19.) The sealing of underground raceways or raceways that are exposed to different temperatures.
Although they are often overlooked, there are specific requirements in the NEC with regard to sealing underground raceways entering buildings or where condensation is created in raceways by the exposure to different temperatures within buildings or where a raceway passes from outdoor to indoor locations.
230.8 requires a service raceway entering a building or structure from an underground distribution system to be sealed in accordance with 300.5(G) (at either or both ends). Water can enter raceways through couplings or it can build up from condensation resulting from the exposure to different temperatures. How ever the water accumulates within the raceway, the intent of the requirement is to prevent water from entering the service equipment via the underground raceway system. A sealant such as duct seal or other types identified for use with the conductor or cable insulation, shield or other components must be used to seal the ends of raceway and it should not have deteriorating effect on the insulation.
300.5 provides requirements for underground installations and 300.5(G) states that conduits or raceways through which moisture can contact live parts shall be sealed or plugged at either or both ends. The requirements of this section are specific to where moisture can contact live parts so consideration must be given to how the conduit or raceway is installed. Not all underground conduits or raceways are installed in a manner that moisture would actually contact a live part. An example would be where an underground raceway enters below the equipment and then bends upward to the bottom of the equipment or a conduit body is used and then the raceway attaches the bottom of the equipment enclosure.
300.7(A) applies to raceways that are exposed to different temperatures. This section requires sealing with an approved material to prevent the circulation of warm air to a colder section of a raceway or sleeve where any portion of the raceway or sleeve is subject to different temperatures and where condensation is known to be a problem. Cold storage areas of buildings, passing from the interior to an exterior location and entering a walk-in cooler are examples of where condensation can occur in raceways due to the difference in temperature. A suitable compound at a conduit body or junction box installed in the raceway system just before it enters the colder location can serve as sealing means. As noted above, the compound must not cause deterioration of the conductor insulation.