Code Change Summary: New definitions were added to Article 690.

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 largest string represents the maximum system voltage which is not permitted to exceed 600 volts for a one or two-family dwelling per 690.7.

Example: Single family dwelling with 3 strings of 10 PV modules, each PV module has an open circuit voltage (VOC) 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 complies with NEC^® 690.7.

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. 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 string inverter is shut off which can be used to comply with the rapid shutdown requirements in 690.12.

Even though the definition of a DC-to-DC Converter was added during the 2014 NEC^® code cycle, it didn’t address the conductors on either side of the device. Defining these conductors is critical in order to apply the correct code sections to the conductors.

In the 2017 NEC^®, the DC-to-DC Converter Source Circuits are the circuits between dc-to-dc converters and from dc-to-dc converters to the common connection point(s) of the dc system.

The DC-to-DC Converter Output Circuits are the circuit conductors between the dc-to-dc converter source circuit(s) and the inverter or dc utilization equipment.

Below is a preview of the NEC^®. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2017 NEC^® edition of NFPA 70.