Voltage Thresholds in the NEC. Moving on up?


There were 120 proposals submitted to raise the 600-volt threshold in the NEC to 1000-volts in the 2014 NEC cycle. These proposals were submitted by the High Voltage Task Group (HVTG), which was appointed by the NEC Correlating Committee. The HVTG was charged to review all NEC requirements and/or the lack of requirements for circuits and systems operating at over 600-volts.

FPN reference to NESC deleted

The origin of this task group began at the end of the 2008 NEC cycle when a fine print note (FPN) [now Informational Notes] referencing the National Electrical Safety Code (NESC) was deleted from 90.2(A)(2). The substantiation provided in the proposal to delete the FPN stated that “conductors on the load side of the service point are under the purview of the NEC, and the FPN sending NEC users to the NESC creates serious confusion for designers, installers and the authority having jurisdiction (AHJ) working on premises wiring at voltage levels over 600-volts”. The proposal was supported by comments that pointed out conflicts that place the AHJ in a very difficult position and the FPN was deleted in the 2008 NEC. The NEC Correlating Committee then appointed the High Voltage Task Group to address issues with installations over 600 volts”.

Article 399 created

In the 2011 NEC, a proposal submitted by the HVTG created a new Article 399, Outdoor Overhead Conductors over 600 Volts. This new article was developed to provide the AHJ with NEC requirements to address the outdoor installations referenced in 90.2(A)(2) without a broad reference to another standard. It is interesting to note that most of the requirements in Article 399 are performance based. The requirements for conductor support and structures outline the installation requirements to consider without being prescriptive. In each case there must be documentation of an engineered design submitted by a licensed professional engineer engaged primarily in the design of such systems. This new article permits an engineer to design the installation in accordance with NESC requirements provided the design is documented and available to the AHJ.

Photo 1. XHHW cable labeled 600 volts

Raising the voltage threshold

The work of the High Voltage Task Group continued into the 2014 NEC cycle with a primary focus on raising the voltage threshold in the NEC from 600 to 1000 volts. This is not the first coordinated attempt to raise the voltage threshold. In the 1990 NEC revision cycle, a Correlating Committee task group tried unsuccessfully to raise the voltage threshold. Finding substantiation on how the NEC settled at 600 volts is difficult. The threshold was raised from 550 to 600 volts in the 1920 NEC. In 1990 it was difficult to substantiate a need to raise the voltage threshold. Today, emerging technologies are operating at just over the 600-volt threshold. We need product standards and installation requirements to facilitate their safe installation. The electrical industry is changing rapidly and codes/standards must keep pace; otherwise, we will be forced to use the International Electrotechnical Commission (IEC*) products and installation standards other than the NEC.

Is going from 600 to 1000 volts the right number? Small wind electric systems often operate at 690 volts AC but solar photovoltaic (PV) systems are currently being installed at dc voltages over 600 volts up to and including 1000 volts, 1200 volts, 1500 volts, and 2000 volts dc. These dc systems are expanding and have become a more integral part of many structures. Small wind electric systems and solar photovoltaic (PV) systems are employed regularly in and on all types of structures from dwellings units to large retail and high rise construction.

Photo 2. Cube fuses labeled 600 VAC or less

The first direction that the Higher Voltage Task Group took was to simply suggest revisions in Chapter 6 of the NEC for Special Equipment. It was quickly understood that changes throughout the NEC were required. Chapter 6 requirements simply modify and/or supplement the rules in Chapters 1 through 4. A review of the UL White Book reveals that UL has many products that are utilized in these systems rated at and above 600 volts, including but not limited to, 1000-V dc PV switches, 1500-Vdc PV fuses, and 2000-V PV wiring. The HVTG realized that the NEC must recognize those products through installation requirements or we will continue to have problems installing and inspecting systems for PV and small wind. The HVTG proposals to raise the voltage threshold recognize emerging technologies that are in many cases operating at just over 600 volts. Everyone needs to play a role in this transition. The present NEC requirements would literally require that a PV system operating at 1000 volts dc utilize a disconnecting means rated at 5 kV. The manufacturers, research and testing laboratories, and the NEC must work together to develop installation requirements and product standards to support these emerging technologies.

Moving the NEC threshold from 600 volts to 1000 volts will not, by itself, allow the immediate installation of systems at 1000 volts. Equipment must first be tested and found acceptable for use at the higher voltage(s). The testing and listing of equipment will not, by itself, allow for the installation of 1000-volt systems. The NEC must include prescriptive requirements to permit the installation of systems that are over 600 but less than or equal to 1000 volts. It will take both tested/listed equipment and changes in our installation code, the NEC, to meet the needs of these emerging technologies that society demands.

Eighty-two percent of the proposals submitted to raise the voltage threshold were accepted in some form. Where a code-making panel felt there was a safety issue or where manufacturers did not want to pursue having their products evaluated at 1000 volts, the Higher Voltage Task Group agreed to reject.

Moving the NEC to 1000 volts is just the beginning. This is just the first step of many to recognize emerging technology with prescriptive requirements to ensure that these systems and products can be safely installed and inspected in accordance with the NEC.

*The IEC is a nonprofit, nongovernmental international standards organization that prepares and publishes international standards for electrical and electronic technologies.