Voltage in a PV array is variable, changing with temperature and operating conditions. There is voltage between the positive and negative PV conductors, regardless of whether one conductor is connected to ground (a system ground). If the negative PV conductor is connected to ground at one point (a negative-grounded system), it will have zero voltage potential to ground, but its potential relative to the positive conductor will still be the array voltage. If instead the positive conductor were to be connected to ground (a positive-grounded system), it would have zero voltage potential to ground, but the potential to negative would still be the array voltage.
If, however, neither the positive nor the negative conductor is connected to ground, both should measure approximately zero volts relative to ground. Because there is not a low-resistance path for current to return to the modules via grounding conductors, there will be little or no electrical potential from the system to ground. Essentially, the PV array is unaware that “ground” exists. However, the voltage measured between positive and negative stays the same—at array voltage—regardless of DC system grounding, or lack thereof. PV systems can be installed and function whether they are positive-grounded, negative-grounded, or ungrounded. The choice of how and whether to ground a system comes down to equipment choices. Some module manufacturers, such as SunPower, may require positive- grounded systems when using their modules, because of possible system efficiencies related to the equipment’s electrical design. Some applications, such as telecom systems, may also require positive-grounded systems. Historically, most PV systems in the United States have been negative- grounded. However, depending on the type of inverter used, an ungrounded PV system may be required.
Because neither DC conductor is grounded in ungrounded PV systems, NEC requirements for ungrounded conductors must be followed. These include overcurrent protection (fusing) requirements per NEC 240.15, color-marking requirements, and disconnecting requirements.
In an ungrounded system, per NEC 690.35(B), each positive and negative conductor must be protected by an overcurrent device. Note that there is an exception in NEC 690.9(A) that doesn’t require overcurrent protection for PV source circuits with limited sources of current that don’t exceed the conductors’ ampacity. An example is one or two paralleled source circuits connected to an inverter that can’t back-feed current. Both positive and negative must be marked with a color other than white or gray, and green or bare—per NEC 200.6 and 250.119, those colors are reserved for grounded and grounding conductors.
For ungrounded DC systems, some PV installers use red to mark positive and black to mark negative, although these colors are also the convention for 120/240 VAC systems and 12 VDC automotive systems. While Code-compliant, using red and black could cause confusion during installation, or make troubleshooting harder if AC and DC conductors are in the same raceway. Other colors that might better be used for marking the positive and negative conductors include yellow, blue, purple, or brown. Also, while the same color could be used for both positive and negative conductors and still be Code-compliant, it is not a good idea. This could lead to polarity mistakes during installation, as well as make future troubleshooting difficult. When trying to determine the system type, a helpful distinction is that a grounded PV array will always have white or gray conductors (for the grounded conductor) on the DC side; an ungrounded array will have no white or gray conductors.
In an ungrounded system, a disconnecting means (a manually operable switch or circuit breaker) must be provided for both the positive and negative conductors, following the disconnecting requirements of NEC 690, Section III (see “Code Corner” in HP119). In addition, the module leads (PV source circuits) and exposed-wire home runs in ungrounded systems must be listed and identified as “PV wire,” which has thicker insulation on the conductors, making it extremely durable and UV resistant. Many modules now come with PV wire leads. But some have leads that are listed only as USE-2/RHW-2, and the NEC prohibits those modules from being used in ungrounded systems. Furthermore, a transformerless inverter cannot usually be retrofitted into a PV system that has modules without PV wire leads—replacing factory-installed module leads will void the module warranty.