CODE CORNER: PV Circuit Calculations

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For PV circuit calculations, just reading through the National Electrical Code (NEC) requirements can be difficult and frustrating. Getting familiar with these figures by reviewing concrete examples can make the rules easier to follow. This example, which uses the electrical specifications below, takes you through Section 690.8 circuit-sizing requirements for a typical residential or small commercial installation and shows where all of the different calculations are applied.

PV Module

  • Pmp = 340 W
  • Voc = 48.0 VDC
  • Vmp = 38.2 VDC
  • Isc = 9.53 A
  • Imp = 8.9 A

Inverter

  • Maximum power = 7,600 W
  • Continuous output current = 32 A

DC Optimizer

  • Maximum output current = 15 A

The example location is Denver, Colorado, where the ASHRAE 2% high temperature is 34°C. The Solar ABCs map data (bit.ly/SolarabcMap) shows two high temperatures. The 0.4% temperature is the air temperature that is exceeded for three hours (not necessarily continuous) in a summer month, June through August. The 2% number represents the temperature exceeded for 14 hours and is the value recommended for use in the calculations.

690.8(A) Calculations

690.8(A)(1)(1)—The maximum source-circuit current for a single series string of PV modules:

Isc × 1.25 = Maximum circuit current

9.53 × 1.25 = 11.91 A

690.8(A)(1)(2)—Doesn’t apply to this example because of the system’s size.

690.8(A)(2)—The maximum output current for the series strings placed in parallel. This is relatively uncommon in modern systems due to the inverter MPPT configurations and the integrated DC combiners on larger inverters. For the sake of the example, let’s say three strings are placed in parallel and we need to calculate the output current of the combiner:

3 × Single String Imax = Total maximum circuit current

3 × 11.91 = 35.73

690.8(A)(3)—The maximum inverter output circuit current is the listed value from the manufacturer: 32 A (no correction factor need be applied).

690.8(A)(4)—The output current for a stand-alone inverter. This is a grid-tied inverter, so this section does not apply.

690.8(A)(5)—The maximum current for the output of a DC-to-DC converter source circuit: 15 A.

690.8(A)(6)—The output circuit current for DC-to-DC converters placed in parallel. If there were three DC-to-DC source circuits placed in parallel prior to the inverter connection:

            15 A × 3 = 45 A

690.8(B) & Conductor Ampacity

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