The module lead’s connector type is important. Often called “quick-connects,” many new products are on the market. The old standard—Multi-Contact (MC) 4—has been joined by Tyco, Radox, Amphenol, and others. The 2011 NEC mandates that these connectors be touch-safe and, for circuits greater than 30 volts, require a tool for opening. Most of these connectors are not cross-compatible, so mixing modules will require properly mating connectors, as well as for wire runs to combiner or pass-through boxes.
Factory-installed module leads will be listed in the spec sheet with wire size, insulation type, and length of the leads (positive and negative leads are not always the same length). Wire diameter generally ranges from 14 AWG to 10 AWG; or they may be listed in square millimeters (mm2). For low-voltage systems, less power will be lost to voltage drop if using modules with heavier-gauge wire.
Insulation type on the conductors may be a single listing, such as PV wire, or have multiple cross-listings, including USE-2, RHW-2, XHHW-2, and/or PV wire. All factory-installed module lead insulation types are tested to be sunlight-resistant and flexible at low temperatures, and are heavily or even double-insulated for installation in extreme outdoor environments. However tough these single conductor leads may be, they still must be protected in a raceway when they leave the vicinity of the array.
A junction box is factory-installed on the back of modules for the connections. Many are sealed and inaccessible to the end user. If it is specified as field-serviceable, the junction box can be opened, and leads and bypass diodes can be installed or replaced. For arrays that are readily accessible (for example, a ground-mounted array), field-accessible and conduit-ready junction boxes can allow for fittings and protective raceways to be installed and meet NEC 690.31(A) code requirements for accessible arrays.
Shading a small part of a PV module can have a disproportionally large effect on its output. Additionally, when a module is partially or completely shaded, the current flowing through the module can reverse direction and create hot spots, which can lead to deterioration of the cell, the internal connections, and the module backsheet. A bypass diode stops the reverse flow of current and also directs electrical flow around the shaded section of the module. Nearly all modules come with factory-installed bypass diodes, with the exception of some thin-film modules. A typical 72-cell module with all the cells in series will have three bypass diodes, each bridging a series of 24 cells that can be bypassed if any or all of those cells are shaded. Depending on where they are located on the module and the type of junction box, diodes may be field-accessible. Regardless of the benefit of diodes, shading should be avoided whenever possible.
Modules per Pallet; Pallets per Container
A pallet of modules isn’t a standard quantity. Details on packing information is important to help calculate point loading if pallets are to be placed on a roof, or for staging large job sites.
Standard test conditions (STC) are the conditions under which a manufacturer tests modules: 1,000 W per m2 irradiance, 25°C (77°F) cell temperature, and 1.5 air mass index. Real-world operating cell temperature is often 20 to 40ºC above the ambient temperature. STC (bright sun and a relatively low cell temperature) are not typical for field operation of modules, but they do provide a consistent standardized reference to compare modules.
An I-V curve (current-voltage) curve is generated at STC for every cell and module manufactured. The I-V curve contains five significant data points (Pmax, Vmp, Voc, Imp, and Isc; discussed below), which are used for system design, troubleshooting, and module comparisons. I-V curves can also be diagrammed for any operating temperature and irradiance level, but the points listed on a module specification sheet and those printed on the back of the module are at STC unless otherwise stated.
Peak Power (Pmax or Pmp)
The specified maximum wattage of a module, the maximum power point (Pmax), sits at the “knee” of the I-V curve, and represents the product of the maximum power voltage (Vmp) and the maximum power current (Imp). This wattage is produced only under a very specific set of operating conditions, and real environmental conditions (changing irradiance and cell temperature) will alter a module’s Pmax.