Recently, I received a call from an electrician friend who wanted to know how to determine the difference between a raceway, nipple and sleeve, and this individual has been an electrician for 20-plus years. I explained the procedure that I felt was used per the National Electrical Code (NEC) to determine when a raceway, nipple or sleeve is used as a wiring method in an electrical system. Because of this inquiry, I wondered if others might have the same problem.
A raceway is considered a run of conduit in lengths of more than 2 feet between the power supply and load served. Couplings connect the lengths together, and connectors terminate the specific type of raceway to enclosures and equipment. When classified as a raceway, only a 40 percent fill area is permitted for two or more conductors, as outlined in Table 1 of Chapter 9. For example, according to Table C.8 from Annex C, 10–12 AWG THHN copper conductors can be enclosed in a 1-inch, rigid metal conduit (RMC). However, if the above conductors are installed as current carrying, the ampacity of each conductor must be derated by 50 percent from the 90-degree column, as required by Table 310.15(B)(2)(a). Note that derating is necessary because four or more current-carrying conductors are routed through the raceway.
Just for the record, when all the conductors are not considered current carrying, the derating factors listed in Table B.310.11 of the NEC can be applied, if the calculations are performed under engineering supervision. The information in Annex B makes it clear that at least half of the conductors—but less than the total number—can carry current without derating the total number, if calculated as illustrated in Examples 1 and 2 of Table B.310.11.
A nipple is a conduit 2 feet or less in length and used to connect enclosures, boxes, gutters, wireways, etc., together. Nipples can be beneficial because derating for too many current-carrying conductors is not required, as there is no derating requirement in Note 4 to Table I of Chapter 9. The nipple installer can even apply a 60 percent fill area instead of the 40 percent fill area per Table 1 and Note 4 of Table 1 to Chapter 9. The fill area for a nipple is not the same as a raceway. The 60 percent fill area for a nipple allows more conductors to occupy the conduit than the 40 percent fill area allotted for a raceway without the application of derating factors.
A sleeve is used when a cord, cable or raceway requires protection from physical damage. For example, a cable can be protected from physical damage by routing it through a 10-foot length (short lengths per 300.12, Ex. and 300.18, Ex.) of conduit that is open on each end (not connected to enclosures, etc.).
Chapter 3 of the NEC, outlined below, discusses how short lengths of raceways can be determined. Note that such a sleeve does not have to be bonded to the equipment-grounding conductor in the protected cable as permitted by the provisions of 250.86, Ex. 2, which clearly states short sections of metal enclosures or raceways used to provide support or protection of cable assemblies from physical damage shall not be required to be grounded. A 50-foot sleeve open on both ends can be used to protect a flexible cord from physical damage when run above ground.
The Ex. to 312.5 (C) recognizes a sleeve at least 18 inches to 10 feet in length, open on one end and connected to a panelboard on the other. Such a sleeve must be installed and used in accordance with the requirements listed in the above section.
Each article in Chapter 3 of the NEC, Part 11, covers the installation requirements needed to install a particular wiring method. The general rule of 344.30 requires RMC to be supported at 10-foot intervals and within 3 feet of each pull box, junction box, enclosure, etc. However, Table 344.30 (B)(2) permits a 1-inch RMC to be supported at 12-foot intervals.
Over the years, many designers, installers and inspectors have interpreted 344.130 as classifying short lengths of raceways as 10 feet or less in length. However, it always is up to the inspector to make the final determination on what is considered a raceway, nipple or sleeve. EC
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA as well as other standards. Contact him at 817.581.2206.