NEC Calculations

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Schematic for Independent Source Circuits
Schematic for independent source circuits
Schematic for rooftop combiner
Schematic for rooftop combiner
Schematic for Independent Source Circuits
Schematic for rooftop combiner

“Code Corner” in HP159 and HP161 detailed the requirements for circuit sizing and overcurrent protection under the 2014 National Electrical Code (NEC). Here are some example calculations to bring that Code language to life. For those who use the 2011 NEC, don’t worry—the calculations are the same.

If you’re rusty on the “whys” behind the calculations, consider reviewing those previous “Code Corner” articles to understand the background information. The calculation examples use:

  • 72-cell, 310 W module; 45.4 Voc, 9.28 A Isc, and 15 A maximum series fuse rating
  • Four PV source circuits connected to a single string inverter
  • The location is Sacramento, California, with an ASHRAE 2% average high temperature of 38°C
  • Exposed rooftop conduit is 2 inches above the roof surface
    The effective temperature inside raceway [from Table 310.15(B)(3)(c)] = 38°C + 22°C = 60°C
    The 90°C conductor insulation correction factor for 60°C from Table 310.15(B)(2)(a) is 0.71
  • The temperature of conductors that run through the attic space is estimated at 55°C
    The 90°C conductor insulation correction factor for 55°C from Table 310.15(B)(2)(a) is 0.76
  • All terminals located in junction and combiner boxes are rated for 75°C
  • All exposed conductors are PV wire and rated at 90°C; all conductors within raceways are THWN-2 and rated at 90°C
  • With four source circuits, there are eight current-carrying conductors in the raceway, and the correction factor is 70% from Table 310.15(B)(3)(a).

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