Flexible Metal Conduit Length, Use of the Term 'Tap' Conductor for High-temp Wires

Since the 1940 National Electrical Code (NEC), there has been a requirement that recessed incandescent fixtures be connected by wire having insulation suitable for the temperature. If the branch circuit conductors were unsuitable, then a splice had to be made in a junction box at least a foot away from the fixture. In addition, a piece of flexible metal conduit not less than 4 feet or more than 6 feet long had to be carried in the high-temperature fixture wire to the junction box, where it was spliced to the branch circuit conductors. This required 4-foot minimum was intended to provide enough length for the heat at the fixture carried by conduction on the metal flex and heat from the fixture carried through the flexible conduit by convection to dissipate by the time it got to the junction box. The junction box is required to be at least 1 foot away so heat radiated from the fixture will dissipate by the time it gets to the junction box. In the 1990 NEC, types AC and MC cable were added as permitted means of connection between recessed fixtures and junction boxes, recognizing that these materials were being supplied as 6-foot “whips,” mainly for recessed fluorescent fixtures. In the 1999 NEC, the minimum length of 4 feet of flex or cable was reduced to 18 inches, because, as the submitter of proposal 18-78 said, the 4-foot length was a nuisance and a waste of material. Three things have happened since 1940 to allow the minimum length of 18 inches instead of 4 feet: 1) The construction of recessed incandescent fixtures has changed. Now most have a terminal connection box factory mounted as part of the fixture and are designed so that the terminal box can accommodate supply conductors having 60 degrees Celsius or, if marked, 75 or 90 degrees Celsius insulation; 2) In 1940, there was no branch circuit conductor rated 90 degrees Celsius. Today, we can run the branch circuit conductors directly to the terminal box on the fixture, making the adjacent junction box unnecessary in many cases; and 3) In 1940, the four-feet minimum length was arbitrary. The change in the 1999 NEC was made in part to correlate with the 18-inch minimum length in the Canadian Electrical Code, which is based on testing, not on guesswork. In Part M of Article 410, Special Provisions for Flush and Recessed Fixtures, appears Section 410-67(c), which reads as follows: “(c) Tap Conductors. Tap conductors of a type suitable for the temperature encountered shall be permitted to run from the fixture terminal connection to an outlet box placed at least 1 foot (305 mm) from the fixture. Such tap conductors shall be in suitable raceway or Type AC or MC cable of at least 18 inches (450 mm) but not more than 6 feet (1.83 m) in length.” Note that the 18-inches and 6-foot lengths are for the raceway (or cable sheath), not the conductor. (The tap lengths in Sec. 240-21 are for the conductors, not the raceways or cables.) The other controversial matter in this Section is the use of the term “tap.” Some believe that the high-temperature wire from the junction box to the fixture must have a lower ampacity than the branch circuit overcurrent protection rating, due to the definition of “tap” in Section 240-3(c), new in the 1999 NEC: “As used in this article, a tap conductor is defined as a conductor, other than a service conductor that has overcurrent protection ahead of its point of supply, that exceeds the value permitted for similar conductors that are protected as described elsewhere in this section.” The “tap” referred to in Sec. 410-67(c) is not the “tap” as defined in Sec. 240-3(c), because that definition starts out with “as used in this article.” Therefore the definition applies only in Article 240, and the tap in Art. 410-67(c) need not have overcurrent protection greater than its ampacity. As a matter of fact, where higher temperature insulation is required, these conductors probably have an ampacity greater than the rating of the branch circuit overcurrent protection. SCHWAN is a NEC consultant in Hayward, Calif. He can be reached via e-mail at bev-schwan@aol.com.

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