If we had to install conductors in a wet location most of us would immediately know to use a type W conductor such as XHHW, THW or THWN. But just what is a wet location? The answer to this can be found in Article 100 under the definition of "Location." Raceways installed in concrete slabs, out-of-doors where exposed to the weather, and installations underground are examples of wet Locations. But what about the inside of the conduit? Is this a wet location? To answer this, we must go to the NFPA NEC Handbook and read the explanatory material. There it states that the intent is that the inside of a raceway located in a wet location is also a wet location. Easy enough, right? But we all know that the explanatory material is not mandatory, regardless of being found in the NFPA Handbook. However, most of us would agree with these explanations. So what is the problem? The problem arises when instrument cables, for instance, Class 2 cables, are installed in conduits located in wet locations. According to U.L., there aren’t any Class 2 cables listed for use in a wet location without also being listed for direct burial. Some will argue, what is the hazard? If I want to install Class 2 cables in wet locations, there is no shock hazard, no ignition hazard, and if the cables are properly listed, no spread of fire hazard. This is true. But there may be other hazards if the cables are used to control an integrated electrical system requiring an orderly shutdown where failure of the cables may result in an oil spill. This is a hazard not normally considered, but may be considered by other authorities such as the Alaska Department of Environmental Conservation. Therefore, the Class 2 cables installed in a wet location may have to be listed for wet locations.

Hazardous Location

If these cables are also installed in a Class 1 Division 2 hazardous location NEC section 725-53(d) requires that the cables be Power Limited Tray Cable (PLTC). However, the only PLTC cables listed for wet locations are the Direct Burial type. An exception to NEC section 725-53(d) allows other types of cables if the circuits are intrinsically safe. We could install Class 2 cables on Class 2 intrinsically safe circuits in a Class 2, Division 2 hazardous location, but, again, if the location is also a wet location the cables may have to be listed for wet locations. Note, we said if the cables are Class 2 and intrinsically safe: Remember, intrinsically safe circuits are not necessarily Class 2 circuits. If the cables are also installed in a cable tray NEC section 725-53(c) states that they must be type PLTC unless the cables are for thermocouple circuits. Therefore, for a location that is wet and Class 2, Division 2, cables in cable tray would have to be PLTC suitable for direct burial since these are the only PLTC cables listed for wet locations.

Other Space used for Environmental Air

If this scenario is expanded to cover other space used for environmental air the cable tray is required by NEC section 300-22(c) to have a solid bottom with a solid cover, or else the cables must be Plenum rated, type Ml, type AC, or type MC without an overall nonmetallic covering. If the cables are plenum rated and not PLTC they cannot be installed in a cable tray by NEC section 725-53(c). Suppliers have had difficulty finding PLTC cables that are plenum rated. When designing these types of installations it is recommended that the authority having jurisdiction be contacted and requested to perform a plan review.