Ungrounded PV Power Systems in the NEC: Page 8 of 12
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Note that residual-current detection does not provide overcurrent or short-circuit protection. On the ac side of the system, this protection is provided by the overcurrent-protection device required in NEC Section 705.12(D). Overcurrent protection for the PV source or output circuits may be required according to Section 690.9.
NEC Section 690.35 contains specific requirements for ungrounded PV systems. These requirements have implications for what products are used and how, from the inverter upstream to the PV array. It is important that electrical engineers and PV system designers understand these requirements so that they can specify the right components in their plans for ungrounded PV systems. Similarly, electricians and PV system installers need to understand these requirements well enough to verify that the correct components are called out in the plans and that suitable materials are available. The inventory requirements for ungrounded and grounded PV systems are meaningfully different. Many common mistakes can be avoided by ensuring that the components called for in ungrounded PV system designs meet the requirements outlined in NEC Section 690.35.
Design and Deployment Considerations
Installing ungrounded PV systems with non-isolated inverters is very similar to installing grounded PV systems with isolated inverters. The same tools and personnel are used. In many cases, the same PV modules and mounting systems are used. The part numbers for the inverters and some of the BOS components may differ, but the parts will be similar to install. According to SMA’s Smith, “Installers with experience installing grounded PV systems will find their existing skill set well suited to installing ungrounded PV systems.” This is because the important differences between ungrounded and grounded PV systems relate to system design and product specification. Smith explains, “While the actual installation differences are few, there are important design differences.”
Inverter. The inverters used in ungrounded PV systems must be tested and listed for the application; see Section 690.35(G). While UL refers to these as non-isolated inverters, it is far more common for manufacturers to refer to these products as transformerless inverters. In the “2012 Grid-Direct String Inverter Specifications” table (pp. 50–62), several manufacturers— including, Advanced Energy, Chint, Growatt, Ingeteam and SMA—use the letters “TL” in the inverter model number to identify their non-isolated products. Other transformerless inverters in the table include Eaton’s PV240, PV250, PV260 and PV270; Exeltech’s XLG18A60; KACO new energy’s blueplanet 6400xi and 7600xi; Power-One’s PVI-3.0, PVI-3.6, PVI-4.2, PVI- 5000 and PVI-6000; and REFUsol’s 012K-UL, 016K-UL, 020K-UL and 024K-UL.
While all inverters for PV applications are listed to UL 1741 and need to conform with the power quality and safety tests found in IEEE 1547.1, the NRTL evaluates non-isolated inverters to a slightly different set of conditions than isolated inverters. Because of the inherent design differences between these products, installers and inspectors need to ensure that inverters used in ungrounded PV systems are listed and labeled as non-isolated inverters. An installer cannot field-configure an isolated inverter to operate on an ungrounded PV system. Similarly, a non-isolated inverter may not be field-modified for any purpose except as indicated in the manufacturer’s installation manual. Always refer to the specific manufacturer’s documentation for installation requirements and allowances.
Inverter input and PV output conductors. One of the easiest mistakes for installers to make is to instinctively grab white wire off the truck for use in an ungrounded PV array. System designers can make the same mistake and specify white wire in their drawings, especially if they start working from existing plans for a grounded PV array. Ungrounded PV power circuits may not use conductors with white insulation.