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Publication Title | Code Corner Flex is Out and the Dodo Diode Isn’t Dead

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Text | Code Corner Flex is Out and the Dodo Diode Isn’t Dead | 001

Code Corner

Flex is Out

and the

Dodo Diode

Isn’t Dead

John Wiles

Sponsored By The Photovoltaic Systems Assistance Center Sandia National Laboratories

The news is not good, but please don’t shoot the messenger. First, an electrical inspector has pointed out that the very popular flexible non- metallic conduit should not be used with PV modules. Second, although some system designers have said that the blocking diode is as dead as the extinct Dodo bird, there may be good reasons for using diodes or fuses in all systems.

Flexible Conduit

Liquidtight flexible nonmetallic conduit as described in Section B of Article 351 of the National Electrical Code (NEC) is often used for containing the wiring from PV modules to nearby junction boxes. This flexible, light- weight, gray plastic conduit is widely available in building supply stores and electrical supply houses in both precut and bulk lengths. The conduit cuts easily with a knife and the termination fittings are easy to install.

Unfortunately, this type of conduit is listed by Underwriters Laboratories (UL) for use at 70 or 80°C when dry and only 60°C when wet. NEC Section 351-23 (b) (2) states that this conduit may not be used where any combination of ambient temperatures and conductor temperatures are in excess of that for which the conduit is approved.

Conduit attached to PV modules is in an exposed location and wet-rated conductors must be used in all conduit that is exposed-see Definitions (location) in the NEC and NEC Handbook. Most PV modules are listed for use with 90°C conductors because junction-box and back-of-module temperatures can exceed 70°C. For these reasons, the conductors used in the installation of PV modules must have a 90°C insulation rating when

wet. Typical cables are THWN-2, RHW-2, and XHHW-2 all having a 90°C insulation rating.

With 90°C cables, high ambient temperatures (70°C+), and possible wet conditions, the NEC says that the liquidtight flexible nonmetallic conduit marked 70°C dry/60°C wet cannot be used. UL is considering revisions to UL Standard 1703 that might require that either the listed PV module or the instructions for the listed module state that the use of this conduit is unacceptable. The flexible nonmetallic conduit can deform at high temperatures and reduce the internal area (needed to maintain the ampacity of the conductor), or the deformation and water can cause the fitting on the end of the conduit to come loose from the conduit.

Alternatives that can be used for PV module connections include liquidtight flexible metal conduit, ridged nonmetallic conduit (PVC), and several of the metal conduits. In addition, where conduit is not needed, outdoor-rated, exposed (not in conduit) single-conductor cables may be used. While these alternatives are, in some cases, more expensive or more difficult to use, they will ensure that the cables are adequately protected and that the protection will not fail due to high temperatures or wet conditions.

The Dodo Diode and The Series Fuse

Each PV module listed to UL Standard 1703 by a recognized testing laboratory is marked with the maximum current value of a series fuse intended to protect the module from overcurrents that may be forced through the module under fault conditions. The NEC requires that all instructions and labeling provided with listed modules be followed. This fuse requirement is in addition to other NEC system requirements to provide proper overcurrent protection for all conductors.

As PV system designs have matured over the last ten years, there has been a divergence of the design and installation practices between high-voltage (over 50- volts) and low-voltage systems. High-voltage systems, usually utility-interactive, have always employed blocking diodes and overcurrent protection for each string of series-connected modules. This NEC-required overcurrent device usually has been sized to provide overcurrent protection for the conductors in the series string and has been generally near the value of the required series fuse (required by UL Standard 1703) used to protect the PV module.

In low-voltage systems, system designers have been very concerned with power losses and voltage drops in these mostly stand-alone, battery-charging systems. Blocking diodes went the way of the dodo bird as designers found that the daytime losses through the diode exceeded the very low losses associated with

74 Home Power #60 • August / September 1997

Image | Code Corner Flex is Out and the Dodo Diode Isn’t Dead

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