Since 2017, the NEC^® has required a wire type equipment grounding conductor for outdoor metal raceways supplying rooftop HVAC equipment supplied by metal raceways made up with compression-type fittings.

The idea is to avoid a common occurrence where a non-threaded raceway such as EMT is installed (serving as the wiring method and the equipment grounding conductor) and the fittings eventually come loose, compromising the effective ground-fault current return path. Having a wire type equipment grounding conductor inside the metal raceway helps to ensure that the effective ground-fault current path is maintained even if the raceway fittings loosen up over time.

The code language exempts Intermediate Metal Conduit (IMC) or Rigid Metal Conduit (RMC) from requiring a wire type equipment grounding conductor since both IMC and RMC are threaded metal raceways and less likely to come apart when jarred or tripped on, however, any IMC or RMC that has been cut and is coupled or connected with a compression fitting would be required to contain the wire type equipment grounding conductor.

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 2020 NEC^® edition of NFPA 70.

2020 Code Language:

440.9 Grounding and Bonding. Where equipment is installed outdoors on a roof, an equipment grounding conductor of the wire type shall be installed in outdoor portions of metallic raceway systems that use compression-type fittings.