Ungrounded PV Power Systems in the NEC: Page 9 of 12
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Conductors with white insulation are commonly used in positively or negatively grounded PV systems to identify the grounded conductor. However, both of the current-carrying conductors in an ungrounded PV system are floating with reference to ground, so there is no grounded conductor. Therefore, according to Section 200.7 of the Code, neither polarity in an ungrounded PV system can be wired using conductors with a continuous white insulation. This section also excludes the use of conductors with continuous gray insulation or with three continuous white stripes for ungrounded PV array circuits. These colors are reserved for grounded circuits. Insulation that is green or green with yellow stripes can be used to identify grounding conductors only; see Section 250.119.
Aside from these restrictions, the NEC does not dictate what color code to use for conductors in an ungrounded PV system. Just be sure to use different colors for PV positive and PV negative conductors to make it easy to maintain the correct polarity throughout the system. Many dc circuits in the US use red for positive and black for negative. Since these colors are probably already in your inventory, this is a logical convention to follow. However, the Code provides installers with a great deal of leeway to do otherwise. In the event that specific color codes do apply, the local AHJ dictates them.
DC disconnects. The brevity with which NEC Section 690.35(A) describes the requirements for dc disconnects in ungrounded PV systems is somewhat misleading: “All photovoltaic source and output circuit conductors shall have disconnects complying with 690, Part III.” In practice, this is one of the most significant changes to how ungrounded PV circuits are wired when compared to grounded systems.
To understand the implications of Section 690.35(A), look at Section 690.13, which is the first section under Part III of Article 690. As a rule, Section 690.13 requires that a means be provided to disconnect the current-carrying conductors in a PV array from other conductors in a building or structure. However, this does not apply to the grounded conductor if opening it with a switch or a circuit breaker would leave “the marked, grounded conductor in an ungrounded and energized state.” This is why disconnects in grounded PV array circuits open ungrounded currentcarrying conductors only, and the grounded conductors are unbroken (except as allowed in Section 690.13 Exceptions No. 1 and No. 2).
In an ungrounded PV system, both the PV positive and the PV negative conductors are ungrounded current-carrying conductors. Since NEC Section 690.13 requires a disconnecting means for all ungrounded current-carrying dc conductors, both the PV positive and the PV negative conductors need to be switched in an ungrounded PV system. Double-pole disconnecting must be used to switch both conductors.
According to a survey of transformerless inverter manufacturer representatives, it is not unheard of for installers used to working with grounded PV systems to make the mistake of switching just one of the dc conductors when wiring a PV disconnect external to the inverter in an ungrounded PV system. Fortunately, this is often an easy fix. In some cases, no additional equipment is required. As Berdner at SolarEdge points out: “Most switches and breakers used today are wired with multiple poles in series, allowing some of the poles to be used for the other conductor. If no additional poles are necessary, then the cost impact of installing a PV disconnect in an ungrounded system versus a grounded system is minimal.”
Designers should be aware that many of the off-the-shelf disconnects commonly used to disconnect multiple PV circuits in grounded systems are not suitable for the same application in ungrounded systems. For example, a three-pole disconnect that is rated to disconnect two or three source circuits in a grounded PV array is able to disconnect only a single circuit in an ungrounded system. Because there is a disparity between ungrounded PV system requirements and commonly available electrical parts, many transformerless inverter manufacturers have proactively taken steps to close this gap.
Many UL-listed string inverters and some central inverters incorporate an integrated dc disconnecting means intended to meet the requirements in NEC Section 690.13. This is also true of non-isolated utility-interactive inverters. Residential system integrators may not need to go to the trouble and additional expense of sourcing and carrying listed multi-pole dc disconnects for use in ungrounded PV systems. Simply use the inverter-integrated PV disconnect as indicated in the manufacturer’s installation instructions, which an NRTL evaluates as part of the product testing and listing process. For larger systems with combiner boxes external to the inverters, appropriate switches need to be sourced.