Question: Can steel raceways be installed as a watertight system?
Answer: No raceway system is really “watertight”, including those that are glued or have sealed joints. Normally raceways will collect condensation. The raceway needs to be installed to allow this moisture to drain out. Specific rules for “arranged to drain” are in NEC 225.22 and 230.53.
The NEC permits galvanized steel EMT, IMC and RMC to be installed in all wet locations both inside and outside. NEC Sections 342.10(D), 344.10(D) and 358.10(D) caution the installer that all hardware items such as straps, bolts, screws etc. be of a corrosion resistant material where used to support raceways in wet locations. Where threadless fittings are used with RMC and IMC, they must be identified and listed for the application. The term “threadless” refers to the method of connecting the fitting to the raceway, even though a threadless fitting can have threads for the securing means, such as a set screw or compression gland. The UL standard for Conduit, Tubing and Cable Fittings, UL 514B, requires that threadless fittings intended for use in wet locations be marked “wet locations” on the fitting or its smallest unit shipping container. Threadless fittings intended for embedment in poured concrete are marked “concrete-tight” or “concrete-tight when taped.” Fittings listed as “wet locations” are also “concrete-tight”, but not all concrete-tight fittings are “wet locations” fittings. For more information, see Section 4.3.3, Fittings for use with RMC, IMC, and EMT in the publication Guidelines for Installing Steel Conduit/Tubing.
NEC, Article 100, defines a wet location as follows:
“Location, Wet. Installations underground or in concrete slabs or masonry in direct contact with the Earth; in locations subject to saturation with water or other liquids, such as vehicle washing areas, and in unprotected locations exposed to weather.”
Wet Locations: Underground
All types of raceways installed underground are considered to be in a wet location. Section 300.5(B) requires that the inside of all raceways installed underground be considered a wet location. Conductors installed in such underground locations must be suitable for use in wet locations and comply with 310.10(C). It is understandable that in some cases where the ground does not percolate well, the water will also seep into the raceway.
If the presence of water is a problem, one of the following steps may help: 1. Install a typical concrete-polymer underground open bottom junction box over a gravel sump. This can be done by excavating a hole; filling it with 1 – 2 inch gravel and placing the box flush with the ground prior to entering the building, or at the lowest location in the conduit run. 2. Install type ECDB or equivalent drain fittings to permit the water to drain out of the conduit or EMT. These would normally be installed in all above-the-ground locations prior to entering each building or piece of equipment.
Wet Location: Above Grade and Indoors
The inside of all above grade raceways installed in a wet location are also considered to be a wet location. Section 300.9 requires conductors and cables installed in these locations to be suitable for use in wet locations in accordance with 310.10(C).
NEC 300.6(D) requires all metallic raceways, boxes and fittings; installed indoors in a wet location, to be mounted so there is at least a 1/4 inch air space between them and the supporting surface to minimize the accumulation of moisture.