Today, designers and contractors face many options for protecting the electrical cables and wiring systems in commercial PV installations. One such option is conduit, which is a type of raceway or closed channel that guards wiring systems running through them against hazards over a system’s lifetime.
There are many factors to take into account to correctly size and select a conduit material, from code-, job- and customer-specific requirements, and making the right selection has a big impact on PV system performance.
Conduit used in solar installations is generally divided into two categories: rigid and flexible. Each category can be further divided into metallic and non-metallic types. Some are UL listed, and those that are not may be considered recognized components.
Non-metallic conduit is made from plastics that have to meet the performance requirements of steel while being much lighter, using materials like Nylon, polypropylene or PVC. Metallic conduit can be made from galvanized steel, stainless steel, brass, aluminum or nickel-plated brass. Some contain both metallic and non-metallic materials to offer low fire hazard and anti-static capabilities, external braiding and other features.
Rigid conduit, sometimes called pipe or tubing, is used on long conduit runs and where extra strength or mechanical protection is needed. Rigid pipe can still be turned, but flexible conduit is normally used in installations that require bending or in areas where vibration will occur.
Each conduit category contains many products. For example, AFC Cable Systems offers three new types of flexible metallic conduit products with extreme temperature resistance called Liquid-Tuff. Gene Brown, liquid-tight and flexible conduit product manager, says 80 to 90 percent of the raceways need to use listed products. The listing tests things like bend radius and UV light performance. For these applications, the hi-low temperature flexible steel conduit could be used in hot and cold climates, and it has a flame-retardant PVC jacket that is sunlight resistant.
But, he says, depending on the installation, some customers don’t require UL listed products and protective tubes, and non-listed liquid-tight conduits can be used. In this case, the Non-UL Extreme Temperature Liquid Tight Flexible Metal Conduit 6800 Series has UV and ozone protection and is one of the better options when exposed to direct sunlight.
For those who need improved flexibility for tight bend radii, and where weight is a consideration, another option (that’s not labeled as conduit) exists: Non-metallic protective tubes (NMPTs). AFC’s Ultraflex Liquid-Tight is an extra-flexible mechanical NMPT non-UL liquid-tight raceway with a corrugated shape, is made from flexible PVC and has a rigid inner core for support.
Factors for sizing conduit
During system design, engineers consider many factors to calculate wiring and conduit sizes such as the type of wire used, voltage drops, type of circuit, conductor designation for indoor/outdoor (or both) and the ambient temperature during sunlight exposure. The temperature rating, conduit fill adjustment factors and National Electrical Code (NEC) tables are important for calculating the ampacity and sizing.
“To size conduit correctly, designers look at the constraints of the specific installation,” Brown says. “They consider the raceway route, how tight the footprint is for connections and the field conditions.” Plus, he says the NEC’s 40 percent fill rate rule is important.
Based on the number of conductors inside, the NEC Code specifies that the outer diameter (OD) has to be filled to a maximum amount of 40 percent. Leaving most of the raceway empty is enough to account for the heat generated in the wires and to allow room to safely pull the conductors through. Knowing the length of the raceway, the OD of each wire and the type of coatings (that may slide differently against one another) allows designers to size everything properly to avoid jamming.
Selecting conduit type and materials
Not paying attention to cable protection can be unsafe, costly and lead to downtime and inefficiency. Selecting the right kind of conduit needed for a job will depend on the application, jobsite and expected risks.
More on selection, common practices and options for the long haul (click next page)
— Solar Builder magazine