PV Circuit Sizing & Current Calculations

Section 690.8 of the National Electrical Code (NEC)  deals with PV circuit sizing and current calculations, and defines how to calculate four maximum circuit current values. These maximum circuit currents are used in additional calculations in sections 690.8(B). But before jumping into calculations, a few NEC definitions will be helpful, since the rules for correction factors and overcurrent requirements can change based on the specific circuit. Working from the array to the inverter, we have:

PV source circuits are conductors between the modules, and from modules to a common point of connection, typically a junction box or combiner box. In industry terms, these are often called the “home runs” from the individual strings.

PV output circuits are conductors between the PV source circuits and the inverter or DC utilization equipment. These are the circuit conductors after a combiner box to the inverter or charge controller. 

Inverter input circuits, in a battery-based system, are the conductors between the inverter and the battery bank. In a grid-tied system, they are the conductors between the inverter and PV output circuits. Typically, these are the conductors between the inverter’s integrated DC disconnect and the inverter’s DC input connection.

Inverter output circuits are the AC conductors from the inverter to the ultimate connection to the AC distribution system for either stand-alone or utility-interactive systems.

Calculations

The first calculation, from 690.8(A)(1), results in the maximum PV source-circuit current. The rated short-circuit current (Isc) is multiplied by 125%. For example, if a PV module has an Isc of 8.8 amps, this calculation is: 8.8 A × 1.25 = 11 A.

Section 690.8(A)(2) covers the maximum current for PV output circuits. For output circuits, multiply the Isc by the number of circuits in parallel, and then by 125%. A common installation method is to keep the source circuits separate until they reach the inverter’s integrated DC combiner and disconnect. In that case, there are no output circuits to consider because the source circuits are not placed in parallel outside of the inverter.

Section 690.8(A)(3) defines the maximum current for the inverter’s output circuit. For utility-interactive inverters, there isn’t a calculation required, since the maximum current is defined as the inverter’s continuous output rating.

Section 690.8(A)(4) shows the calculation for the highest input current of a stand-alone inverter. This value helps determine the conductor size and overcurrent protection device (OCPD) rating between the batteries and the inverter. Divide the inverter’s continuous power output rating by its lowest DC operating voltage, and then multiply by the inverter’s rated efficiency under those conditions.

Part 5 of 690.8(A), added to the 2014 Code, defines the maximum output current of DC-to-DC converters as the rated output per the manufacturer’s specifications. No additional calculations are required.

In the 2014 NEC, 690.8(B), which outlines the rules for calculating minimum conductor sizes in PV circuits, is titled “Conductor Ampacity.” The OCPD section has been relocated to 690.9. The method for conductor sizing has not changed, although the 2014 sections incorporate some clarifications.

In 690.8(B)—690.8(B)(2) in the 2011 edition—two calculations must be run; the circuit conductor size must be based on the larger of the two values calculated. The first calculation is in 690.8(B)(1)—690.8(B)(2)(a) in the 2011 edition. Because PV system currents are considered continuous, the maximum currents calculated in 690.8(A) must be multiplied by 125% to calculate the minimum conductor size. This calculation ensures that the conductors do not carry more than 80% of the continuous current value (0.8 is the inverse of 1.25), a standard procedure in earlier Code articles. In the PV industry, the result of this calculation is commonly referred to as the “156% factor.” When this rule is applied, the module’s rated Isc has been multiplied by 156% (125% × 125% = 156%). However, don’t just multiply everything by 156%. Inverter output circuits were not multiplied by 125% originally, so the 156% factor doesn’t apply to them.  This calculation is done before applying any adjustment and correction factors, commonly referred to as “conditions of use,” which include corrections for conductors exposed to temperatures in excess of 30°C or more than three current-carrying conductors within a conduit. The ampacity of the conductor, at a minimum, then, needs to be greater than or equal to the maximum current in 690.8(A) × 1.25.