Going Transformer-less

Some photovoltaic (PV) inverter manufacturers have designed and built transformer-less inverters to add to their existing line of transformer-type inverters for installation in the United States. Transformer-less inverters have been popular in Europe for quite some time. In comparison to the heavier same-wattage transformer-style inverters, they weigh much less, ultimately cost less, do not suffer the same energy losses, and are usually installed as ungrounded systems. However, some issues with transformer-less inverters must be understood, because they may affect the decision to install them or instead go with the conventional grounding-type transformer inverters.

Section 690.35 of the 2014 National Electrical Code (NEC) provides requirements for the installation and application of ungrounded PV direct current (DC) source and DC output circuits. Source-circuit conductors are defined as the circuits between modules or from the modules to the DC system’s common connection points, such as the combiner box. PV-output circuits are the circuit conductors between the source circuit (modules) and the inverter, such as the combiner box to the inverter. 

The ungrounded power systems must comply with certain requirements. Since both the positive and negative conductors are ungrounded, they must have overcurrent protection sized based on 690.9 of the NEC. Both negative and positive ungrounded conductors must be able to be simultaneously disconnected in accordance with Part III of Article 690, rather than disconnecting just the positive ungrounded conductor in a grounded system.

All ungrounded PV source and PV-output circuits must be provided with a listed ground-fault protection device or system that detects ground faults in the PV DC current-carrying conductors and components with indication that a ground fault has occurred. The ground-fault protection system must automatically disconnect all conductors or cause the inverter or charge controller connected to the faulted circuit to automatically cease supplying power to any output circuits. Ground-fault detection systems are required by 250.21 for all ungrounded alternating current (AC) systems of 50 to 1,000 volts and by 250.167(A) for ungrounded DC systems. 

Similar to the marking requirements in 250.167(C) for marking grounding types at the DC source or the first disconnecting means, 690.35(F) requires the power source to be labeled at each junction box, combiner box, disconnect and device where energized and exposed ungrounded circuits conductors could be serviced. 

The label must read as follows: “Warning. Electric shock hazard. The DC conductors of this PV system are ungrounded and may be energized.” Conductors for the ungrounded system must be metallic or nonmetallic jacketed multiconductor cables; installed in raceways; listed and identified as PV wire installed as exposed, single conductors; or direct-buried and identified for direct-burial use.

The most important part of installing an ungrounded, transformer-less PV system is to ensure the proper grounding electrode system adheres to 690.47(B), which references the requirements in 250.169 for ungrounded systems. Ungrounded DC separately derived systems are covered in 250.169, which states that an ungrounded DC system supplied from a standalone power source, such as a PV system, must have a grounding electrode conductor connected to an electrode that complies with Part III of Article 250 to provide for grounding of metal enclosures, raceways, cables and exposed noncurrent­-­carrying metal parts of equipment. The grounding electrode conductor connection must be made to the metal enclosure at any point on the separately derived system from the source (the PV modules) to the first system disconnecting means or overcurrent device, or the connection must be made at the source of the separately derived system that does not have a disconnecting means.

The size of the grounding electrode conductor for the DC side of the ungrounded transformer-less inverter is based on requirements in 250.166. The size of the grounding electrode conductor connected to the enclosure or the disconnecting means for the PV DC system is based on 250.166(B), (C) or (D). Section 250.166(B) requires it to be no smaller than the largest conductor supplied by the system and not smaller than 8 AWG copper or 6 AWG aluminum. If a rod, pipe or plate electrode is used, the grounding electrode conductor is not required to be larger than a 6 AWG copper or a 4 AWG aluminum. Where connected to a concrete-encased electrode, the grounding electrode conductor is not required to be larger than a 4 AWG copper wire, and where connected to a ground ring, the grounding electrode conductor is not required to be larger than the conductor used for the ground ring. New to the 2014 NEC: in no case shall the grounding electrode conductor for a DC system be required to be larger than 3/0 copper or 250 kcmil aluminum.

Be sure to follow the grounding electrode requirements for ungrounded transformer-­less inverter systems.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and Mark.C.Ode@ul.com.

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