Code Corner: 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).

Comments (1)

Robert Pollock_2's picture

I just installed a 5.1 kw PV system with SolarEdge 7600 Inverter plus LG Chem Resu 10 battery. The City rejected my Single Line Drawing concerning the above calculations. (cut and pasted from solaredge.com)
Now we're headed to court, to get an injunction to force them to inspect and sign off. The system uses Optimizers because it's the best set up for hot places (Palm Springs) and the wire's were all sized to the maximum that the conduits can accept, which is code and over code regarding the ground wire (#8 used) I re-built the roof under the system (couldn't afford to do the whole roof) by removing the heat absorbing tiles and replacing them with a radiant barrier, airspace and a reflective, light weight, energy dispersing steel roof, with a continuous air vent at the top. The electrical engineer who disapproved the plan didn't like one detail, so small I forget what it was. My reason for getting a court injunction, is mostly to make permit acquisition for homeowners easier, and suggests that the engineer could have flagged the deficiency, so the inspector would be sure it was addressed. Thus facilitating the process, not impeding it.

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