Raceway Selection and Installation for PV Systems: Part One
Inside this Article
The NEC recognizes a variety of conduits and tubing that hold wires and cables. How do you select the right raceway type for a particular application, and how do you design and install it efficiently, in a Code-compliant and long-lasting manner?
PV systems, like other electrical power systems, use electrical conductors to route power from sources to loads. It is often necessary to enclose these conductors and protect them along their path. The 2011 NEC lists several methods of getting conductors from here to there, including about 20 different types of raceways and almost as many types of cable, a multiconductor assembly with a covering or jacket.
In this article I focus on raceways commonly used for PV systems. In Part One, to aid in specification and selection, I describe various types of raceways and discuss the differences among them. In the next issue, Part Two will present practical design considerations regarding specific applications of given raceway types and cover installation techniques.
Definitions and Jargon
Unfortunately, the topic of raceways is an area full of trade shorthand and jargon. I can clear up some of this terminology.
Solid or stranded electrical wires are generally known as conductors. Individual conductors can appear by themselves, as in USE-2 underneath a PV array, or several can be assembled in the factory into a cable or installed together in the field into a raceway.
Conduits, tubing and even square wireways are all types of raceways, according to the NEC. However, the NEC does not consider auxiliary gutters raceways, although they are similar to square wireways. While Code does not define the terms conduit and tubing, it does define the term raceway and includes the types of conduit and tubing discussed in this article. For clarity, I use the term raceway as defined in Article 100 to generally indicate “an enclosed channel of metal or nonmetallic materials designed expressly for holding wires, cables, or busbars.”
The trade often refers to particular raceway types with shorthand. For example, among the common flexible raceways, flex usually refers to type FMC (flexible metal conduit; see NEC Article 348), liquidtight usually refers to type LFMC (liquidtight flexible metal conduit; see NEC Article 350) and Sealtite usually refers to type LFNC (liquidtight flexible nonmetallic conduit; see NEC Article 356).
The real confusion happens with the nonflexible circular raceways: These common raceways are rigid (not flexible), but the term rigid often refers to type RMC (rigid metal conduit; see NEC Article 344). Thinwall refers to type EMT (electrical metallic tubing: see NEC Article 358), which in turn is slightly confusing because it is called tubing instead of conduit. Fortunately, both tubing and conduit are types of raceway. Schedule 40 and Schedule 80 usually refer to PVC (polyvinyl chloride) conduits, as per NEC Article 352. Steel or galvanized are ambiguous terms because either could refer to RMC, IMC (intermediate metal conduit; see NEC Article 342) or EMT. The term rigid is even more ambiguous because it could include both metallic and nonmetallic raceway materials, such as PVC. Technically, RMC can also be made from stainless steel, red brass or aluminum, but the vast majority of RMC is galvanized steel.
This article—and hopefully PV project specifications and plan sets—refers to the various raceways by their NEC designators. If you are bidding on a project with drawings or specifications calling for an ambiguous raceway type such as “rigid,” make sure to get formal, written clarification through the Request for Information (RFI) process to determine the intent of the specification. If you assume one type of raceway and the intent was for the other, you may substantially over- or underbid.
PV systems are generally similar to other electrical power systems, but they do have certain features that affect raceway selection and installation. For example, the bulk of a typical PV system is installed in an exterior location with high exposure to sunlight and other environmental phenomena.