NEC 2017 Updates for PV Systems: Page 3 of 4
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SIMULATING VOLTAGE AND CURRENT
NEC 2017 provides new options for calculating voltage and current in a PV array. Specifically, Section 690.7(A) now allows a licensed engineer to use a simulation to calculate the maximum PV source and output voltage for a PV system with a capacity greater than 100 kW. A revision to 690.8 similarly allows engineers to simulate the maximum PV source and output current for systems over 100 kW. When an engineer uses a simulation for current, the calculated value may not be less than 70% of the value as determined by the traditional approach (1.25 x Isc).
The benefit of simulating voltage and current is that it enables much more accurate calculations. Array ampacity, for example, is based on continuous load, defined in Article 100 as “the maximum current expected to continue for 3 hours or more.” Computer modeling can accurately simulate this maximum 3-hour current value for a specific PV array based on its location and orientation. By comparison, the traditional method of calculating PV circuit currents significantly oversizes conductors, especially given recent improvements in short-circuit protection. The new calculation method will reduce conductor and conduit costs, which make up an increasing percentage of the overall costs in large PV systems.
The process of updating Section 690.12, “Rapid Shutdown of PV Systems on Buildings,” was by far the most contentious part of the 2017 revision cycle related to Article 690. The rapid-shutdown requirements in NEC 2014 represent a compromise between fire service representatives, who wanted to control conductors within the array, and PV industry stakeholders, who felt that it was too early to do so, based on concerns about technology costs and reliability. As expected, the 2017 revision cycle reopened this debate about whether to impose additional requirements for reducing hazards within the PV array.
Fire service representatives submitted a proposal to control conductors within the array to 80 V or less. The Solar Energy Industries Association (SEIA) submitted a competing proposal to refine the NEC 2014 requirements and make them more enforceable. This heated debate continued during the public comment period, as stakeholders developed new concepts for addressing electrical hazards within the PV array. As a result of this debate, the fire service and SEIA proposals found more common ground by the end of the comment period. The SEIA proposal focused on reducing hazards within the PV array by requiring listed and labeled or field-labeled rapid-shutdown PV arrays, a concept that NFPA’s Fire Fighter Safety and PV Systems Task Group developed. The fire service included this same requirement as a compliance option added to its original proposal.
After the public comment period, it was up to CMP 4 to decide how to proceed. After much deliberation—far more than for any other topic—CMP 4 combined the SEIA and fire service proposals and added language allowing for building-integrated glass or polymeric PV arrays with completely concealed wiring. The revised rapid-shutdown requirements in 690.12(B)(2) provide three compliance options for reducing hazards within a PV array.
- Option 1: List and label or field-label PV array as a rapid-shutdown PV array.
- Option 2: Limit control conductors within the array boundary to 80 V or less within 30 seconds of rapid-shutdown initiation.
- Option 3: Install nonmetallic PV array with no exposed wiring and array more than 8 feet from any grounded metal parts.
To implement Option 1, industry stakeholders need to develop a product safety standard for rapid-shutdown PV arrays. To allow time for this standard’s development process, CMP 4 added a delayed adoption date, specifying that 690.12(B)(2) “shall become effective January 1, 2019.” One benefit of codifying Options 2 and 3 is that these provide stakeholders with some guidance on developing a consensus for the rapid-shutdown PV array certification standard.
Labeled vs. identified. The NEC 2017 requirements for “listed and labeled” rapid-shutdown equipment meaningfully revises NEC 2014, which requires “listed and identified” equipment. Article 100 defines the term identified as “suitable for the specific purpose, function, use, environment, application, and so forth.” By contrast, the definition of labeled in Article 100 “indicates compliance with appropriate standards or performance in a specified manner.” The latter is more prescriptive and narrowly defined than the former.
In practice, this means that installers can use off-the-shelf electrical components to meet NEC 2014 rapid-shutdown requirements so long as the conditions of use are consistent with the equipment ratings. For example, under NEC 2014, you could locate a contactor combiner at the edge of a PV array and use this to meet Section 690.12 as long as the combiner was rated for the outdoor environment and the PV voltage and current characteristics. NEC 2017 will require that this contactor combiner be specifically listed to a rapid-shutdown PV array standard and labeled accordingly. Revised language in 690.12(D) states: “Equipment that performs the rapid-shutdown functions, other than initiation devices such as listed disconnect switches, circuit breakers or control switches, shall be listed and labeled for providing rapid-shutdown protection.”
This is an important distinction. Some jurisdictions— including New Jersey, New Mexico and Washington—have misinterpreted NEC 2014 requirements and asked installers and vendors to prove that rapid-shutdown solutions comply with a rapid-shutdown safety standard. As of today, no such standard exists. Until the NEC 2017 is adopted, there is no requirement that equipment used for rapid shutdown be listed and labeled specifically for the rapid shutdown of PV arrays. Installers can use any listed equipment to provide rapid shutdown, so long as they field the equipment in a manner consistent with the product listing.
Plaque or directory. One of the most important parts of the rapid-shutdown requirements is properly communicating the level of hazard to emergency responders. Section 690.56(C) details the revised field-labeling requirements for PV systems equipped with rapid shutdown, which will help first responders differentiate between systems designed to meet NEC 2014 versus NEC 2017. In the event that a building hosts PV systems built to different Code standards—such as no rapid shutdown (pre NEC 2014), NEC 2014 compliant or NEC 2017 compliant—the field-applied plaque or directory needs to show a plan view of the building with a dotted line around array areas that remain energized after initiation of rapid shutdown, as illustrated in Figure 4.