CODE CORNER: Circuit Calculations

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A recurring and confounding topic is circuit calculations and conductor sizing. Except for PV systems greater than 100 kW, the methods for calculating voltages and currents haven’t changed in recent Code cycles, yet the topic remains confusing. In HP182, I reviewed the major changes in Part II of Article 690. In this article, I’ll clarify the calculations in Section 690.8 to illustrate how to apply the Code requirements.

The calculations required for the circuit current and overcurrent protective devices (OCPDs) are outlined in NEC 690.8 and 690.9. The first step in this process is to calculate the maximum circuit current per 690.8(A). This calculation is dependent upon which portion of the circuit is being evaluated. Understanding the six subsections is important.

Section 690.8(A)(1)(1) or (2) calculates the PV source-circuit currents. The second subsection is new to 2017 and only applies to PV systems greater than 100 kW, which is beyond the scope of this article.

Section 690.8(A)(1)(1) dictates that we multiply the PV source-circuit short-circuit current (Isc) values by 125%. This will typically be a single module, or a series string of modules, that creates a PV source circuit. The 125% multiplier accounts for scenarios in which the irradiance is greater than the STC conditions, such as edge-of-cloud effects, increased irradiance at higher elevations, and reflectance from snow.

Section 690.8(A)(2) applies to the PV output circuits. The value calculated in the first subsection is multiplied by the number of circuits in parallel. This applies to cases in which a DC combiner box was used to place multiple source circuits in parallel and provides a single output into an inverter or charge controller.

Section 690.8(A)(3) pertains to the inverter output circuit. There is no multiplier to use. The inverter’s continuous output current rating is the value used for the maximum inverter output-circuit current. Regardless of the PV input, inverters cannot output more current than they are designed for. Section 690.8(A)(4) covers stand-alone inverter input circuit current requirements, which are not discussed in this  “Code Corner.”

Sections 690.8(A)(5) and (6) are for systems that use DC-to-DC converters, such as optimizers. Similar to inverters, these power electronics control the circuit’s current output, so the maximum source circuit current is the rated value of the converter’s continuous output. If multiple circuits are placed in parallel, the output circuit is the number of circuits in parallel multiplied by the continuous output rating.

Section 690.8(B) deals with the calculation of conductor ampacities and requirements for conductor sizing. This section dictates that PV circuits shall be considered continuous and calls for conductors to be sized based on the largest conductor calculated in the first two subsections, or where protected by an adjustable OCPD, as dictated in the third subsection. The third subsection is new to 2017 and a method unlikely to be used in residential and small commercial applications. Large-scale PV arrays may benefit from the adjustable OCPDs, but are special cases.

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