348 and Art. 710.
Code Making Panel (CMP) 13 has never been able to agree on more specific wording in Sec. 710-4(a), although recent actions certainly seem to demonstrate a lack of consensus in favor of over-600V applications. In the 1993 cycle, CMP 13 entertained a public comment opposing the use of EMT over 600V from one of its members (E. E. Carlton). He cited adverse results from testing in the late 1960s. The comment related to proposals that had been submitted that would have expressly allowed EMT. In each case, the panel unanimously rejected wording that would have expressly allowed EMT over 600V.
On the final vote, however, the voting NEMA alternate representative (David Barnard) advised that although he was supporting the panel action, NEMA did not accept Mr. Carlton's substantiation. Mr. Barnard questioned the reliability of the documentation on the testing cited in that comment. Meanwhile, the panel had earlier rejected a proposal to delete the words "other identified raceways" as excessively vague. The submitter noted that Sec. 300-2(a) permitted Chapter 3 wiring methods over 600V "where specifically permitted elsewhere in the Code," and the cited phrase was hardly specific.
The final result of all this was to leave the back door open just enough to allow UL to revisit the long-standing "Green Book" prohibition against use over 600V. UL brought the question up at the 1994 meeting of its Electrical Council, which consists of inspection authorities in major jurisdictions. UL wanted to know what they would be looking for to determine if EMT were "identified" for this purpose, because it had reopened this question.
None of the inspectors brought up major objections or the Carlton comment, and UL decided to go ahead and change the guide card. UL also noted that the mechanical properties of EMT were at least equal to the armor on Type MC cable, which has been recognized for use over 600V since the 1940 NEC. In addition, some large jurisdictions in the West had been allowing it for some time with no reported problems. The most recent change reflects UL's confidence in the suitability of this wiring method for over 600V applications.