Code Change Summary: A new definition of a DC-to-DC Converter was added to 690.2.

A DC-to-DC Converter is a device installed in the PV source circuit or PV output circuit that can provide a DC output voltage and current at a higher or lower value than the DC input voltage and current. In other words, the DC-to-DC Converter can change and regulate the DC output voltage and current.

In PV systems without a DC-to-DC Converter, the open circuit voltage of each PV module wired in series is corrected for cold weather expectations, and then added together for each module in the series string. The summed voltage of all modules in the same string cannot exceed 600 volts for a one or two-family dwelling per 690.7(C).

Example: Single family dwelling with 3 strings of 10 PV modules, each PV module with an open circuit voltage of 60 volts (after cold weather correction factors). Total of 30 modules.

60 volts X 10 modules = 600 volts. Each string will have a maximum of 600 volts which is also the maximum overall PV system voltage and compliant with NEC^® 690.7(C).

By using DC-to-DC Converters and following the manufacturer instructions, it may be possible to configure the same system as 2 strings of 15 modules and the DC-to-DC Converters would control the output voltage to keep it at 600 volts or less to comply with 690.7(C). Less series strings combined in parallel means smaller PV output circuit conductors. Also, many DC-to-DC Converters have a feature that will reduce the module output voltage to 1 volt when the central inverter is shut off which can be used to comply with the new rapid shutdown requirements in 690.12.

Below is a preview of the NEC^®.  See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).