Conduits and the Code: Unraveling the mystery
This column is in response to requests to unravel the mysteries of the NEC 2005...
This column is in response to requests to unravel the mysteries of the NEC 2005 relating to acceptable conduits for extending unlisted optical fiber and unlisted communications cable inside the building beyond the exterior wall.
So far, there are at least three strong opinions: Only rigid conduit is acceptable; Electrical metallic tubing is also acceptable; and, the NEC is so loosely written, you can read anything you want into it.
Each group, of course, cites sections of the Code to support their arguments.
Today, we use three basic types of steel conduit. Galvanized rigid metal, intermediate metal, and electrical metallic tubing are non-combustible and can be used indoors, outdoors, underground, concealed or exposed.
Rigid metal conduit, or RMC, is a round threaded metal raceway. It is the heaviest-weight and thickest-wall steel conduit, and is listed to UL 6 Standard for Electrical Rigid Metal Conduit-Steel.
Developed in the 1970s, intermediate metal conduit (IMC) is a round, threaded metal raceway. When compared to RMC, it has a reduced wall thickness and weighs about one-third less. IMC-Steel is listed to UL 1242 Standard for Electrical Intermediate Metal Conduit-Steel.
IMC is interchangeable with galvanized RMC. Both have threads with a ¾-inch per foot taper, use the same couplings and fittings, have the same support requirements, and are permitted in the same locations.
Electrical metallic tubing (EMT) is a round metal raceway, but is unthreaded (due to the thin wall of the material). Lengths of EMT are connected by using set-screw or compression-type couplings. EMT is also permitted to have an integral coupling, which is comprised of an expanded “belled” shape of tube on one end with set screws. Electrical Metallic Tubing is listed to UL 797 Standard for Electrical Metallic Tubing-Steel.
Before we can begin to unravel this web of code, you will need to read (at a minimum) the following sections in NEC 2005: 770.2 Point of entrance; 770.3; 770.113; 800.2 Point of entrance; 800.3; 800.113; and 300.22.
Bottom line-the definitions for Point of entrance are pretty much the same, and both 770 and 800 reference 300.22.
The real fork in the road appears between 770.113 and 800.113. To explain how we got to this fork, we need to go back to NEC 2002.
The morphing exceptions of 770
NEC 2002 770.50 Listing, Marking, and Installation of Optical Fiber Cables states that “Optical fiber cables in a building shall be listed…” …and gives three exceptions to that requirement:
Exception No. 1: Optical-fiber cables shall not be required to be listed and marked where the length of the cable within the building, measured from its point of entrance, does not exceed 15 m (50 ft.) and the cable enters the building from the outside and is terminated in an enclosure.
Exception No. 2: Conductive optical-fiber cable shall not be required to be listed and marked where the cable enters the building from the outside and is run in rigid metal conduit or intermediate metal conduit, and such conduits are grounded to an electrode in accordance with 800.40(B).
Exception No. 3: Non-conductive optical-fiber cables shall not be required to be listed and marked where the cable enters the building from the outside and is run in a raceway installed in compliance with Chapter 3.
During the process of reviewing the proposals for the NEC 2005, many of the articles were renumbered, hence 770.50 (2002) is now 770.113 (2005).
And there was the proposal to narrow the list of choices in Exception No. 3 (2002). Exception No. 2 (2002) disappeared all together and Exception No. 3 (2002) morphed into Exception No. 2 (2005).
While Exception No. 3 (2002) accepted anything mentioned in Chapter 3, Exception No. 2 (2005) narrows the field by specifically listing IMC, RMC, RNC, and EMT as raceway types that are accepted for use with unlisted non-conductive optical-fiber cable that “enters the building from the outside.”
Notice that the “enters the building from the outside” and not “point of entrance” language is used here. Now… it is time to re-read NEC 2005 Section 770.2 Point of entrance. Notice that the only way to extend the point of entrance beyond the building wall is by choosing RMC or IMC grounded to an electrode in accordance with 800.100(B).
Article 800: Four becomes two
NEC 2002 800.50 Listing, Marking, and Installation of Communications Wires and Cables states that “Communications wires and cables installed as wiring within buildings shall be listed…” and gives four exceptions to that requirement:
Exception No. 1: Voltage markings shall be permitted where the cable has multiple listings and voltage marking is required for one or more of the listings.
Exception No. 2: Listing and marking shall not be required where the cable enters the building from the outside and is continuously enclosed in a rigid metal conduit system or an intermediate metal conduit system, and such conduit systems are grounded to an electrode in accordance with 800.40(B).
Exception No. 3: Listing and marking shall not be required where the length of the cable within the building, measured from its point of entrance, does not exceed 15 m (50 ft.) and the cable enters the building from the outside and is terminated in an enclosure or on a listed primary protector.
Splice cases or terminal boxes, both metallic and plastic types, are typically used as enclosures for splicing or terminating telephone cables.
This exception limits the length of unlisted outside plant cable to 15 m (50 ft.), while 800.30(B) requires that the primary protector be located as close as practicable to the point at which the cable enters the building. Therefore, in installations requiring a primary protector, the outside plant cable may not be permitted to extend 15 m (50 ft.) into the building if it is practicable to place the primary protector closer than 15 m(50 ft.) to the entrance point.
Exception No. 4: Multipurpose cables shall be considered as being suitable for the purpose and shall be permitted to substitute for communications cables as provided for in 800.53(G).
Just to keep things interesting, 800.50 (2002) is now 800.113 (2005). Exception No. 2 and No. 4 (2002) disappeared altogether, and Exception No. 3 (2002) morphed into Exception No. 2 (2005).
Come on guys! Having the type of raceway be determined by whether a term, or point of entrance in this case, is used in an exception to a listing requirement is just too tortuous, even for the NEC. For NEC 2008, please consider revising so that the user can easily understand what is acceptable and what is not.
(First of a two-part article.-Eds.)
DONNA BALLAST is BICSI’s standards representative, and a BICSI registered communications distribution designer (RCDD). Send your questions to Donna via e-mail: firstname.lastname@example.org