Code Change Summary: A new code section was added to address underground wiring methods at a commercial repair garage.

For quite some time there has been a gray area in the NEC^® as far as the wiring methods permitted under slab at a commercial repair garage. Section 511.4(A) provided rules on the wiring methods allowed “in” Class I locations while 511.7 provided rules on wiring methods and equipment used “above” a Class I location. What about under slab?

In the 2017 NEC^®, a new section was added which requires threaded rigid metal conduit (RMC) or intermediate metal conduit (IMC) for underground wiring. For these methods, Table 300.5 requires a minimum cover depth of zero inches where installed under a building.

A new exception was added that aligns with similar language in 514.8 which allows specific nonmetallic raceways under slab as long as certain conditions are met.

The exception allows PVC conduit, Type RTRC conduit, and Type HDPE conduit where buried under not less than 2 feet of cover. There’s one stipulation; if PVC, RTRC, or HDPE conduit is used, threaded RMC or threaded steel IMC must be used for the last 2 ft. of the underground run to emergence or to the point of connection to the aboveground raceway, and an equipment grounding conductor must be included to provide electrical continuity of the raceway system and for grounding of non–current-carrying metal parts.

Below is a preview of the NEC^®. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2017 NEC^® edition of NFPA 70.